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I was looking at how JPEG compression works, and to me it seems to be really similar to what H.264 (and other video codecs) does with its intraframe prediction step.

In both algorithms the image frame is "chroma-downsampled", divided into small blocks, and then DCT and quantization are applied. Finally the frame/picture is encoded into a bitstream using Huffman coding.

Is this correct? Are there important differences between JPEG and H.264 intraframe compressions that I'm missing?

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This is a complex topic, so best to link and excerpt

From https://sonnati.wordpress.com/2010/10/19/h-264-for-image-compression/

H.264 contains a number of new features that allow it to compress images much more efficiently than JPG.

New transform design

Differently from JPG, an exact-match integer 4×4 spatial block transform is used instead of the well known 8×8 DCT. It is conceptually similar to DCT but with less ringing artifacts. There is also a 8×8 spatial block transform for less detailed areas and chroma.

A secondary Hadamard Transform (2×2 on chroma and 4×4 on luma) can be usually performed on “DC” coefficients to obtain even more compression in smooth regions.

There is also an optimized quantization and two possible zig-zag pattern for Run Length Encoding of transformed coefficients.

Intra-frame compression

H.264 introduces complex spatial prediction for intra-frame compression. Rather than the “DC”-only prediction found in MPEG2 and the transform coefficient prediction found in H.263+, H.264 defines 6 prediction directions (modes) to predict spatial information from neighbouring blocks when encoded using 4×4 transform. The encoder tries to predict the block interpolating the color value of adiacent blocks. Only the delta signal is therefore transmitted.

There are also 4 prediction modes for smooth color zones (16×16 blocks). Residual data are coded with 4×4 trasforms and a further 4×4 Hadamard trasform is used for DC coefficients.

Improved quantization

A new logarithmic quantization step is used (compound rate 12%). It’s also possible to use Frequency-customized quantization scaling matrices selected by the encoder for perceptual-based quantization optimization.

Inloop deblocking filter

An adaptive deblocking filter is applied to reduce eventual blocking artifacts at high compression ratio.

Advanced Entropy Coding

H.264 can use the state of the art in entropy coding: Context Adaptive Binary Arithmetic Coding (CABAC) which is much more efficient than the standard Huffman coding used in JPG.

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