Alternatives to glReadPixels

[gaurav-fueled]

Readme contains : Experiment with alternatives to glReadPixels(). Add a PBO speed test. (Doesn't seem to be a way to play with eglCreateImageKHR from Java.)

but It seems it has not been added yet.

I am trying to use https://github.com/CyberAgent/android-gpuimage for some image processing tasks. There seems to a speed bottle neck at https://github.com/CyberAgent/android-gpuimage/blob/febdf4900b437b2069661fd371d5469196e26f18/library/src/jp/co/cyberagent/android/gpuimage/PixelBuffer.java#L194
If there is some faster alternatives to glReadPixels it would greatly enhance the image processing time.

[fadden]

The performance of glReadPixels() seems to vary widely between different devices and different driver releases. This is why I added a trivial benchmark to Grafika. (What values do you get?)

http://bigflake.com/mediacodec/#ExtractMpegFrames shows some timing values for extracting video frames with glReadPixels(); on a Nexus 5 the function ran quickly, and most of the time was spent elsewhere (primarily PNG compression in that case).

(The convertToBitmap() function you reference could be sped up by inverting the Y axis in the GL rendering, rather than the bitmap copy, and it'd annoy the GC less if the pixel buffers were allocated once and re-used. But if that's not the bottleneck then there's no reason to mess with it.)

One partially-completed attempt to use non-public APIs from the NDK can be found at http://stackoverflow.com/questions/21151259/replacing-glreadpixels-with-egl-khr-image-base-for-faster-pixel-copy/21199078#21199078 ; I think with the final "edit" it would actually work, but I never got around to trying it out (and at this point it's unlikely I ever will).

The fastest way to get images to disk is to use the MP4 video encoder, but that's usually not appropriate unless your input is video.

[gaurav-fueled]

Hey @fadden,

The time glReadPixels in convertToBitmap() method takes on Nexus 5 (Android L) for 3264 X 2448 size image comes out to be 689 milli seconds.

In grafika 3264 X 2448 size image takes 51.416 milli seconds on Nexus 5 with on Android L.

[fadden]

That's a lot of pixels. Even so, I suspect you may be hitting a slow path in the driver to get performance that bad.

In any event, this is a bit out of scope for Grafika, since there isn't yet a public Java-language API that will make this faster.