Do H264 P&B frames contain pixels values?

Question

From what I understand, the I-frames are the ones containing the actual pixels values, and the P&B frames just contain some kind of indicators on how to construct the next frames from the already decoded frames.

With that being said, shouldn't I get a completely blue video, if I've only changed the I-frames to be completely blue pictures?

Thank you!

Answer 1

No frames contain pixel values. After the DCT, the image is transformed and recorded in the frequency domain. B/P frames must reference something. Even if it uninitiated memory. The result of decoding in this case will be determined by the values in this memory, called the decoded picture buffer. A zeroed out DPB will be displayed as green (assumening YUV). It will not be completely green after applying the residuals from a intraframe, you will see some elements of the frame information.

Answer 2

No. When decoding P- and B-frames, the decoding first does a prediction based on earlier frames, but on top of this it adds a so called residual which is encoded in the P- and B-frames. It's not plain pixel values, but the difference between the prediction and the intended picture.

If the reference frames is changed to a plain color, the residual still alters color of pixels. It won't necessarily resemble the original picture much though, as it only is the difference to the original expected prediction.