[FFmpeg-user] remap filter, unmapped pixels

Paul B Mahol onemda at gmail.com
Thu Feb 20 14:47:29 EET 2020


On 2/20/20, Michael Koch <astroelectronic at t-online.de> wrote:
> Am 20.02.2020 um 12:49 schrieb Paul B Mahol:
>> On 2/20/20, Michael Koch <astroelectronic at t-online.de> wrote:
>>> Am 20.02.2020 um 11:07 schrieb Paul B Mahol:
>>>> On 2/20/20, Michael Koch <astroelectronic at t-online.de> wrote:
>>>>> I'd like to understand how the new "fill" option in the remap filter
>>>>> works.
>>>>> What is an "unmapped pixel"? How is it possible that pixels are
>>>>> unmapped, if the xmap and ymap files have the same size as the output
>>>>> image? Is a pixel "unmapped" if it has invalid coordinates (negative or
>>>>> too big) in the xmap or ymap files?
>>>> Previously all unmapped pixels were set to black for rgba or dark
>>>> green for y420p.
>>>> It is very obvious to understand even for every little kid in the block.
>>> Finally I got it working. A pixel can be declared as "unmapped" by
>>> setting the values to 65535 in the xmap and ymap files. However using a
>>> negative value (for example -1) doesn't work. In this case the pixel is
>>> mapped to the top left pixel of the input image.
>>> This behaviour might be obvious for little kids, but not for me. Should
>>> be added to documentation.
>> Nope. Documentation does not need trivial information.
>> Obviously by having even limited programming experience one would
>> found out that remaps are unsigned short, thus can not be negative.
>
> I that case I did make the PGM file with C# code and some coordinates
> were set to -1. Obviously I shouldn't have done that because the PGM
> specification doesn't allow negative values. But FFmpeg did not report
> any errors when reading this file. May be a warning should be added if
> negative values are found.

Negative values can not be represented in PGM, they are cast to some
other positive value depending on implementation.

>
> Michael
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