CODEWITHSUNDEEP
iosopengl-es-2.0framebuffer
Jonas Sourlier
Jonas Sourlier

Reputation: 14435

Is it possible to render to an 8-bit texture framebuffer on OpenGL ES 2.0 / iOS?

I need to render something to an off-screen buffer and read the pixels back into "CPU memory" by calling glReadPixels. My code works great when compiled to normal OpenGL under Windows, but to make it run under OpenGL ES 2.0 in iOS, I had to replace the GL_ALPHA (or GL_LUMINANCE) texture with an GL_RGBA one, meaning

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA,  width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);

instead of

glTexImage2D(GL_TEXTURE_2D, 0, GL_ALPHA,  width, height, 0, GL_ALPHA, GL_UNSIGNED_BYTE, NULL);

I wasn't able to run the code with an 8-bit texture.

But since I don't need the full colors on this rendering, those extra 3 bytes per pixel are wasted. Before I try it again, here my question:

Has anyone succeeded to off-screen-render to an 8-bit texture and reading the bytes back into CPU memory? Is it possible at all?

Upvotes: 2

Views: 1850

Answers (2)

xuguo
xuguo

Reputation: 1826

To make it run in OpenGL 3.0, you could use GL_R8 for buffer buffers:

glGenFramebuffers(1, &_framebuffer);
glBindFramebuffer(GL_FRAMEBUFFER, _framebuffer);

glGenRenderbuffers(1, &_renderBuffer);
glBindRenderbuffer(GL_RENDERBUFFER, _renderBuffer);
glRenderbufferStorage(GL_RENDERBUFFER, GL_R8, 640, 480);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
                          GL_RENDERBUFFER, _renderBuffer);

GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER) ;
if(status != GL_FRAMEBUFFER_COMPLETE) {
  NSLog(@"failed to make complete framebuffer object %x", status);
}

Upvotes: 0

sup
sup

Reputation: 807

Set the glRenderbufferStorage format to GL_R8_EXT, and the glReadPixels/glTexImage2D formats to GL_RED_EXT.

http://www.khronos.org/registry/gles/extensions/EXT/EXT_texture_rg.txt

http://developer.apple.com/library/ios/#releasenotes/General/WhatsNewIniPhoneOS/Articles/iOS5.html#//apple_ref/doc/uid/TP30915195-SW47

Edit: I've tested this and it works, but only on A5 and above (iPad2/3/4/mini, iPhone4S/5, iPod touch 5th gen). Unfortunately it's not available on A4 and older where it's needed most (iPhone4, iPod touch 4th gen, 3GS, iPad1).

Upvotes: 2

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