I want to dump raw texture data to disk (to read back later), and I'm not sure about glReadPixel will read from the currently bound texture.
How can I read the buffer from my texture?
glReadPixels function reads from framebuffers, not textures. To read a texture object you must use glGetTexImage but it isn't available in OpenGL ES :(
If you want to read the buffer from your texture then you can bind it to an FBO (FrameBuffer Object) and use glReadPixels:
//Generate a new FBO. It will contain your texture.
glGenFramebuffersOES(1, &offscreen_framebuffer);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, offscreen_framebuffer);
//Create the texture
glGenTextures(1, &my_texture);
glBindTexture(GL_TEXTURE_2D, my_texture);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);
//Bind the texture to your FBO
glFramebufferTexture2DOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_TEXTURE_2D, my_texture, 0);
//Test if everything failed
GLenum status = glCheckFramebufferStatusOES(GL_FRAMEBUFFER_OES);
if(status != GL_FRAMEBUFFER_COMPLETE_OES) {
printf("failed to make complete framebuffer object %x", status);
}
Then, you only must call to glReadPixels when you want to read from your texture:
//Bind the FBO
glBindFramebufferOES(GL_FRAMEBUFFER_OES, offscreen_framebuffer);
// set the viewport as the FBO won't be the same dimension as the screen
glViewport(0, 0, width, height);
GLubyte* pixels = (GLubyte*) malloc(width * height * sizeof(GLubyte) * 4);
glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE, pixels);
//Bind your main FBO again
glBindFramebufferOES(GL_FRAMEBUFFER_OES, screen_framebuffer);
// set the viewport as the FBO won't be the same dimension as the screen
glViewport(0, 0, screen_width, screen_height);