How can I dump OpenGL RenderBuffer to PNG or JPG image?

Question

I'm working on a project which uses OpenGL framebuffer/renderbuffer to draw freehand lines. This App uses OpenGL ES 1.x and I can't figure out how to dump current screen to PNG or JPG image.

Thanks for reading this question.

From OpenGL Newbie.

Codes used to create frame buffer and render buffer.

// Generate IDs for a framebuffer object and a color renderbuffer
glGenFramebuffersOES(1, &viewFramebuffer);
glGenRenderbuffersOES(1, &viewRenderbuffer);

glBindFramebufferOES(GL_FRAMEBUFFER_OES, viewFramebuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer);

// This call associates the storage for the current render buffer with the EAGLDrawable (our CAEAGLLayer)
// allowing us to draw into a buffer that will later be rendered to screen wherever the layer is (which corresponds with our view).
[context renderbufferStorage:GL_RENDERBUFFER_OES fromDrawable:(id<EAGLDrawable>)self.layer];
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, viewRenderbuffer);

glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_WIDTH_OES, &backingWidth);
glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_HEIGHT_OES, &backingHeight);

// For this sample, we also need a depth buffer, so we'll create and attach one via another renderbuffer.
glGenRenderbuffersOES(1, &depthRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, depthRenderbuffer);
glRenderbufferStorageOES(GL_RENDERBUFFER_OES, GL_DEPTH_COMPONENT16_OES, backingWidth, backingHeight);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_DEPTH_ATTACHMENT_OES, GL_RENDERBUFFER_OES, depthRenderbuffer);

Answer 1

-(UIImage *)dumpImage
{
    GLubyte *buffer = (GLubyte *) malloc(backingWidth * backingHeight * 4);
    GLubyte *buffer2 = (GLubyte *) malloc(backingWidth * backingHeight * 4);

    GLvoid *pixel_data = nil;
    glReadPixels(0, 0, backingWidth, backingHeight, GL_RGBA, GL_UNSIGNED_BYTE, (GLvoid *)buffer);

    /* make upside down */

    for (int y=0; y<backingHeight; y++) {
        for (int x=0; x<backingWidth*4; x++) {
            buffer2[y * 4 * backingWidth + x] = buffer[(backingHeight - y - 1) * backingWidth * 4 + x];
        }
    }

    // make data provider from buffer
    CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, buffer2, backingWidth * backingHeight * 4, freeImageData);

    // set up for CGImage creation
    int bitsPerComponent = 8;
    int bitsPerPixel = 32;
    int bytesPerRow = 4 * backingWidth;
    CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
    CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault;
    // Use this to retain alpha
    //CGBitmapInfo bitmapInfo = kCGImageAlphaPremultipliedLast;
    CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;
    CGImageRef imageRef = CGImageCreate(backingWidth, backingHeight, bitsPerComponent, bitsPerPixel, bytesPerRow, colorSpaceRef, bitmapInfo, provider, NULL, NO, renderingIntent);

    // make UIImage from CGImage
    UIImage *newUIImage = [UIImage imageWithCGImage:imageRef];

    return newUIImage;
}

freeImageData() frees data later.

void freeImageData(void *info, const void *data, size_t size) 
{
    //printf("freeImageData called");
    free((void*)data);
}