Grab Mac OS Screen using GL_RGB format

Question

Hi,

I'm using the glgrab code to try and grab a full-screen screenshot of the Mac screen. However, I want the bitmap data to be in the GL_RGB format. That is, each pixel should be in the format:

0x00RRGGBB

The original code specified the GL_BGRA format. However, changing that to GL_RGB gives me a completely blank result. The total source code I'm using is:

CGImageRef grabViaOpenGL(CGDirectDisplayID display, CGRect srcRect)
{
    CGContextRef bitmap;
    CGImageRef image;
    void * data;
    long bytewidth;
    GLint width, height;
    long bytes;
    CGColorSpaceRef cSpace = CGColorSpaceCreateWithName (kCGColorSpaceGenericRGB);

    CGLContextObj    glContextObj;
    CGLPixelFormatObj pixelFormatObj ;
    GLint            numPixelFormats ;
    //CGLPixelFormatAttribute
    int attribs[] =
    {
//      kCGLPFAClosestPolicy,
        kCGLPFAFullScreen,
        kCGLPFADisplayMask,
        NULL,    /* Display mask bit goes here */
        kCGLPFAColorSize, 24,
        kCGLPFAAlphaSize, 0,
        kCGLPFADepthSize, 32,
        kCGLPFASupersample,
        NULL
    } ;


    if ( display == kCGNullDirectDisplay )
        display = CGMainDisplayID();
    attribs[2] = CGDisplayIDToOpenGLDisplayMask(display);


    /* Build a full-screen GL context */
    CGLChoosePixelFormat( (CGLPixelFormatAttribute*) attribs, &pixelFormatObj, &numPixelFormats );
    if ( pixelFormatObj == NULL )    // No full screen context support
    {
        // GL didn't find any suitable pixel formats. Try again without the supersample bit:
        attribs[10] = NULL;
        CGLChoosePixelFormat( (CGLPixelFormatAttribute*) attribs, &pixelFormatObj, &numPixelFormats );
        if (pixelFormatObj == NULL)
        {
            qDebug("Unable to find an openGL pixel format that meets constraints");
            return NULL;
        }
    }
    CGLCreateContext( pixelFormatObj, NULL, &glContextObj ) ;
    CGLDestroyPixelFormat( pixelFormatObj ) ;
    if ( glContextObj == NULL )
    {
        qDebug("Unable to create OpenGL context");
        return NULL;
    }


    CGLSetCurrentContext( glContextObj ) ;
    CGLSetFullScreen( glContextObj ) ;


    glReadBuffer(GL_FRONT);


    width = srcRect.size.width;
    height = srcRect.size.height;


    bytewidth = width * 4; // Assume 4 bytes/pixel for now
    bytewidth = (bytewidth + 3) & ~3; // Align to 4 bytes
    bytes = bytewidth * height; // width * height

    /* Build bitmap context */
    data = malloc(height * bytewidth);
    if ( data == NULL )
    {
        CGLSetCurrentContext( NULL );
        CGLClearDrawable( glContextObj ); // disassociate from full screen
        CGLDestroyContext( glContextObj ); // and destroy the context
        qDebug("OpenGL drawable clear failed");
        return NULL;
    }
    bitmap = CGBitmapContextCreate(data, width, height, 8, bytewidth,
                                   cSpace, kCGImageAlphaNoneSkipFirst /* XRGB */);
    CFRelease(cSpace);


    /* Read framebuffer into our bitmap */
    glFinish(); /* Finish all OpenGL commands */
    glPixelStorei(GL_PACK_ALIGNMENT, 4); /* Force 4-byte alignment */
    glPixelStorei(GL_PACK_ROW_LENGTH, 0);
    glPixelStorei(GL_PACK_SKIP_ROWS, 0);
    glPixelStorei(GL_PACK_SKIP_PIXELS, 0);

    /*
     * Fetch the data in XRGB format, matching the bitmap context.
     */
    glReadPixels((GLint)srcRect.origin.x, (GLint)srcRect.origin.y, width, height,
                 GL_RGB,
#ifdef __BIG_ENDIAN__
                 GL_UNSIGNED_INT_8_8_8_8_REV, // for PPC
#else
                 GL_UNSIGNED_INT_8_8_8_8, // for Intel! http://lists.apple.com/archives/quartz-dev/2006/May/msg00100.html
#endif
                 data);
    /*
     * glReadPixels generates a quadrant I raster, with origin in the lower left
     * This isn't a problem for signal processing routines such as compressors,
     * as they can simply use a negative 'advance' to move between scanlines.
     * CGImageRef and CGBitmapContext assume a quadrant III raster, though, so we need to
     * invert it. Pixel reformatting can also be done here.
     */
    swizzleBitmap(data, bytewidth, height);


    /* Make an image out of our bitmap; does a cheap vm_copy of the bitmap */
    image = CGBitmapContextCreateImage(bitmap);

    /* Get rid of bitmap */
    CFRelease(bitmap);
    free(data);


    /* Get rid of GL context */
    CGLSetCurrentContext( NULL );
    CGLClearDrawable( glContextObj ); // disassociate from full screen
    CGLDestroyContext( glContextObj ); // and destroy the context

    /* Returned image has a reference count of 1 */
    return image;
}

I'm completely new to OpenGL, so I'd appreciate some pointers in the right direction. Cheers!

Update:

After some experimentation, I have managed to narrow my problem down. My problem is that while I don't want the alpha component, I Do want each pixel to be packed to 4-byte boundaries. Now, when I specify GL_RGB or GL_BGR formats to the glReadPixels call, I get the bitmap data packed in 3 byte blocks. When I specify GL_RGBA, or GL_BGRA, I get four byte blocks, but always with the alpha channel component last.

I then tried changing the value passed to

bitmap = CGBitmapContextCreate(data, width, height, 8, bytewidth,cSpace, kCGImageAlphaNoneSkipFirst /* XRGB */);

however, no variations of AlphaNoneSkipFirst or AlphaNoneSkipLast puts the alpha channel at the start of the pixel byte block.

Any ideas?

Answer 1

Try with GL_UNSIGNED_BYTE instead of GL_UNSIGNED_INT_8_8_8_8_REV / GL_UNSIGNED_INT_8_8_8_8.

Although it seems you want GL_RGBA instead -- then it should work with either 8_8_8_8_REV or 8_8_8_8 instead.

Answer 2

I'm not a Mac guy, but if you can get RGBA data and want XRGB, can't you just bitshift each pixel down eight bits?

foreach( unsigned int* RGBA_pixel, pixbuf )
{
    (*RGBA_pixel) = (*RGBA_pixel) >> 8;
}

Answer 3

When I use GL_BGRA the data is returned pre-swizzled which is confirmed because the colors look correct when i display the result in a window.

Contact me if you want the project I created. Hope this helps.