[FFmpeg-trac] #10640(ffmpeg:new): Incorrect interpretation of PNG data in "monob" pixel format

[FFmpeg]

#10640: Incorrect interpretation of PNG data in "monob" pixel format
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             Reporter:  Joey Sodergren  |                     Type:  defect
               Status:  new             |                 Priority:  normal
            Component:  ffmpeg          |                  Version:  6.0
             Keywords:                  |               Blocked By:
             Blocking:                  |  Reproduced by developer:  0
Analyzed by developer:  0               |
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 **Summary of the bug:**
 Since this summer, I've been doing a lot of experimentation with PNG
 optimization. As part of my process, I started running files through
 FFMPEG to re-encode them as a sort of "encoding normalization" step.
 (Don't ask why this is necessary... I experimentally proved it helped the
 rest of my system produce better / smaller results.) I have recently
 discovered a bug in FFMPEG's ability to read some PNG files. While FFMPEG
 ''does'' appear to be aware that images in the {{{pal8}}} pixel format can
 contain transparency data, it does not appear to be aware that images in
 the {{{monob}}} pixel format can also contain transparency data, and is
 improperly crushing it to opaque pixels.

 **How to reproduce:**
 1. The files {{{Palette_Mode.png}}} and {{{Monob_Mode.png}}} are encoded
 with different pixel formats, but they both store their pixels as single
 bits.
   * These two files should be attached to this ticket.
   * If you download them on Windows, inspect their Properties and open the
 Details tab. You should see {{{Bit depth    1}}}.
 1. The files {{{Palette_Mode.png}}} and {{{Monob_Mode.png}}} should appear
 visually identical: fully opaque white glyphs on a fully transparent
 background.

 {{{
 % ffprobe -hide_banner -v error -show_entries frame=pix_fmt
 Palette_Mode.png

 [FRAME]
 pix_fmt=pal8
 [/FRAME]

 % ffprobe -hide_banner -v error -show_entries frame=pix_fmt Monob_Mode.png

 [FRAME]
 pix_fmt=monob
 [/FRAME]

 % ffmpeg -hide_banner -v error -i Palette_Mode.png -c png -pix_fmt rgba
 -update 1 Pal_Expand.png

 % ffmpeg -hide_banner -v error -i Monob_Mode.png -c png -pix_fmt rgba
 -update 1 Mono_Expand.png
 }}}

 At this point, inspect the files {{{Pal_Expand.png}}} and
 {{{Mono_Expand.png}}}. The file {{{Pal_Expand.png}}} should appear
 visually identical to the input. The file {{{Mono_Expand.png}}}, however,
 should not. Its transparent background will be improperly crushed to
 opaque black.
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/10640>
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