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So I'm trying to get the camera pixel data, monitor any major changes in luminosity and then save the image. I have decided to use open gl as I figured it would be quicker to do the luminosity checks in the fragment shader.

I bind a surface texture to the camera to get the image to the shader and am currently using glReadPixels to get the pixels back which I then put in a bitmap and save. The bottle neck on the glReadPixels is crazy so I looked into other options and saw that EGL_KHR_image_base was probably my best bet as I'm using OpenGL-ES 2.0.

Unfortunately I have no experience with extensions and don't know where to find exactly what I need. I've downloaded the ndk but am pretty stumped. Could anyone point me in the direction of some documentation and help explain it if I don't understand fully?

bradz44
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2 Answers2

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Copying pixels with glReadPixels() can be slow, though it may vary significantly depending on the specific device and pixel format. Some tests with using glReadPixels() to save frames from video data (which is also initially YUV) found that 96.5% of the time was in PNG compression and file I/O on a Nexus 5.

In some cases, the time required goes up substantially if the source and destination formats don't match. On one particular device I found that copying to RGBA, instead of RGB, reduced the time required.

The EGL calls can work but require non-public API calls. And it's a bit tricky; see e.g. this answer. (I think the comment in the edit would allow it to work, but I never got back around to trying it, and I'm not in a position to do so now.)

Community
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fadden
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  • Thanks for the answer, I can't use EGLImageKHR. Is there a specific import I am missing? I'm including the following relevent imports: android.graphics.Bitmap; android.graphics.SurfaceTexture; android.hardware.Camera; android.opengl.GLES20; android.opengl.GLSurfaceView; android.opengl.Matrix; javax.microedition.khronos.egl.EGLConfig; javax.microedition.khronos.egl.EGLContext; javax.microedition.khronos.egl.EGLDisplay; javax.microedition.khronos.egl.EGLSurface; javax.microedition.khronos.opengles.GL10; – bradz44 Sep 25 '14 at 08:34
  • There is no Java-language wrapper for it. You can create your own with the NDK, or just do all of the work in native code. – fadden Sep 25 '14 at 17:35
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The only solution would be using Pixel Pack Buffer (PBO), where the reading is asynchronous. However, to utilize this asynchronous, you need to have PBO and use it as ping pong buffer.

I refer to http://www.jianshu.com/p/3bc4db687546 where I reduce the read time for 1080p from 40ms to 20ms.

Qiang Zhang
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