CaptureScreen in Objective-C using OpenGL on an IMac - objective-c

Question

I have a multidisplay system, 27 inch (2560 x 1440) and Apple Cinema Display (1920 x 1200) with ATI Radeon HD 5750 1024 MB graphics card. I used to be able to capture screen shots with my previous iMac with a Nvidia card, also multidisplay systems. Now I get an OpenGL error.
This GDB was configured as "x86_64-apple-darwin".tty /dev/ttys001
[Switching to process 28434 thread 0x0] 2011-08-20 19:07:42.040
DetectObjectColor[28434:c03] invalid fullscreen drawable 2011-08-20
19:07:42.043 DetectObjectColor[28434:c03] * Assertion failure in
-[OpenGLScreenReader readPartialScreenToBuffer:bufferBaseAddress:],
/Documents/Personal/DetectObjectColor/OpenGLScreenReader.m:230
2011-08-20 19:07:42.043 DetectObjectColor[28434:c03] * Terminating
app due to uncaught exception 'NSInternalInconsistencyException',
reason: 'OpenGL error 0x0506'
I call this from my main program:
OpenGLScreenReader *mOpenGLScreenReader;
mOpenGLScreenReader = [[OpenGLScreenReader alloc] init];
[mOpenGLScreenReader readRectScreen:CGRectMake(570, 265, 35, 800)];
This is my init method
-(id) init
{
if (self = [super init])
{
// Create a full-screen OpenGL graphics context
// Specify attributes of the GL graphics context
NSOpenGLPixelFormatAttribute attributes[] = {
NSOpenGLPFANoRecovery,
NSOpenGLPFAFullScreen,
NSOpenGLPFAScreenMask,
CGDisplayIDToOpenGLDisplayMask(kCGDirectMainDisplay),
(NSOpenGLPixelFormatAttribute) 0
};
NSOpenGLPixelFormat *glPixelFormat = [[NSOpenGLPixelFormat alloc] initWithAttributes:attributes];
if (!glPixelFormat)
{
return nil;
}
// Create OpenGL context used to render
mGLContext = [[[NSOpenGLContext alloc] initWithFormat:glPixelFormat shareContext:nil] autorelease];
// Cleanup, pixel format object no longer needed
[glPixelFormat release];
if (!mGLContext)
{
[self release];
return nil;
}
[mGLContext retain];
// Set our context as the current OpenGL context
[mGLContext makeCurrentContext];
// Set full-screen mode
[mGLContext setFullScreen];
NSRect mainScreenRect = [[NSScreen mainScreen] frame];
mWidth = mainScreenRect.size.width;
mHeight = mainScreenRect.size.height;
mByteWidth = mWidth * 4; // Assume 4 bytes/pixel for now
mByteWidth = (mByteWidth + 3) & ~3; // Align to 4 bytes
mData = malloc(mByteWidth * mHeight);
NSAssert( mData != 0, #"malloc failed");
}
return self;
}
This is my readRectScreen method which is giving me the error on OpenGL when it executes the readPartialScreenToBuffer method
- (void) readRectScreen:(CGRect) srcRect
{
mWidth = srcRect.size.width;
mHeight = srcRect.size.height;
mByteWidth = mWidth * 4; // Assume 4 bytes/pixel for now
mByteWidth = (mByteWidth + 3) & ~3; // Align to 4 bytes
mData = malloc(mByteWidth * mHeight);
[self readPartialScreenToBuffer:srcRect bufferBaseAddress: mData];
}
- (void) readPartialScreenToBuffer: (CGRect) srcRect bufferBaseAddress: (void *) baseAddress
{
// select front buffer as our source for pixel data
GLint width, height;
width = srcRect.size.width;
height = srcRect.size.height;
glReadBuffer(GL_FRONT);
//Read OpenGL context pixels directly.
// For extra safety, save & restore OpenGL states that are changed
glPushClientAttrib(GL_CLIENT_PIXEL_STORE_BIT);
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);
//Read a block of pixels from the frame buffer
glReadPixels((GLint)srcRect.origin.x, (GLint)srcRect.origin.y, width, height, GL_BGRA, GL_UNSIGNED_INT_8_8_8_8_REV,baseAddress);
glPopClientAttrib();
//Check for OpenGL errors
GLenum theError = GL_NO_ERROR;
theError = glGetError();
NSAssert1( theError == GL_NO_ERROR, #"OpenGL error 0x%04X", theError);
}
Can anybody please help... I am totally lost as to where the error might be.

Answer 1

I guess your new computer has Mac OS X 10.7 (Lion) installed. Lion does not allow this method of screen capture (reading the front buffer) anymore. Applications must migrate to CGDisplayCaptureImage() API, which was introduced in 10.6.

Answer 2

Your issue seems to be, that you don't get/have a valid OpenGL context. But that's not your real problem:
OpenGL is a drawing/rendering library. glReadPixels is meant to read back the image generated by your program. OpenGL is not a general purpose graphics API (though some/many people confuse it for that).
For some time one could exploit the side effects between OpenGL contexts, windows and framebuffers to take screenshots using glReadPixels. On modern systems with compositing window managers this no longer works!
You're barking up the wrong tree, sorry.