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glGrab for screen capture on mac os

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#1 NewDisplayName   Members   -  Reputation: 351


Posted 04 July 2012 - 08:45 AM


I am trying to reuse the glGrab code for writing a screen capture tool of my own, on Mac OS.
I am able to get the context right, with proper height, width, bits per component, etc, as I could check using CG methods.

But glReadPixels (which is also not returning any gl error) always creates a buffer with all color value being 0.

The code snippet of the glReadPixels from glGrab below :

glReadPixels((GLint)srcRect.origin.x, (GLint)srcRect.origin.y, width, height, GL_BGRA, #if defined(__BIG_ENDIAN__) GL_UNSIGNED_INT_8_8_8_8_REV, #elif defined(__LITTLE_ENDIAN__) GL_UNSIGNED_INT_8_8_8_8, #else #error Do not know the endianess of this architecture #endif data);

I am checking the value of each pixel on the buffer using below code snippet from here and it always gives a value of 0.

for(size_t row = 0; row < height; row++)
for(size_t col = 0; col < width; col++)
const uint8_t* pixel =
&bytes[row * bpr + col * bytes_per_pixel];

for(size_t x = 0; x < bytes_per_pixel; x++)
printf("%d", (int)pixel[x]);
if( x < bytes_per_pixel - 1 )

if( col < width - 1 )
printf(", ");

That's probably the reason that everytime I render the captured screen inside my local renderer, I receive a black screen.

Please suggest if there is something that I have missed about how to use the glGrab code.
Also, on a side note is it safe to assume that the destination bytestream generated by glReadPixels is the same as the one generated by GetDIBits on Windows?



#2 mhagain   Crossbones+   -  Reputation: 7431


Posted 04 July 2012 - 01:29 PM

Can you show the definition of your "data" variable?

Also be aware that the GL spec indicates that there should be no endian considerations with the 8_8_8_8 types. It specifies explicit bit locations for each component within a generic 32-bit value, so if you need to take endianness into account here then you've got a non-conformant implementation (which is highly unlikely as these types go back to GL 1.2).

It appears that the gentleman thought C++ was extremely difficult and he was overjoyed that the machine was absorbing it; he understood that good C++ is difficult but the best C++ is well-nigh unintelligible.

#3 NewDisplayName   Members   -  Reputation: 351


Posted 04 July 2012 - 10:21 PM

Please find the exact code for glGrab below. I am using it as it is, as of now. (In between tried to replave void *data with GLubyte, unsigned char and what not, but it didn't work.) All I am trying to do is simple use this "data" stream to create my own video frame. The same thing I have achieved on windows using GDI, as I mentioned before, by using "data" stream which is output from GetDIBits. Which also makes me question that is it safe to assume that both these data streams are identical in terms of their properties?
Regardless, of this, even the CGImage won't do any good as long as glReadPixels doesn't read something into the data stream. :-(
Another thing of note, the screen which I am trying to capture has a resolution of 1600x900, so fairly big for glReadPixels, but shouldn't really be a reason for it to fill all 0s in pixels.

[source lang="cpp"]// from http://lists.apple.com/archives/cocoa-dev/2005/Aug/msg00901.html// slightly modified by Ryan Poling, 9/2006./* * glGrab.c *//* IMPORTANT: This Apple software is supplied to you by Apple Computer, Inc. ("Apple") in consideration of your agreement to the following terms, and your use, installation or modification of this Apple software constitutes acceptance of these terms. If you do not agree with these terms, please do not use, install or modify this Apple software. In consideration of your agreement to abide by the following terms, and subject to these terms, Apple grants you a personal, non-exclusive license, under Apple\xd5s copyrights in this original Apple software (the "Apple Software"), to use and modify the Apple Software, with or without modifications, in source and/or binary forms; provided that if you redistribute the Apple Software in its entirety and without modifications, you must retain this notice and the following text and disclaimers in all such redistributions of the Apple Software. Neither the name, trademarks, service marks or logos of Apple Computer, Inc. may be used to endorse or promote products derived from the Apple Software without specific prior written permission from Apple. Except as expressly tated in this notice, no other rights or licenses, express or implied, are granted by Apple herein, including but not limited to any patent rights that may be infringed by your derivative works or by other works in which the Apple Software may be incorporated. The Apple Software is provided by Apple on an "AS IS" basis. APPLE MAKES NO WARRANTIES, EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION THE IMPLIED WARRANTIES OF NON-INFRINGEMENT, MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, REGARDING THE APPLE SOFTWARE OR ITS USE AND OPERATION ALONE OR IN COMBINATION WITH YOUR PRODUCTS. IN NO EVENT SHALL APPLE BE LIABLE FOR ANY SPECIAL, INDIRECT, INCIDENTAL OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) ARISING IN ANY WAY OUT OF THE USE, REPRODUCTION, MODIFICATION AND/OR DISTRIBUTION OF THE APPLE SOFTWARE, HOWEVER CAUSED AND WHETHER UNDER THEORY OF CONTRACT, TORT (INCLUDING NEGLIGENCE), STRICT LIABILITY OR OTHERWISE, EVEN IF APPLE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. */#include <CoreFoundation/CoreFoundation.h>#include <ApplicationServices/ApplicationServices.h>#include <OpenGL/OpenGL.h>#include <OpenGL/gl.h>#include "glGrab.h"/* * 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); while (top < bottom) { topP = (void *)((top * rowBytes) + (intptr_t)base); bottomP = (void *)((bottom * rowBytes) + (intptr_t)base); /* * Save and swap scanlines. * * This code does a simple in-place exchange with a temp buffer. * If you need to reformat the pixels, replace the first two bcopy() * calls with your own custom pixel reformatter. */ bcopy( topP, buffer, rowBytes ); bcopy( bottomP, topP, rowBytes ); bcopy( buffer, bottomP, rowBytes ); ++top; --bottom; } free(buffer);}/* * Given a display ID and a rectangle on that display, generate a CGImageRef * 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, release it * using CFRelease(). * Returns NULL on an error. */CGImageRef grabViaOpenGL(CGDirectDisplayID display, CGRect srcRect) { CGContextRef bitmap; CGImageRef image; void * data; long bytewidth; GLint width, height; long bytes; //CGColorSpaceRef cSpace = CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB); // (9/2006 changed this line because the colors looked a bit too saturated. Guessed right on first try!) // http://lists.apple.com/archives/quartz-dev/2006/Feb/msg00113.html CGColorSpaceRef cSpace = CGColorSpaceCreateDeviceRGB(); CGLContextObj glContextObj; CGLPixelFormatObj pixelFormatObj; long numPixelFormats; CGLPixelFormatAttribute attribs[] = { kCGLPFAFullScreen, kCGLPFADisplayMask, 0, /* Display mask bit goes here */ 0 } ; if ( display == kCGNullDirectDisplay ) display = CGMainDisplayID(); attribs[2] = CGDisplayIDToOpenGLDisplayMask(display); /* Build a full-screen GL context */ CGLChoosePixelFormat( attribs, &pixelFormatObj, &numPixelFormats ); if ( pixelFormatObj == NULL ) // No full screen context support return NULL; CGLCreateContext( pixelFormatObj, NULL, &glContextObj ) ; CGLDestroyPixelFormat( pixelFormatObj ) ; if ( glContextObj == NULL ) 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 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. */// this call is different on Intel and PPC becaused of endian issues// see below URL for why we need to do this// http://developer.apple.com/documentation/MacOSX/Conceptual/universal_binary/universal_binary_tips/chapter_5_section_25.html glReadPixels((GLint)srcRect.origin.x, (GLint)srcRect.origin.y, width, height, GL_BGRA,#if defined(__BIG_ENDIAN__) GL_UNSIGNED_INT_8_8_8_8_REV,#elif defined(__LITTLE_ENDIAN__) GL_UNSIGNED_INT_8_8_8_8,#else#error Do not know the endianess of this architecture#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;}[/source]

Edited by Utkarsh Khokhar, 04 July 2012 - 10:23 PM.

#4 NewDisplayName   Members   -  Reputation: 351


Posted 05 July 2012 - 11:37 AM

And just to add the way I am getting the parameters to pass to grabViaOpenGL is as follows :

CGDirectDisplayID display = CGMainDisplayID();
CGRect srcRect = screen bounds of display;

#5 NewDisplayName   Members   -  Reputation: 351


Posted 05 July 2012 - 11:02 PM

Just in case, if nobody here is familiar with glGrab and know some other better and working method for screen capture on mac using opengl, I am all ears. :-)
But that makes glGrab all the more interesting. People claim that it has always worked, but what can be the issue now.

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