Writing to then reading from an offscreen FBO on i

问题:

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);

回答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);