How Can I Compress a Texture in OpenGL ES on iPhone/iPad?

I'm making an iPad app which needs OpenGL to do a flip animation. I have a front image texture and a back image texture. Both the two textures are screenshots.

// Capture an image of the screen
UIGraphicsBeginImageContext(view.bounds.size);
[view.layer renderInContext:UIGraphicsGetCurrentContex开发者_JS百科t()];
image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

// Allocate some memory for the texture
GLubyte *textureData = (GLubyte*)calloc(maxTextureSize*4, maxTextureSize);

// Create a drawing context to draw image into texture memory
CGContextRef textureContext = CGBitmapContextCreate(textureData, maxTextureSize, maxTextureSize, 8, maxTextureSize*4, CGImageGetColorSpace(image.CGImage), kCGImageAlphaPremultipliedLast);
CGContextDrawImage(textureContext, CGRectMake(0, maxTextureSize-size.height, size.width, size.height), image.CGImage);
CGContextRelease(textureContext);
// ...done creating the texture data

[EAGLContext setCurrentContext:context];

glGenTextures(1, &textureToView);
glBindTexture(GL_TEXTURE_2D, textureToView);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, maxTextureSize, maxTextureSize, 0, GL_RGBA, GL_UNSIGNED_BYTE, textureData);

// free texture data which is by now copied into the GL context
free(textureData);

Each of the texture takes up about 8 MB of memory, which is unacceptable for an iPhone/iPad app. Could anyone tell me how I can compress the texture to reduce memory usage?

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Later versions OpenGL support compresses textures. You can have them being compressed by OpenGL upon uploading the texture data, or do it yourself and provide the pre-compressed data to OpenGL.

The ARB specification for compressed texture support in OpenGL http://www.opengl.org/registry/specs/ARB/texture_compression.txt

And here's the description of one particular compression format http://www.opengl.org/registry/specs/ARB/texture_compression_rgtc.txt

And specific to OpenGL ES this compression format: http://www.khronos.org/registry/gles/extensions/OES/OES_compressed_ETC1_RGB8_texture.txt