pad filter fails in certain edge cases because of rounding

[Ivan Middleton]

In certain cases, the pad filter fails to configure itself properly. Here's an example, using imagemagick to create an appropriate test file:

$ convert -size 15x15 xc:red -sampling-factor 4:2:0 red.jpg
$ ffmpeg -i red.jpg -vf pad=iw+16:ih+16:ow-iw:oh-ih pad.png
...
...Input area 16:16:31:31 not within the padded area 0:0:30:30...

It seems like ow-iw and oh-ih should always work as values of x and y; this should put the image in the lower right corner.

The failure occurs because the values of w:h:x:y are first computed as 31:31:16:16 and then rounded down (via ff_draw_round_to_sub) to 30:30:16:16 due to chroma considerations. Since the values of w and h have shrunk but x and y have not, the image is no longer validly placed.

A possible solution is to round off w and h before evaluating x and y. In my example, this would result in w:h being evaluated as 31:31 and rounded to 30:30, then x:y being evaluated as 15:15 and rounded to 14:14.

[Carl Eugen Hoyos]

For future tickets, please remember to always provide the FFmpeg command line you tested together with the complete, uncut console output.

Looks like a duplicate of ticket #1618