JP7812892B2 - Prediction device, image encoding device, image decoding device, and program - Google Patents

Description

The present invention relates to an intra-prediction device, an image encoding device, an image decoding device, and a program.

Research is being conducted into video coding methods to compress the amount of data used to transmit and store still and moving images. In recent years, ultra-high-resolution video, such as 8K-SHV, has become increasingly popular in video coding technology, and coding methods such as AVC/H.264 and HEVC/H.265 are known as methods for transmitting massive amounts of moving image data.

The evaluation software (VTM) for VVC, the next-generation video coding standard being jointly standardized by MPEG and ITU, uses intra-prediction, which takes advantage of spatial correlation within a frame (see non-patent document 1). Using decoded reference pixels surrounding the image block to be coded, the encoder selects the optimal mode from a total of 67 prediction modes, including planar prediction, DC prediction, and 65 directional predictions, and sends this information to the decoder.

As a method for improving the prediction accuracy of intra prediction, a prediction image synthesis method has been proposed in which a prediction image is generated using two intra prediction modes and each pixel of the two prediction images is added together to generate a new prediction image (see Non-Patent Document 2). Specifically, a prediction image is generated by adding a directional prediction image, which is a prediction image generated using one of the 65 directional prediction modes mentioned above, and a planar prediction image, which is a prediction image generated using the planar mode.

JVET-L1001 “Versatile Video Coding (Draft 3)” JVET-M0458 “Non-CE3: Combined-Hypothesis Intra-Prediction”

By the way, directional prediction has the disadvantage that while it has high prediction accuracy for pixels close to the reference pixel, the prediction accuracy can decrease as the pixel moves away from the reference pixel.

However, the predicted image synthesis method described in Non-Patent Document 2 simply averages directional predicted images and planar predicted images on a pixel-by-pixel basis, and does not take into account the drawbacks of directional prediction, leaving room for improvement in terms of further improving the prediction accuracy of intra prediction.

Therefore, the present invention aims to provide an intra prediction device, an image encoding device, an image decoding device, and a program that further improve the prediction accuracy of intra prediction using a predicted image synthesis technique.

An intra prediction device according to a first aspect performs intra prediction on image blocks obtained by dividing an original image. The intra prediction device includes a first predicted image generation unit that predicts the image block using a first intra prediction mode, which is directional prediction, to generate a first predicted image; a second predicted image generation unit that predicts the image block using a second intra prediction mode, which is non-directional prediction, to generate a second predicted image; a weighting coefficient determination unit that determines a weighting coefficient to be used when weighting and combining the first predicted image and the second predicted image based on at least one of the block size and block shape of the image block; and an image synthesis unit that weights and combines the first predicted image and the second predicted image using the weighting coefficient determined by the weighting coefficient determination unit.

The image encoding device according to the second aspect includes the intra prediction device according to the first aspect.

The image decoding device according to the third aspect includes the intra prediction device according to the first aspect.

A program according to the fourth aspect causes a computer to function as the intra prediction device according to the first aspect.

The present invention provides an intra prediction device, an image encoding device, an image decoding device, and a program that further improve the prediction accuracy of intra prediction using a predicted image synthesis technique.

FIG. 1 is a diagram illustrating a configuration of an image encoding device according to an embodiment. FIG. 10 is a diagram illustrating prediction modes of intra prediction according to the embodiment. FIG. 2 is a diagram illustrating a configuration of an intra prediction unit of an image encoding device according to an embodiment. FIG. 10 is a diagram illustrating an example of the operation of an intra prediction unit according to the embodiment. FIG. 10 is a diagram illustrating a first example of the operation of a weighting coefficient determination unit according to the embodiment. FIG. 10 is a diagram illustrating a second example of the operation of the weighting coefficient determination unit according to the embodiment. FIG. 1 is a diagram illustrating a configuration of an image decoding device according to an embodiment. FIG. 2 is a diagram illustrating a configuration of an intra prediction unit of an image decoding device according to an embodiment. FIG. 10 is a diagram illustrating an example of an operation flow of an intra prediction unit according to the embodiment.

An image encoding device and an image decoding device according to an embodiment will be described with reference to the drawings. The image encoding device and the image decoding device according to the embodiment encode and decode moving images, respectively, as typified by MPEG. In the following description of the drawings, the same or similar parts are designated by the same or similar reference numerals.

<Configuration of Image Encoding Device>
First, an image encoding device according to this embodiment will be described. Fig. 1 is a diagram showing the configuration of an image encoding device 1 according to this embodiment.

As shown in FIG. 1, the image coding device 1 includes a block division unit 100, a subtraction unit 110, a transformation/quantization unit 120, an entropy coding unit 130, an inverse quantization/inverse transformation unit 140, a synthesis unit 150, a memory 160, and a prediction unit 170.

The block division unit 100 divides an original image, which is an input image in units of frames (or pictures) that make up a moving image, into multiple image blocks and outputs the image blocks obtained by division to the subtraction unit 110. The size of the image blocks is, for example, 32 x 32 pixels, 16 x 16 pixels, 8 x 8 pixels, or 4 x 4 pixels. The shape of the image blocks is not limited to squares and may be rectangular (rectangular). An image block is a unit of encoding performed by the image encoding device 1 (block to be encoded) and a unit of decoding performed by the image decoding device (block to be decoded). Such an image block is sometimes called a CU (Coding Unit).

The subtraction unit 110 calculates a prediction residual that represents the difference (error) between the current block to be coded input from the block division unit 100 and the predicted image obtained by the prediction unit 170 predicting the current block to be coded. Specifically, the subtraction unit 110 calculates the prediction residual by subtracting each pixel value of the predicted image from each pixel value of the block, and outputs the calculated prediction residual to the transformation/quantization unit 120.

The transform/quantization unit 120 performs orthogonal transform processing and quantization processing on a block-by-block basis. The transform/quantization unit 120 includes a transform unit 121 and a quantization unit 122.

The transform unit 121 performs orthogonal transform processing on the prediction residuals input from the subtraction unit 110 to calculate orthogonal transform coefficients, and outputs the calculated orthogonal transform coefficients to the quantization unit 122. Examples of orthogonal transform include the discrete cosine transform (DCT), discrete sine transform (DST), and Karhunen-Loeve transform (KLT).

The quantization unit 122 quantizes the orthogonal transform coefficients input from the transform unit 121 using a quantization parameter (Qp) and a quantization matrix, and outputs the quantized orthogonal transform coefficients to the entropy coding unit 130 and the inverse quantization and inverse transform unit 140. The quantization parameter (Qp) is a parameter that is commonly applied to each orthogonal transform coefficient in a block and determines the coarseness of quantization. The quantization matrix is a matrix whose elements are the quantization values used when quantizing each orthogonal transform coefficient.

The entropy coding unit 130 performs entropy coding on the orthogonal transform coefficients input from the quantization unit 122, compresses the data, generates coded data (bitstream), and outputs the coded data to the outside of the image coding device 1. Entropy coding can use Huffman coding or CABAC (Context-based Adaptive Binary Arithmetic Coding), among others. The entropy coding unit 130 also receives information such as syntax related to prediction from the prediction unit 170 and performs entropy coding on the input information.

The inverse quantization and inverse transform unit 140 performs inverse quantization and inverse orthogonal transform processing on a block-by-block basis. The inverse quantization and inverse transform unit 140 includes an inverse quantization unit 141 and an inverse transform unit 142.

The inverse quantization unit 141 performs inverse quantization processing corresponding to the quantization processing performed by the quantization unit 122. Specifically, the inverse quantization unit 141 restores the orthogonal transform coefficients by inverse quantizing the orthogonal transform coefficients input from the quantization unit 122 using a quantization parameter (Qp) and a quantization matrix, and outputs the restored orthogonal transform coefficients to the inverse transform unit 142.

The inverse transform unit 142 performs inverse orthogonal transform processing corresponding to the orthogonal transform processing performed by the transform unit 121. For example, if the transform unit 121 performs a discrete cosine transform, the inverse transform unit 142 performs an inverse discrete cosine transform. The inverse transform unit 142 performs inverse orthogonal transform processing on the orthogonal transform coefficients input from the inverse quantization unit 141 to restore the prediction residual, and outputs the restored prediction residual, which is the restored prediction residual, to the synthesis unit 150.

The synthesis unit 150 synthesizes, on a pixel-by-pixel basis, the reconstructed prediction residual input from the inverse transform unit 142 with the predicted image input from the prediction unit 170. The synthesis unit 150 adds each pixel value of the reconstructed prediction residual to each pixel value of the predicted image to reconstruct (decode) the block to be coded, and outputs the decoded image on a block-by-block basis to the memory 160. Such a decoded image is sometimes called a reconstructed image.

Memory 160 stores the decoded image input from synthesis unit 150. Memory 160 stores the decoded image on a frame-by-frame basis. Memory 160 outputs the stored decoded image to prediction unit 170. Note that a loop filter may be provided between synthesis unit 150 and memory 160. Note that part of memory 160 may be included in prediction unit 170.

The prediction unit 170 performs prediction on a block-by-block basis. The prediction unit 170 includes an inter prediction unit 171, an intra prediction unit 172, and a switching unit 173.

The inter prediction unit 171 uses the decoded image stored in the memory 160 as a reference image to calculate a motion vector using a technique such as block matching, predicts the block to be coded, generates an inter prediction image, and outputs the generated inter prediction image to the switching unit 173.

The inter prediction unit 171 selects the optimal inter prediction method from among inter prediction using multiple reference images (typically bi-prediction) and inter prediction using a single reference image (unidirectional prediction), and performs inter prediction using the selected inter prediction method. The inter prediction unit 171 outputs information related to the inter prediction (motion vectors, etc.) to the entropy coding unit 130.

The intra prediction unit 172 generates an intra prediction image by referencing decoded pixel values surrounding the block to be coded among the decoded images stored in the memory 160, and outputs the generated intra prediction image to the switching unit 173. The intra prediction unit 172 also outputs syntax related to the selected prediction mode to the entropy coding unit 130. Hereinafter, the block to be intra prediction is referred to as the intra prediction target block.

The intra prediction unit 172 selects the optimal prediction mode to apply to the block to be intra-predicted from among multiple prediction modes, and predicts the block to be intra-predicted using the selected prediction mode.

Figure 2 is a diagram showing prediction modes for intra prediction according to this embodiment. As shown in Figure 2, there are 67 prediction modes, from 0 to 66. Prediction mode mode "0" is planar prediction, prediction mode mode "1" is DC prediction, and prediction modes "2" to "66" are directional prediction. In directional prediction, the direction of the arrow indicates the prediction direction, the starting point of the arrow indicates the position of the pixel to be predicted, and the end point of the arrow indicates the position of the reference pixel used to predict this pixel to be predicted. Modes "2" to "33" are prediction modes that primarily reference reference pixels on the left side of the block to be intra predicted. On the other hand, modes "35" to "66" are prediction modes that primarily reference reference pixels on the top side of the block to be intra predicted.

The switching unit 173 switches between the inter-predicted image input from the inter-prediction unit 171 and the intra-predicted image input from the intra-prediction unit 172, and outputs either of the predicted images to the subtraction unit 110 and the synthesis unit 150.

Figure 3 shows the configuration of the intra prediction unit 172 according to this embodiment. The intra prediction unit 172 corresponds to the intra prediction device provided in the image encoding device 1.

As shown in FIG. 3, the intra prediction unit 172 includes a memory 160a, a first predicted image generation unit 172a, a second predicted image generation unit 172b, a weighting coefficient determination unit 172c, and an image synthesis unit 172d.

Memory 160a is part of memory 160 shown in Figure 1. Memory 160a stores reference pixels, which are decoded pixels that are referenced during intra prediction.

The first predicted image generation unit 172a predicts the block to be intra-predicted using a first intra-prediction mode, which is directional prediction, to generate a directional predicted image (first predicted image), and outputs the generated directional predicted image to the image synthesis unit 172d. Specifically, the first predicted image generation unit 172a references reference pixels stored in memory 160a and generates a directional predicted image using one of 65 directional prediction modes.

The second predicted image generation unit 172b predicts the block to be intra-predicted using a second intra prediction mode, which is non-directional prediction, to generate a predicted image (second predicted image), and outputs the generated predicted image to the image synthesis unit 172d. Specifically, the second predicted image generation unit 172b references reference pixels stored in memory 160a and generates a directional predicted image using a predetermined non-directional second intra prediction mode.

The second intra prediction mode may be any non-directional prediction mode, but in this embodiment, an example will be described in which the second intra prediction mode is planar prediction.

The weighting coefficient determination unit 172c determines a weighting coefficient based on at least one of the block size and block shape of the block to be intra-predicted, and outputs the determined weighting coefficient to the image synthesis unit 172d. The weighting coefficient output by the weighting coefficient determination unit 172c is used when weighting and synthesizing the directional predicted image generated by the first predicted image generation unit 172a and the planar predicted image generated by the second predicted image generation unit 172b.

The image synthesis unit 172d uses the weighting coefficients input from the weighting coefficient determination unit 172c to weight-synthesize the directional prediction image input from the first prediction image generation unit 172a and the planar prediction image input from the second prediction image generation unit 172b, and outputs the weighted-synthesized prediction image as an intra-prediction image.

Figure 4 is a diagram showing an example of the operation of the intra prediction unit 172 according to this embodiment. Figure 4 illustrates an example in which the block to be intra predicted is a square of 8x8 pixels, but the block to be intra predicted does not have to be square. Also, the black circles in Figure 4 represent reference pixels.

As shown in Figure 4(a), the second predicted image generation unit 172b predicts the intra-prediction target block using planar prediction to generate a planar predicted image. Specifically, planar prediction generates predicted pixel values by interpolation prediction using the four reference pixels shown in Figure 4(a).

As shown in Figure 4(b), the first predicted image generation unit 172a predicts the block to be intra-predicted using a directional prediction mode to generate a directional predicted image. Directional prediction generates predicted pixel values by extrapolating reference pixels along the prediction direction, so while prediction accuracy is high for pixels close to the reference pixels, it has the disadvantage that prediction accuracy can decrease as pixels move farther away from the reference pixels.

The weighting factor determination unit 172c determines weighting factors based on at least one of the block size and block shape of the block to be intra-predicted. In this embodiment, the weighting factor determination unit 172c determines the weighting factor α (second weighting factor) to be applied to the planar predicted image and the weighting factor β (first weighting factor) to be applied to the directional predicted image.

As shown in FIG. 4(c), the image synthesis unit 172d applies a weighting factor β to each pixel of the directional predicted image generated by the first predicted image generation unit 172a, and applies a weighting factor α to each pixel of the planar predicted image generated by the second predicted image generation unit 172b, synthesizes the directional predicted image and the planar predicted image on a pixel-by-pixel basis, and outputs the synthesized predicted image as an intra-predicted image.

Figure 5 shows a first example of the operation of the weighting coefficient determination unit 172c according to this embodiment. In Figure 5, the "length of the shortest side" is the length of the shortest side of the rectangular block to be intra-predicted, expressed in number of pixels. In other words, the "length of the shortest side" represents the block size of the block to be intra-predicted. Meanwhile, "α:β" in Figure 5 represents the ratio of weighting coefficient α to weighting coefficient β.

Here, "the length of the smallest side" is presented as an example of the block size of the block to be intra-predicted, but other examples such as "block width + block height" or "block width x block height" can also be used.

As shown in Figure 5, the weighting coefficient determination unit 172c determines the weighting coefficients α and β so that the ratio of the weight (α) of the planar prediction image to the weight (β) of the directional prediction image increases as the block size of the block to be intra-predicted increases.

In the example shown in Figure 5, when the block size is "8", the weight (α) of the planar predicted image and the weight (β) of the directional predicted image are equal. In this case, the image synthesis unit 172d outputs the predicted image obtained by averaging the planar predicted image and the directional predicted image as the intra predicted image. Hereinafter, the block size "8" will be referred to as the reference block size.

When the block size is "4" which is smaller than the reference block size, the weighting coefficient determination unit 172c determines the weighting coefficients α and β so that the weight of the planar predicted image (α = 1) is smaller than the weight of the directional predicted image (β = 2). In this case, the proportion of the planar predicted image in the intra predicted image output by the image synthesis unit 172d becomes smaller.

On the other hand, as the block size becomes larger than the reference block size, the weighting coefficient determination unit 172c determines the weighting coefficients α and β so that the weight (α) of the planar predicted image becomes larger than the weight (β) of the directional predicted image. In this case, the proportion of planar predicted images in the intra predicted images output by the image synthesis unit 172d increases.

Here, in directional prediction, the larger the block size of the block to be intra-predicted, the lower the prediction accuracy of pixels farther from the reference pixel. Therefore, prediction accuracy can be improved by determining the weighting factors α and β so that the proportion of planar predicted images is increased and the proportion of directional predicted images is decreased.

Note that the method for determining the weighting coefficients α and β shown in Figure 5 is just one example, and the specific values of the weighting coefficients α and β are not limited to the example shown in Figure 5.

Figure 6 is a diagram showing a second example of the operation of the weighting coefficient determination unit 172c according to this embodiment. In Figure 6, the "larger of width/height or height/width" refers to the larger of the width/height or height/width values for the block to be intra-predicted. In other words, the "larger of width/height or height/width" is a value that increases as the shape of the block to be intra-predicted becomes more elongated. Meanwhile, "α:β" in Figure 6 represents the ratio of weighting coefficient α to weighting coefficient β.

As shown in Figure 6, the weighting coefficient determination unit 172c determines the weighting coefficients α and β so that the ratio of the weight (α) of the planar predicted image to the weight (β) of the directional predicted image decreases as the ratio of the long side to the short side of the rectangular intra-prediction target block increases. Note that the short side refers to the shorter side of the rectangle, and the long side refers to the longer side of the rectangle.

In other words, the weighting coefficient determination unit 172c determines the weighting coefficients α and β so that the ratio of the weight (α) of the planar prediction image to the weight (β) of the directional prediction image becomes smaller the more elongated the shape of the block to be intra-predicted.

In the example shown in Figure 6, if the "larger of width/height or height/width" is "1," that is, if the target block for intra prediction is square, the weight (α) of the planar predicted image and the weight (β) of the directional predicted image are equal. In this case, the image synthesis unit 172d outputs the predicted image obtained by averaging the planar predicted image and the directional predicted image as the intra predicted image. Hereinafter, the square target block for intra prediction is referred to as the reference shape.

As the intra-prediction target block changes from its reference shape (square) to an elongated shape, the weighting coefficient determination unit 172c determines the weighting coefficients α and β so that the weight (α) of the planar prediction image becomes smaller than the weight (β) of the directional prediction image. In this case, the proportion of the planar prediction image in the intra-prediction image output by the image synthesis unit 172d decreases.

Here, in directional prediction, the more elongated the block to be intra-predicted is, the less likely it is to predict pixels farther away from the reference pixel, potentially resulting in higher prediction accuracy. Therefore, prediction accuracy can be improved by determining the weighting factors α and β so that the proportion of planar predicted images is reduced and the proportion of directional predicted images is increased.

Note that the method for determining the weighting coefficients α and β shown in Figure 6 is just one example, and the specific values of the weighting coefficients α and β are not limited to the example shown in Figure 6.

Furthermore, operation example 1 and operation example 2 of the weighting coefficient determination unit 172c may be combined and implemented. For example, a table like that shown in Figure 6 may be defined separately for each value of the "length of the smallest side" (i.e., block size). Alternatively, a table like that shown in Figure 5 may be defined separately for each value of the "larger of width/height or height/width."

<Configuration of Image Decoding Device>
Next, an image decoding device according to this embodiment will be described. Fig. 7 is a diagram showing the configuration of an image decoding device 2 according to this embodiment.

As shown in FIG. 7, the image decoding device 2 has an entropy decoding unit 200, an inverse quantization/inverse transform unit 210, a synthesis unit 220, a memory 230, and a prediction unit 240.

The entropy decoding unit 200 decodes the coded data generated by the image coding device 1 and outputs the quantized orthogonal transform coefficients to the inverse quantization and inverse transform unit 210. The entropy decoding unit 200 also acquires syntax related to prediction (intra prediction and inter prediction) and outputs the acquired syntax to the prediction unit 240.

The inverse quantization and inverse transform unit 210 performs inverse quantization processing and inverse orthogonal transform processing on a block-by-block basis. The inverse quantization and inverse transform unit 210 includes an inverse quantization unit 211 and an inverse transform unit 212.

The inverse quantization unit 211 performs inverse quantization processing corresponding to the quantization processing performed by the quantization unit 122 of the image encoding device 1. The inverse quantization unit 211 reconstructs the orthogonal transform coefficients of the block to be decoded by inverse quantizing the quantized orthogonal transform coefficients input from the entropy decoding unit 200 using a quantization parameter (Qp) and a quantization matrix, and outputs the reconstructed orthogonal transform coefficients to the inverse transform unit 212.

The inverse transform unit 212 performs inverse orthogonal transform processing corresponding to the orthogonal transform processing performed by the transform unit 121 of the image encoding device 1. The inverse transform unit 212 performs inverse orthogonal transform processing on the orthogonal transform coefficients input from the inverse quantization unit 211 to restore the prediction residual, and outputs the restored prediction residual (restored prediction residual) to the synthesis unit 220.

The synthesis unit 220 reconstructs (decodes) the original block by synthesizing the prediction residual input from the inverse transform unit 212 and the predicted image input from the prediction unit 240 on a pixel-by-pixel basis, and outputs the decoded image on a block-by-block basis to the memory 230.

The memory 230 stores the decoded image input from the synthesis unit 220. The memory 230 stores the decoded image in frame units. The memory 230 outputs the decoded image in frame units to the outside of the image decoding device 2. Note that a loop filter may be provided between the synthesis unit 220 and the memory 230. Furthermore, a portion of the memory 230 may be included in the prediction unit 240.

The prediction unit 240 performs prediction on a block-by-block basis. The prediction unit 240 has an inter prediction unit 241, an intra prediction unit 242, and a switching unit 243.

The inter prediction unit 241 predicts the current block to be decoded by inter prediction, using the decoded image stored in the memory 230 as a reference image. The inter prediction unit 241 generates an inter prediction image by performing inter prediction in accordance with the syntax, motion vectors, etc. input from the entropy decoding unit 200, and outputs the generated inter prediction image to the switching unit 243.

The intra prediction unit 242 references the decoded image stored in the memory 230 and generates an intra prediction image by predicting the block to be decoded using intra prediction based on the syntax input from the entropy decoding unit 200, and outputs the generated intra prediction image to the switching unit 243.

The switching unit 243 switches between the inter-predicted image input from the inter-prediction unit 241 and the intra-predicted image input from the intra-prediction unit 242, and outputs one of the predicted images to the synthesis unit 220.

Figure 8 shows the configuration of the intra prediction unit 242 according to this embodiment. The intra prediction unit 242 corresponds to the intra prediction device provided in the image decoding device 2. The intra prediction unit 242 performs operations similar to those of the intra prediction unit 172 provided in the image encoding device 1.

As shown in FIG. 8, the intra prediction unit 242 has a memory 230a, a first predicted image generation unit 242a, a second predicted image generation unit 242b, a weighting coefficient determination unit 242c, and an image synthesis unit 242d.

Memory 230a is part of memory 230 shown in Figure 7. Memory 230a stores reference pixels, which are decoded pixels that are referenced during intra prediction.

The first predicted image generation unit 242a predicts the block to be intra-predicted using a first intra-prediction mode, which is directional prediction, to generate a directional predicted image (first predicted image), and outputs the generated directional predicted image to the image synthesis unit 242d. Specifically, the first predicted image generation unit 242a references reference pixels stored in the memory 230a and generates a directional predicted image using the directional prediction mode indicated by the syntax input from the entropy decoding unit 200.

The second predicted image generation unit 242b predicts the block to be intra-predicted using a second intra prediction mode, which is non-directional prediction, to generate a predicted image (second predicted image), and outputs the generated predicted image to the image synthesis unit 242d. Specifically, the second predicted image generation unit 242b references reference pixels stored in the memory 230a and generates a directional predicted image using a predetermined non-directional second intra prediction mode. In this embodiment, the second intra prediction mode is planar prediction.

The weighting coefficient determination unit 242c determines a weighting coefficient based on at least one of the block size and block shape of the block to be intra-predicted, and outputs the determined weighting coefficient to the image synthesis unit 242d. The block size and block shape of the block to be intra-predicted are input to the weighting coefficient determination unit 242c from the entropy decoding unit 200. The operation of the weighting coefficient determination unit 242c is the same as that of operation examples 1 and 2 described above.

The image synthesis unit 242d uses the weighting coefficients input from the weighting coefficient determination unit 242c to weight-synthesize the directional prediction image input from the first prediction image generation unit 242a and the planar prediction image input from the second prediction image generation unit 242b, and outputs the weighted-synthesized prediction image as an intra-prediction image.

<Example of intra prediction operation flow>
Next, an example of the operation flow of intra prediction according to this embodiment will be described. The operation of intra prediction is the same in the image encoding device 1 and the image decoding device 2, but here, the operation of intra prediction (intra prediction unit 242) in the image decoding device 2 will be described. Fig. 9 is a diagram showing an example of the operation flow of the intra prediction unit 242.

First, the entropy decoding unit 200 decodes syntax indicating the intra prediction mode selected by the image encoding device 1. If this syntax indicates a mode other than a directional prediction mode (DC prediction or planar prediction) (step S1: NO), the intra prediction unit 242 performs intra prediction similar to conventional methods (step S7).

Second, the entropy decoding unit 200 decodes syntax indicating whether or not to apply a predictive image synthesis method. If this syntax indicates that a predictive image synthesis method is not to be applied (step S2: NO), the intra prediction unit 242 performs intra prediction similar to conventional methods (step S7). However, if the intra prediction mode selected by the image encoding device 1 is a directional prediction mode, the predictive image synthesis method may always be applied. In this case, there is no need to signal syntax indicating whether or not to apply a predictive image synthesis method.

Third, if the intra prediction mode selected by the image encoding device 1 is directional prediction (step S1: YES) and a predicted image synthesis method is applied (step S2: YES), the weighting coefficient determination unit 242c determines the weighting coefficients α and β based on at least one of the block size and block shape of the block to be intra predicted (step S3).

Fourth, the first predicted image generation unit 242a predicts the intra-prediction target block using the directional prediction mode selected by the image encoding device 1 to generate a directional predicted image (step S4). Furthermore, the second predicted image generation unit 242b predicts the intra-prediction target block using planar prediction to generate a planar predicted image (step S5).

Fifth, the image synthesis unit 242d uses the weighting coefficients α and β determined in step S3 to weight and synthesize the directional prediction image generated in step S4 and the planar prediction image generated in step S5, and outputs the weighted and synthesized prediction image as an intra-prediction image (step S6).

<Summary of the embodiment>
The image encoding device 1 and the image decoding device 2 according to this embodiment determine weighting factors based on at least one of the block size and block shape of a block to be intra-predicted, and weight-combine a directional predicted image and a planar predicted image using the determined weighting factors. This allows for higher prediction accuracy of intra-prediction compared to a case where the directional predicted image and the planar predicted image are simply averaged in the predicted image combining technique.

<Other embodiments>
In the above-described embodiment, an example in which the second intra prediction mode is planar prediction has been mainly described. However, the second intra prediction mode may be planar prediction. When the second intra prediction mode is planar prediction, the term "planar prediction" in the above-described embodiment may be read as "planar prediction."

Alternatively, the second intra prediction mode may be DC prediction. When the second intra prediction mode is DC prediction, "Planar prediction" in the above-described embodiment may be read as "DC prediction."

A program may be provided that causes a computer to execute each process performed by the image encoding device 1. A program may be provided that causes a computer to execute each process performed by the image decoding device 2. The program may be recorded on a computer-readable medium. Using a computer-readable medium makes it possible to install the program on a computer. Here, the computer-readable medium on which the program is recorded may be a non-transitory recording medium. The non-transitory recording medium is not particularly limited, and may be, for example, a CD-ROM, DVD-ROM, or other recording medium.

The circuits that perform each process performed by the image encoding device 1 may be integrated, and the image encoding device 1 may be configured as a semiconductor integrated circuit (chip set, SoC). The circuits that perform each process performed by the image decoding device 2 may be integrated, and the image decoding device 2 may be configured as a semiconductor integrated circuit (chip set, SoC).

The above describes the embodiments in detail with reference to the drawings, but the specific configuration is not limited to that described above, and various design changes can be made without departing from the spirit of the invention.

1: Image encoding device 2: Image decoding device 100: Block division unit 110: Subtraction unit 120: Transform/quantization unit 121: Transform unit 122: Quantization unit 130: Entropy encoding unit 140: Inverse quantization/inverse transform unit 141: Inverse quantization unit 142: Inverse transform unit 150: Combining unit 160: Memory 160a: Memory 170: Prediction unit 171: Inter prediction unit 172: Intra prediction unit 172a: First predicted image generation unit 172b: Second predicted image generation unit 172c: Weighting coefficient determination unit 172d: Image composition unit 173: Switching unit 200: Entropy decoding unit 210: Inverse quantization/inverse transform unit 211: Inverse quantization unit 212: Inverse transform unit 220: Combining unit 230: Memory 230a : Memory 240 : Prediction unit 241 : Inter prediction unit 242 : Intra prediction unit 242 a : First predicted image generation unit 242 b : Second predicted image generation unit 242 c : Weighting coefficient determination unit 242 d : Image synthesis unit 243 : Switching unit

Claims (2)

A decoding device that performs intra prediction on image blocks obtained by dividing an original image,
an entropy decoding unit for obtaining an intra-prediction mode;
a first predicted image generation unit that predicts the image block using directional prediction to generate a first intra predicted image;
a second predicted image generation unit that predicts the image block using non-directional prediction to generate a second intra predicted image;
an image synthesis unit that weights and synthesizes the first intra- predicted image and the second intra -predicted image based on the block shape of the image block ,
The entropy decoding unit does not acquire information that specifies the non-directional prediction.
Decryption device. A program for causing a computer to function as the decoding device according to claim 1.