On the flip side, it's probably slow compared to what the core
video system could do itself; reading pixels one by one is not as
fast as getting down-and-dirty with cacheline reads and such, and
certainly isn't as fast as copying from one buffer on the video
card to another without involving the CPU at all, which is what the
system perhaps does (or should do, or someday will do) on at least
some accelerated video cards.
Absolutely! The best way to handle the abstraction of the hardware,
and get good performance, is probably to go with the OpenGL
software. Now, using OpenGL isn't exactly obvious, though, so some
sample code is in order.
Finally, it's uncomfortably dependent upon the layout of video
memory; Josh's code only works for 16-bit and 32-bit color, and
only with interleaved (non-planar) data, which is fine for now
AFAIK, but Apple has made no guarantees about sticking with only
those memory layouts in the future that I am aware of. It's
possible right now to switch the screen to 8-bit mode, and many
games do, but Josh's code wouldn't be able to take a snapshot of
the screen in that state, while my CopyBits-base code probably
would (haven't tried). And so forth.
Again, OpenGL can help here, as it includes the ability to convert
pixel formats, and can even deal with hardware-specific pixel reading
problems such as microtiling, or getting the correct framebuffer when
full screen buffer swapping (the GL version of page flipping) is in
effect. OpenGL will also use a DMA transfer for reading the
framebuffer content, where available.
OpenGL provides both a synchronous read path suitable for simple
screen captures using glReadPixels, and a fast asynchronous path
using the texture read mechanism, which is a bit more complex, but
works well for repeated capture operations, such as screencasting or
recording a movie from the screen.
Here's a simple mechanism that can be used to read a rectangle from a
display to a CGImageRef. It's more comment than code:
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* perform an in-place swap from Quadrant 1 to Quadrant III format
* (upside-down PostScript/GL to right side up QD/CG raster format)
* We do this in-place, which requires more copying, but will touch
* only half the pages. (Display grabs are BIG!)
* Pixel reformatting may optionally be done here if needed.
static void swizzleBitmap(void * data, int rowBytes, int height)
int top, bottom;
void * buffer;
void * topP;
void * bottomP;
void * base;
top = 0;
bottom = height - 1;
base = data;
buffer = malloc(rowBytes);
* Save and swap scanlines.
* This code does a simple in-place exchange with a temp
* If you need to reformat the pixels, replace the first two
* calls with your own custom pixel reformatter.
bcopy( topP, buffer, rowBytes );
bcopy( bottomP, topP, rowBytes );
bcopy( buffer, bottomP, rowBytes );
free( buffer );
* Given a display ID and a rectangle on that display, generate a
* containing the display contents.
* srcRect is display-origin relative.
* This function uses a full screen OpenGL read-only context.
* By using OpenGL, we can read the screen using a DMA transfer
* when it's in millions of colors mode, and we can correctly read
* a microtiled full screen OpenGL context, such as a game or full
* screen video display.
* Returns a CGImageRef. When you are done with the CGImageRef,
* using CFRelease().
* Returns NULL on an error.
CGImageRef grabViaOpenGL(CGDirectDisplayID display, CGRect srcRect)
void * data;
GLint width, height;
CGColorSpaceRef cSpace = CGColorSpaceCreateWithName
CGLPixelFormatObj pixelFormatObj ;
long numPixelFormats ;
CGLPixelFormatAttribute attribs =
0, /* Display mask bit goes here */
/* Build bitmap context */
data = malloc(height * bytewidth);
if ( data == NULL )
CGLSetCurrentContext( NULL );
CGLClearDrawable( glContextObj ); // disassociate from
CGLDestroyContext( glContextObj ); // and destroy the
bitmap = CGBitmapContextCreate(data, width, height, 8, bytewidth,
kCGImageAlphaNoneSkipFirst /* XRGB */);
/* Read framebuffer into our bitmap */
glFinish(); /* Finish all OpenGL commands */
glPixelStorei(GL_PACK_ALIGNMENT, 4); /* Force 4-byte
* Fetch the data in XRGB format, matching the bitmap context.
* glReadPixels generates a quadrant I raster, with origin in
the lower left
* This isn't a problem for signal processing routines such as
* as they can simply use a negative 'advance' to move between
* 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
image = CGBitmapContextCreateImage(bitmap);
/* Get rid of bitmap */
/* Get rid of GL context */
CGLSetCurrentContext( NULL );
CGLClearDrawable( glContextObj ); // disassociate from full
CGLDestroyContext( glContextObj ); // and destroy the context
/* Returned image has a reference count of 1 */
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