[Libav-user] avcodec_decode_video2 doesn't get pal8 palette and how to get "closest" pix_fmt

Matthew Einhorn moiein2000 at gmail.com
Tue Aug 30 20:01:56 CEST 2011

On Thu, Jul 28, 2011 at 8:15 AM, Matthew Einhorn <moiein2000 at gmail.com> wrote:
> On Thu, Jul 28, 2011 at 6:02 AM, Stefano Sabatini
> <stefano.sabatini-lala at poste.it> wrote:
>> On date Tuesday 2011-07-26 13:58:00 -0400, Matthew Einhorn encoded:
>>> On Sun, Jul 24, 2011 at 9:41 AM, Stefano Sabatini
>>> <stefano.sabatini-lala at poste.it> wrote:
>>> > On date Sunday 2011-07-24 04:51:49 -0400, Matthew Einhorn encoded:
>> [...]
>>> Upon spending some time with the debugger I've isolated the problem
>>> into a very weird corner. But first a bit more about my code (partly
>>> attached). My code is a dll wrapper to the ffmpeg dlls. Upon one dll
>>> call you create an object for a video and initialize all the
>>> format/conversion/codec contexts (open function). Then with further
>>> dll calls you request the next frame. As said, this works fine now for
>>> all video formats I tested with except pal8 (rawvideo). With pal8,
>>> calling avcodec_decode_video2(m_pCodecCtx, m_pFrame, &nFrameFinished,
>>> m_pAVPacket) copies a palette of zero into m_pFrame->data[1]
>>> (palette_has_changed is also zero). So this has nothing to do with the
>>> sws_scale, because sws_scale gets a bad palette. So the question is
>>> why avcodec_decode_video2 doesn't read the palette. The video file
>>> isn't bad because of the following.
>> So far so good.
>>> I was able to fix this if before returning from the open function I
>>> added one call to avcodec_decode_video2 (and of course before that to
>>> av_read_frame). That is, if I asked ffmpeg to decode the first frame
>>> before I returned from the function that initialized frames, context
>>> etc. the palette was read correctly in the first and subsequent frames
>>> (palette_has_changed was one). But if I requested the first frame
>>> after returning from my open frame function, in a separate function,
>>> the palette isn't read properly.
>>> Now, this smells of something going out of context and closed when my
>>> open function returns. It cannot be my variables because all of my
>>> variables are created as class variables beforehand which stay put. I
>>> also don't use any smart pointers or such. So it must be (I think)
>>> that one of the av alloc functions clears something if I don't decode
>>> a frame before returning from the function that called the av alloc
>>> function. I think it's something with the decoder, possibly a buffer?
>>> My dlls are called from the same thread every time they are called and
>>> the dll doesn't unload or move between calls. Now ffplay does all its
>>> work from one central main function with calls to other functions (and
>>> it's not a dll) so that's why I think ffplay doesn't have an issue
>>> with it.
>>> Now, I understand that this might be difficult to debug so I'm mostly
>>> asking for clues and what to look at. I.e. in all the format/codec
>>> contexts structs is there some function pointer or member variables
>>> that are responsible for getting the palettes and will help me track
>>> down the issue?
>> Why the
>>                av_free_packet(m_pAVPacket);
>> in cDecodeFrame()?
>> This looks suspicious.
> The reason for the av_free_packet call after decoding the frame is
> that that's how the dranger example did it. But ffplay also does the
> same thing at line 1773:
> http://www.ffmpeg.org/doxygen/trunk/ffplay_8c-source.html#l01771
> When I removed the av_free_packet call it introduced a memory leak
> into the application and the memory use of the app grew with each call
> to get next frame and it also didn't fix the pal8 palette issue.
>>> avcodec_decode_video2 ends up calling some function
>>> pointer so I couldn't follow through the code to see where it's
>>> actually read. It could also be that the problem is with the the
>>> zeranoe dlls in which case this might not be the best place to solve
>>> it, but I doubt it because it works fine for all the other videos.
>>> >> In particular, from what I seemed to have read and seen of ffmpeg, for
>>> >> pal8 AVFrame data[0] is the data, while data[1] is the palette. When
>>> >> calling avcodec_decode_video2 on a pal8 video, data[0] is indeed data
>>> >> (bunch of different values), while data[1] is an array with all
>>> >> elements zero. Indeed, when I edited data[1] to some random values the
>>> >> sws_scale output image was not black anymore and you could see the
>>> >> remnants of my picture.
>>> >>
>>> >
>>> >> So I'm wondering, is the video file broken and that's why the palette
>>> >> doesn't show up? Or did I miss a flag when initializing codec/format
>>> >> context etc. so that the palette isn't read?
>>> >
>>> > AFAIK you don't need any special hacks for working with palette
>>> > formats.
>>> >
>>> >> 2. I'm looking for a function similar to avcodec_find_best_pix_fmt.
>>> >> What I want is to pass in a list of formats and the function would
>>> >> return what's the closest format. For example, say the source format
>>> >> is pal8 and I pass in as possible destination formats: RGB24 and
>>> >> GRAY8. Then the function should return GRAY8.
>>> >> avcodec_find_best_pix_fmt would return in that case RGB24 which "is"
>>> >> the best format, but in this case would waste 2 extra bytes since pal8
>>> >> is only 8 bytes depth and gray to start with.
>>> >>
>>> >> Does a function like this exist? Would it be easy for me to write such
>>> >> a function using the ffmpeg API? And if so can I get some pointers?
>>> >
>>> > Should be easy to hack the logic of avcodec_find_best_pix_fmt() for
>>> > implementing an avcodec_find_closest_pix_fmt() or such.
>>> >
>>> I looked through the code for the above functions and I think as is,
>>> the avcodec_find_best_pix_fmt function should return the closest pix
>>> format like I want. I think the only reason it doesn't (I think) is
>>> because the pal8 format in particular might be set wrongly.
>>> If you look at the pix_fmt_info array that the
>>> avcodec_find_best_pix_fmt1 func is referring to, you'll see this
>>> definition for pal8:
>>> [PIX_FMT_PAL8] = {
>>>     .is_alpha = 1,
>>>     .color_type = FF_COLOR_RGB,
>>>     .depth = 8,
>>> },
>>> shouldn't it be .color_type = FF_COLOR_GRAY? Because it's set to
>>> FF_COLOR_RGB, the avcodec get loss function returns a chroma and
>>> colorspace loss when converting from pal8 to gray8. That's why RGB24
>>> gets picked over gray8. But I thought that pal8 is already gray (B/W)
>>> so there shouldn't be any loss? Admittedly, I don't know too much
>>> about the pix formats.
>> Pal8 works by storing a palette in data[1], which maps an integer in
>> the range 0-255 to an RGBA 32-bits entry.
>> The actual chromatic image features can be guessed only by analyzing
>> the palette itself, in case you have all the RGBA entries set to a
>> gray color then it will be gray, but in general a PAL8 image will
>> contain a colored (R != G != B) data.
> I didn't know that. The pal8 videos I saw was gray so I assumed all of
> them were (although if they were all gray a palette wouldn't really be
> needed...). So avcodec_find_best_pix_fmt should work fine for me then.
> Thanks,
> Matt

FYI this seems to have been fixed in one of the recent gits.

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