#2452: decoding yuv420p to rgb shifts luminance down
-------------------------------------+-------------------------------------
Reporter: burek | Owner:
Type: defect | Status: new
Priority: normal | Component:
Version: | undetermined
unspecified | Keywords:
Blocked By: | Blocking:
Reproduced by developer: 0 | Analyzed by developer: 0
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Original post on the forum:
http://ffmpeg.gusari.org/viewtopic.php?f=11&t=884
Summary of the bug:
"I'm recording video from a grayscale camera into an MPEG4 video file
using FFMPEG 1.2. Recently I noticed a weird effect: if I decode the file
using FFMPEG or windows media player, the output frames are noticeably
darker (by about 10/12 brightness values) than the original source."
How to reproduce:
{{{
ffmpeg -ss 0.5 -i video.avi -vframes 1 -t 1 -s 1280x680 -pix_fmt gray
gray.bmp
ffmpeg version 1.2 Copyright (c) 2000-2013 the FFmpeg developers
built on Apr 4 2013 12:40:58 with gcc 4.6.2 (GCC)
configuration: --enable-w32threads
libavutil 52. 18.100 / 52. 18.100
libavcodec 54. 92.100 / 54. 92.100
libavformat 54. 63.104 / 54. 63.104
libavdevice 54. 3.103 / 54. 3.103
libavfilter 3. 42.103 / 3. 42.103
libswscale 2. 2.100 / 2. 2.100
libswresample 0. 17.102 / 0. 17.102
Input #0, avi, from 'video.avi':
Metadata:
encoder : Lavf53.32.100
Duration: 00:00:07.47, start: 0.000000, bitrate: 16532 kb/s
Stream #0:0: Video: mpeg4 (Simple Profile) (FMP4 / 0x34504D46),
yuv420p, 128
0x720 [SAR 1:1 DAR 16:9], 30 tbr, 30 tbn, 30 tbc
Output #0, image2, to 'gray.bmp':
Metadata:
encoder : Lavf54.63.104
Stream #0:0: Video: bmp, gray, 1280x680 [SAR 17:18 DAR 16:9], q=2-31,
200 kb
/s, 90k tbn, 30 tbc
Stream mapping:
Stream #0:0 -> #0:0 (mpeg4 -> bmp)
Press [q] to stop, [?] for help
frame= 1 fps=0.0 q=0.0 Lsize=N/A time=00:00:00.03 bitrate=N/A
video:851kB audio:0kB subtitle:0 global headers:0kB muxing overhead
-100.002524%
}}}
"I thought the encoding step was doing this until I opened the same file
in VLC and it gave me back the correct result. I played around with FFMPEG
command lines to decode a single frame and realized that if I decode a
frame into GRAY8 pixel format, the brightness/luminance values are
preserved."
"This happens with every single grayscale video that I encode to MPEG4
with FFMPEG. My best guess so far is it has to do with how the container
pixel format gets converted to/from. Since I'm using MPEG4, the file pixel
format is YUV420P. I have no idea how ffmpeg encodes from GRAY8 to
YUV420P, but maybe it stores just the luminance values in Y... if this
happens, then decoding from this to RGB could produce darker pixels due to
the shift factor that is applied to the luminance on decoding?
To sum it up:
1) How can encoding a grayscale video to YUV420P with FFMPEG and decoding
back produce wrong (darker) brightness values when decoding to RGB versus
GRAY8? Presumably once the frames are in YUV420P format it shouldn't
matter whether the source is actually grayscale or not so the result
should be equivalent, no?
2) How does VLC avoid this situation? I was under the impression that VLC
used FFMPEG as well for video decoding, but somehow they managed to figure
out how to produce the correct values without requiring me to indicate
explicitly that the video was grayscale."
--
Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/2452>
FFmpeg <http://ffmpeg.org>
FFmpeg issue tracker
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