screenShot code Not working on ipad, works on iphone - ios

Question

i am having this strange problem...
i had to capture the screen data and convert it into an image using the following code..this code is working fine over iphone/ipad simulator and on iphone device but not on iPad only .
iphone device is having ios version 3.1.1 and ipad is ios 4.2...
- (UIImage *)screenshotImage {
CGRect screenBounds = [[UIScreen mainScreen] bounds];
int backingWidth = screenBounds.size.width;
int backingHeight =screenBounds.size.height;
NSInteger myDataLength = backingWidth * backingHeight * 4;
GLuint *buffer = (GLuint *) malloc(myDataLength);
glReadPixels(0, 0, backingWidth, backingHeight, GL_RGBA4, GL_UNSIGNED_BYTE, buffer);
for(int y = 0; y < backingHeight / 2; y++) {
for(int xt = 0; xt < backingWidth; xt++) {
GLuint top = buffer[y * backingWidth + xt];
GLuint bottom = buffer[(backingHeight - 1 - y) * backingWidth + xt];
buffer[(backingHeight - 1 - y) * backingWidth + xt] = top;
buffer[y * backingWidth + xt] = bottom;
}
}
CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, buffer, myDataLength, releaseScreenshotData);
const int bitsPerComponent = 8;
const int bitsPerPixel = 4 * bitsPerComponent;
const int bytesPerRow = 4 * backingWidth;
CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault;
CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;
CGImageRef imageRef = CGImageCreate(backingWidth,backingHeight, bitsPerComponent, bitsPerPixel, bytesPerRow, colorSpaceRef, bitmapInfo, provider, NULL, NO, renderingIntent);
CGColorSpaceRelease(colorSpaceRef);
CGDataProviderRelease(provider);
UIImage *myImage = [UIImage imageWithCGImage:imageRef];
CGImageRelease(imageRef);
// myImage = [self addIconToImage:myImage];
return myImage;}
Any idea whats going wrong ..??

Answer 1

Those two lines don't match
NSInteger myDataLength = backingWidth * backingHeight * 4;
glReadPixels(0, 0, backingWidth, backingHeight, GL_RGBA4, GL_UNSIGNED_BYTE, buffer);
GL_RGB4 means 4 bits per channel, however you're allocating for 8 bits per channel. The proper token is GL_RGB8. On the iPhone GL_RGB4 may be unsupported and falls back to GL_RGBA.
Also make sure you're reading from the correct buffer (front vs. left vs. any (accidently) bound FBOs). I recommend reading from the back buffer before doing the buffer swap.

Answer 2

for ios 4 or later i m using Multi-Sampling technique for anti-aliasing ....glReadpixels() cannot read directly from multiSampled FBO you need to resolve it to Single sampled buffer and then try reading it...Please refer to the following post :-
Reading data using glReadPixel() with multisampling

Answer 3

Screenshot from openGL ES apple documentation
- (UIImage*)snapshot:(UIView*)eaglview
{
GLint backingWidth, backingHeight;
// Bind the color renderbuffer used to render the OpenGL ES view
// If your application only creates a single color renderbuffer which is already bound at this point,
// this call is redundant, but it is needed if you're dealing with multiple renderbuffers.
// Note, replace "_colorRenderbuffer" with the actual name of the renderbuffer object defined in your class.
glBindRenderbufferOES(GL_RENDERBUFFER_OES, _colorRenderbuffer);
// Get the size of the backing CAEAGLLayer
glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_WIDTH_OES, &backingWidth);
glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_HEIGHT_OES, &backingHeight);
NSInteger x = 0, y = 0, width = backingWidth, height = backingHeight;
NSInteger dataLength = width * height * 4;
GLubyte *data = (GLubyte*)malloc(dataLength * sizeof(GLubyte));
// Read pixel data from the framebuffer
glPixelStorei(GL_PACK_ALIGNMENT, 4);
glReadPixels(x, y, width, height, GL_RGBA, GL_UNSIGNED_BYTE, data);
// Create a CGImage with the pixel data
// If your OpenGL ES content is opaque, use kCGImageAlphaNoneSkipLast to ignore the alpha channel
// otherwise, use kCGImageAlphaPremultipliedLast
CGDataProviderRef ref = CGDataProviderCreateWithData(NULL, data, dataLength, NULL);
CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
CGImageRef iref = CGImageCreate(width, height, 8, 32, width * 4, colorspace, kCGBitmapByteOrder32Big | kCGImageAlphaPremultipliedLast,
ref, NULL, true, kCGRenderingIntentDefault);
// OpenGL ES measures data in PIXELS
// Create a graphics context with the target size measured in POINTS
NSInteger widthInPoints, heightInPoints;
if (NULL != UIGraphicsBeginImageContextWithOptions) {
// On iOS 4 and later, use UIGraphicsBeginImageContextWithOptions to take the scale into consideration
// Set the scale parameter to your OpenGL ES view's contentScaleFactor
// so that you get a high-resolution snapshot when its value is greater than 1.0
CGFloat scale = eaglview.contentScaleFactor;
widthInPoints = width / scale;
heightInPoints = height / scale;
UIGraphicsBeginImageContextWithOptions(CGSizeMake(widthInPoints, heightInPoints), NO, scale);
}
else {
// On iOS prior to 4, fall back to use UIGraphicsBeginImageContext
widthInPoints = width;
heightInPoints = height;
UIGraphicsBeginImageContext(CGSizeMake(widthInPoints, heightInPoints));
}
CGContextRef cgcontext = UIGraphicsGetCurrentContext();
// UIKit coordinate system is upside down to GL/Quartz coordinate system
// Flip the CGImage by rendering it to the flipped bitmap context
// The size of the destination area is measured in POINTS
CGContextSetBlendMode(cgcontext, kCGBlendModeCopy);
CGContextDrawImage(cgcontext, CGRectMake(0.0, 0.0, widthInPoints, heightInPoints), iref);
// Retrieve the UIImage from the current context
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
// Clean up
free(data);
CFRelease(ref);
CFRelease(colorspace);
CGImageRelease(iref);
return image;
}