Grab Mac OS Screen using GL_RGB format

问题:

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?

回答1:

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; } 

回答2:

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.

回答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.

文章来源: Grab Mac OS Screen using GL_RGB format