Skip Mode

[koliva]

I know that one of the modes used in H.264 is skip mode but I couldnt understand it exactly. How can it decide that the current block is in skip mode? What is the criteria?

[Dust Signs]

During encoding or during decoding? In the latter case the mb_skip_flag will tell you. During encoding skip mode is mostly used for blocks which merely change. What "merely" means is of couse subject to interpretation and trial (woohoo, magic numbers ;)

Dust Signs

[koliva]

During encoding or during decoding? In the latter case the mb_skip_flag will tell you. During encoding skip mode is mostly used for blocks which merely change. What "merely" means is of couse subject to interpretation and trial (woohoo, magic numbers ;)

Dust Signs

I asked for encoding process.

To be clear, when we are searching a true match between current block in I frame and a block in P frame, if there is not remarkable changing, we can say that this block is in skip mode. this changing completely depends on a certain value. Is that explanation correct?

[Dust Signs]

This highly depends on what you want to achieve. You can theoretically encode all macroblocks which are allowed to be skip macroblocks (as defined by the standard) as such, but you'll have to ask yourself if it is reasonable.
Most encoders I know work with a best practice threshold. If the residual to be encoded (most of the time determined in a kind of fast ME or ME pre-pass) is below that threshold the block is encoded in skip mode, otherwise the "full" ME process is carried out. Please consider that coding the same block multiple times in skip mode can lead to severe blurring (due to the loop deblocking filter) and pumping (in subsequent I/IDR pictures due to the switch between inter and intra prediction modes) artefacts.

Dust Signs