I'm making a movie player with libav. I have decoding video packets working, I have play in reverse working, I have seeking working. All this works no an x86 android emulator, but fails to work on a real android phone (arm64-v8a)
The failure is in sws_scale() - it returns 0. The video frames continue to be decoded properly with no errors.
There are no errors, warnings, alerts from libav. I have connected an avlog_callback
void log_callback(void *ptr, int level, const char *fmt, va_list vargs) {
if (level<= AV_LOG_WARNING)
__android_log_print( level, LOG_TAG, fmt, vargs);
}
uint64_t openMovie( char* path, int rotate, float javaDuration )
{
av_log_set_level(AV_LOG_WARNING);
av_log_set_callback(log_callback);
The code to do the sws_scale() is:
int JVM_getBitmapBuffer( JNIEnv* env, jobject thiz, jlong av, jobject bufferAsInt, jbyte transparent ) {
avblock *block = (avblock *) av;
if (!block) {
__android_log_print(ANDROID_LOG_ERROR, LOG_TAG, " avblock is null");
return AVERROR(EINVAL);
}
if (!block->pCodecCtx) {
__android_log_print(ANDROID_LOG_ERROR, LOG_TAG, " codecctx is null");
return AVERROR(EINVAL);
}
int width = block->pCodecCtx->width;
int height = block->pCodecCtx->height;
if (NULL == block->sws) {
__android_log_print( ANDROID_LOG_ERROR, LOG_TAG, "getBitmapBuffer:\n *** invalid sws context ***" );
}
int scaleRet = sws_scale( block->sws,
block->pFrame->data,
block->pFrame->linesize,
0,
height,
block->pFrameRGB->data,
block->pFrameRGB->linesize
);
if (scaleRet == 0 ) {
__android_log_print(ANDROID_LOG_ERROR, LOG_TAG, " scale failed");
__android_log_print(ANDROID_LOG_ERROR, LOG_TAG, " pframe linesize %d", block->pFrame->linesize[0]);
__android_log_print(ANDROID_LOG_ERROR, LOG_TAG, " pframergb linesize %d", block->pFrameRGB->linesize[0]);
__android_log_print(ANDROID_LOG_ERROR, LOG_TAG, " height %d",
height);
return AVERROR(EINVAL);
}
Setting up the codex and avframes:
//i have tried every combination of 1, 8, 16, and 32 for these values
int alignRGB = 32;
int align = 16;
int width = block->pCodecCtx->width;
int height = block->pCodecCtx->height;
block->pFrame = av_frame_alloc();
block->pFrameRGB = av_frame_alloc();
block->pFrameRGBBuffer = av_malloc(
(size_t)av_image_get_buffer_size(AV_PIX_FMT_RGB32, width, height, alignRGB)
);
av_image_fill_arrays(
block->pFrameRGB->data,
block->pFrameRGB->linesize,
block->pFrameRGBBuffer,
AV_PIX_FMT_RGB32,
width,
height,
alignRGB
);
block->pFrameBuffer = av_malloc(
(size_t) av_image_get_buffer_size(block->pCodecCtx->pix_fmt,
width, height, align
)
);
av_image_fill_arrays(
block->pFrame->data,
block->pFrame->linesize,
block->pFrameBuffer,
block->pCodecCtx->pix_fmt,
width, height,
align
);
block->sws = sws_getContext(
width, height,
AV_PIX_FMT_YUV420P,
width, height,
AV_PIX_FMT_RGB32,
SWS_BILINEAR, NULL, NULL, 0
);
Wildcards are that:
Other notes:
block is a simple C struct with pointers to relevant AV memory structures.I'm pretty certain it has something to do with the pixel alignment. But documentation is practically non-existent on this topic. It could also be the difference between pixel formats RGBA and RGB32, or possibly little-endian vs big-endian.
This is a known bug in FFMPEG on ARM architecture.
A workaround was posted by mythtv that involves subtracting 1 from the destination width in order to bypass broken optimization code.
https://code.mythtv.org/trac/ticket/12888
https://code.mythtv.org/trac/changeset/7de03a90c1b144fc0067261af1c9cfdd8d358972/mythtv
Reported against FFMPEG 3.2.1 http://trac.ffmpeg.org/ticket/6192
Still exists in FFMPEG 4.3.2
int new_width = width;
#if ARCH_ARM
// The ARM build of FFMPEG has a bug that if sws_scale is
// called with source and dest sizes the same, and
// formats as shown below, it causes a bus error and the
// application core dumps. To avoid this I make a -1
// difference in the new width, causing it to bypass
// the code optimization which is failing.
if (pix_fmt == AV_PIX_FMT_YUV420P
&& dst_pix_fmt == AV_PIX_FMT_BGRA)
new_width = width - 1;
#endif
d->swsctx = sws_getCachedContext(d->swsctx, width, height, pix_fmt,
new_width, height, dst_pix_fmt,
SWS_FAST_BILINEAR, NULL, NULL, NULL);
Building mobile-ffmpeg 4.3.2 with --debug option produces a working binary.
The --debug flag does not seem to be working anymore. Must've been some unclean builds. This does seem to be related to alpha channel.
The only non-alpha RGB format supported by both Android.Bitmap.Config and ffmpeg is RGB_565. Using 565 works with even numbered destination widths.
in libswscale/yuv2rgb.c:
SwsFunc ff_yuv2rgb_get_ptr(SwsContext *c) {
...
switch(c->dstFormat) {
case AV_PIX_FMT_RGBA:
return (CONFIG_SWSCALE_ALPHA & isALPHA(c->srcFormat)) ? yuva2rgb_c: yuv2rgb_c_32;
....
case AV_PIX_FMT_RGB565:
return yuv2rgb_c_16_ordered_dither;
...
My source is YUV420P, and does not support alpha channels (like yuv A 420p). When my destination format is RGB565, this method seems to not have any problems.
It's a shame that I can't produce any alpha channel for Android. I guess the real answer is to just use OpenGL and use a surface that supports blitting YUV directly.
I have found the codepaths in libswscale/swscale_unscaled.c and libswscale/yuv2rgb.c
If you scale the output then you can convert to RGBA dest format.
If you add the flag SWS_ACCURATE_RND to your SwsContext then you also avoid the bad codepath.