Qt Multimedia status on Android

BogDan Vatra taipanromania at gmail.com
Thu May 3 16:07:28 UTC 2012

Hello everyone,

 For a couple of weeks me and a good friend tried to provide basic
multimedia for Qt on
Android, but sadly we didn't succeed at all, so I'd like to share with
you our findings.

  Our first track (and the most important one)  was getting ffmpeg or
gstream compiled
on Android and use it to encode/decode the videos, I was misled by MoboPlayer[1]
which claimed to use ffmpeg to do the job, so everything seems to good
to be true.
After a long period of time of hard working, we manage to get ffmpeg
compiled on all android
supported processors (armv5, armv7 and armv7 with neon) [2], and to
create a simple video
player on top of ffmpeg, then we've discovered that arm processors are
not fast enough to
decode H264 frames, my HTC Desire HD phone (with a 1Ghz processor) was
not able to
decode a 720p frame in less than 90ms (using amrv7+neon), 150ms using
armv7 (without
neon) and 300ms using armv5 which is not enough to have a realtime
decoding.Even worse
after the decoding is finished the image still needs to be converted
from YUV420P to RGB,
this operations takes ages ! On my phone it takes from 300ms to 600ms
! So, what we've
done wrong and why MoboPlayer is so damn fast? After I recover from
that shock I started to
investigate why MoboPlayer is that fast, soon I've discover that
MoboPlayer is cheating
big time! They are not using ffmpeg to decode the video ! They are
using Android's media
player for this job and they are using ffmpeg ONLY to get the
subtitles and to play other
formats which are not supported by Android. This discovery was a
disaster for me, because I
could not believe my eyes, I begin to search for another player which
is open source and
which I can check what is doing under the hood, I found Dolphin Player
[3][4], after I install it
my fears came true, the player was not able to play in realtime none
of my videos recorder
with my phone ! I dig deeper intro ffmpeg sources than I found that
they can use Android
stagefright[5] library to decode H264 frames, sadly this library can
not be used on all Android
devices and it only decodes to a YUV420P image, so we still need to
convert it to RGB.

After I recovered from that heart attack I begin to investigate how
the hell Android is doing it that
fast, and if I can cheat OpenMAX to render into a buffer which I can
read it instead to render into
an Android surface. After I check almost all OpenMAX implementation I
found that it is possible
to send a SurfaceTextureClient[7]object to OpenMAX[6]. Again it seemed
to be too easy and too
good to be true, the same technique could be used also for camera
preview, even it needed non-public
APIs to be used in order to get it working (by the end of this year
all major Android vendors will have
upgrades to 4.x)!
Basically what I needed to do was just to implement ISurfaceTexture
interface[8] and to read the image.
The Image should contain one of these PixelFormat[9] image. Of course
nothing is that easy as it sounds,
and again it was a dead end, because Android is using *NONE* of these
pixel formats [10] !
For the name of God why somebody uses an unknown (useless) pixel
format ? So, I started to dig
deeper and deeper, and after a long and painful period of time, I
think I found the answer:
These images uses some "special" [11] pixel formats which are hardware
vendor specific, these images
can be pushed to the video card and the video card converts them very
very fast into a gl texture, practically
Android uses this textures to draw the frames onto a Surface, is
similar with this[12] test example. Now
the problem is how to use this texture in Qt? In order to display it
into a Qt 4.x application we need somehow
to get the pixels, we can use it to fill a FBO and read the pixels
using glReadPixels, but glReadPixels
is way too slow, or we can use it to fill a PBO which is a little bit
faster but not enough, actually is not
even close to what we need for a realtime video play !

So whats next ? Sadly I have no idea how to create a realtime video
recoder/player on Android using
Qt 4.x ! So if nobody comes with another idea, I'm going to remove
multimedia plugin from alpha4 release.
It seems Qt 5.x will be able to use the texture directly [13] but I'm
not very sure !
Alpha4 is the last release were it comes with new features for Qt 4.x
after this release next releases
which will target Qt 4.x will contain only bugfixes. After we'll ship
alpha4 I'll focus on getting Qt 5 ported.


[1] https://play.google.com/store/apps/details?id=com.clov4r.android.nil
[2] https://gitorious.org/android-ffmpeg/android-ffmpeg
[3] http://code.google.com/p/dolphin-player/
[4] https://play.google.com/store/apps/details?id=com.broov.player
[5] https://gitorious.org/android-ffmpeg/android-ffmpeg/blobs/master/libavcodec/libstagefright.cpp
[6] http://androidxref.com/source/xref/system/media/wilhelm/src/android/MediaPlayer_to_android.cpp#android_Player_setNativeWindow
[7] http://androidxref.com/source/xref/frameworks/base/include/gui/SurfaceTextureClient.h
[8] http://androidxref.com/source/xref/frameworks/base/include/gui/ISurfaceTexture.h
[9] http://androidxref.com/source/xref/frameworks/base/include/ui/PixelFormat.h#63
[10] http://androidxref.com/source/xref/system/core/include/system/graphics.h#43
[11] http://androidxref.com/source/xref/system/core/include/system/graphics.h#54
[12] http://androidxref.com/source/xref/frameworks/base/opengl/tests/gl2_yuvtex/gl2_yuvtex.cpp
[13] http://qt-project.org/wiki/Qt-5-Alpha

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