Picture boundary in HEVC/H265

[dumdum23]

Hi,
As per some papers on hevc standard:
"the boundaries of the picture are defined in units of the minimum allowed luma CB size. As a result, at the right andbottom edges of the picture, some CTUs may cover regions that are partly outside the boundaries of the picture. This condition is detected by the decoder, and the CTU quadtree is implicitly split as necessary to reduce the CB size to the point where the entire CB will fit into the picture"

Can anyone please explain me this? I mean shouldn't the boundary be detected in terms of number of CTBs only?

Thanks in advance

[Parabola]

Hopefully a picture helps. CTUs (usually more accurate name than CTB) are relatively large and often overhang the bottom of the active image. So a conformance window is specified in the headers.

http://www.parabolaresearch.com/Doom9ForumPictureBoundaryExample.JPG

[pieter3d]

Not quite, Here is a picture that may make more sense:
http://i.imgur.com/FPV5gEA.png

You can see the CTBs that are partially outside the right and bottom edge have implied splits (dashed lines), and the CUs that fall outside completely are simply skipped (not coded)

[Parabola]

Pieter shows coding_quadtree syntax element in blue, coding_tree_unit (CTU) rows and columns indicated with numbers. I like the use of dashed lines to show splits that were inferred rather than signalled in the bitstream.

My overlay shows slice_segment_data (yellow), tiles (with ticks), coding_tree_unit's (CTUs/thick orange) and coding_unit (CUs/thin orange). In our software the overlays respond to hover/click/zoom, i.e. interactive.

As per Pieter's dashed lines, if your source dimensions are not an integer multiple of CTU size then CQT splits will be inferred as appropriate at the right and/or bottom of picture such that no CU is bisected by the right or bottom picture edge.

If your source is not an integer multiple of min CU size then you additionally have to specify a conformance window which is, IIRC, a luma-pixel-precision crop.

[pieter3d]

Conformance window is in 2-pixel units in the syntax, since otherwise you'd have half chroma pels.