Writing to then reading from an offscreen FBO on i

Question

I'm trying to do some image manipulation on the iPhone, basing things on the GLImageProcessing example from Apple.

Ultimately what I'd like to do is to load an image into a texture, perform one or more of the operations in the example code (hue, saturation, brightness, etc.), then read the resulting image back out for later processing/saving. For the most part, this would never need to touch the screen, so I thought that FBOs might be the way to go.

To start with, I've cobbled together a little example that creates an offscreen FBO, draws to it, then reads the data back out as an image. I was psyched when this worked perfectly in the simulator, then bummed as I realized I just got a black screen on the actual device.

Disclaimer: my OpenGL is old enough that I've had quite a bit of a learning curve going to OpenGL ES, and I've never been much of a texture wizard. I do know that the device has different characteristics from the simulator in terms of framebuffer access (mandatory offscreen FBO and swap on the device, direct access on the simulator), but I haven't been able to find what I've been doing wrong, even after a fairly extensive search.

Any suggestions?

// set up the offscreen FBO sizes

int renderBufferWidth = 1280;
int renderBufferHeight = 720;

// now the FBO
GLuint  fbo = 0;
glGenFramebuffersOES(1, &fbo);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, fbo);

GLuint  renderBuffer = 0;
glGenRenderbuffersOES(1, &renderBuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, renderBuffer);
glRenderbufferStorageOES(GL_RENDERBUFFER_OES,
                         GL_RGBA8_OES,
                         renderBufferWidth,
                         renderBufferHeight);

glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES,
                             GL_COLOR_ATTACHMENT0_OES,
                             GL_RENDERBUFFER_OES,
                             renderBuffer);

GLenum status = glCheckFramebufferStatusOES(GL_FRAMEBUFFER_OES);
if (status != GL_FRAMEBUFFER_COMPLETE_OES) {
    NSLog(@"Problem with OpenGL framebuffer after specifying color render buffer: %x", status);
}

// throw in a test drawing  
glClearColor(0.5f, 0.5f, 0.5f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);


static const GLfloat triangleVertices[] = {
    -0.5f,  -0.33f,
    0.5f,  -0.33f,
    -0.5f,   0.33f
};

static const GLfloat triangleColors[] = {
    1.0, 0.0, 0.0, 0.5,
    0.0, 1.0, 0.0, 0.5,
    0.0, 0.0, 1.0, 0.5
};

GLint backingWidth = 320;
GLint backingHeight = 480;

NSLog(@"setting up view/model matrices");
glMatrixMode(GL_PROJECTION);
glLoadIdentity();

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();


glVertexPointer(2, GL_FLOAT, 0, triangleVertices);
glEnableClientState(GL_VERTEX_ARRAY);
glColorPointer(4, GL_FLOAT, 0, triangleColors);
glEnableClientState(GL_COLOR_ARRAY);



// draw the triangle
glDrawArrays(GL_TRIANGLE_STRIP, 0, 3);


// Extract the resulting rendering as an image
int samplesPerPixel = 4; // R, G, B and A
int rowBytes = samplesPerPixel * renderBufferWidth;
char* bufferData = (char*)malloc(rowBytes * renderBufferHeight);
if (bufferData == NULL) {
    NSLog(@"Unable to allocate buffer for image extraction.");
}

// works on simulator with GL_BGRA, but not on device
glReadPixels(0, 0, renderBufferWidth,
             renderBufferHeight,
             GL_BGRA,
             GL_UNSIGNED_BYTE, bufferData);
NSLog(@"reading pixels from framebuffer");

// Flip it vertically - images read from OpenGL buffers are upside-down
char* flippedBuffer = (char*)malloc(rowBytes * renderBufferHeight);
if (flippedBuffer == NULL) {
    NSLog(@"Unable to allocate flipped buffer for corrected image.");
}

for (int i = 0 ; i < renderBufferHeight ; i++) {   
    bcopy(bufferData + i * rowBytes,
          flippedBuffer + (renderBufferHeight - i - 1) * rowBytes,
          rowBytes);
}

// unbind my FBO
glBindFramebufferOES(GL_FRAMEBUFFER_OES, 0);

// Output the image to a file


CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
int bitsPerComponent = 8;

CGBitmapInfo bitmapInfo = kCGImageAlphaNoneSkipFirst | kCGBitmapByteOrder32Host;
CGContextRef contextRef = CGBitmapContextCreate(flippedBuffer,
                                                renderBufferWidth,
                                                renderBufferHeight,
                                                bitsPerComponent,
                                                rowBytes, colorSpace, bitmapInfo);
if (contextRef == nil) {
    NSLog(@"Unable to create CGContextRef.");
}

CGImageRef imageRef = CGBitmapContextCreateImage(contextRef);

if (imageRef == nil) {
    NSLog(@"Unable to create CGImageRef.");
} else {
    if (savedImage == NO) {
        UIImage *myImage = [UIImage imageWithCGImage:imageRef];
        UIImageWriteToSavedPhotosAlbum(myImage, nil, nil, nil);
        savedImage = YES;
    }   
}

Edit:

The answer, of course, was that the bitmap format should be GL_RGBA, not GL_BGRA:

// works on simulator with GL_BGRA, but not on device
glReadPixels(0, 0, renderBufferWidth,
         renderBufferHeight,
         **GL_RGBA**,
         GL_UNSIGNED_BYTE, bufferData);

Answer 1

As Andrew answered himself:

The answer, was that the bitmap format should be GL_RGBA, not GL_BGRA

// works on simulator with GL_BGRA, but not on device
glReadPixels(0, 0, renderBufferWidth,
             renderBufferHeight,
             GL_RGBA, // <--
             GL_UNSIGNED_BYTE, bufferData);