iOS7 How to take OpenGLES screenshot. - ios

Question

I'm using VLCKit to play video in my app, and I need to be able to take a screenshot of the video at certain points. This is the code I'm using:
-(NSData*)generateThumbnail{
int s = 1;
UIScreen* screen = [UIScreen mainScreen];
if ([screen respondsToSelector:#selector(scale)]) {
s = (int) [screen scale];
}
GLint viewport[4];
glGetIntegerv(GL_VIEWPORT, viewport);
int width = viewport[2];
int height = [_profile.resolution integerValue];//viewport[3];
int myDataLength = width * height * 4;
GLubyte *buffer = (GLubyte *) malloc(myDataLength);
GLubyte *buffer2 = (GLubyte *) malloc(myDataLength);
glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE, buffer);
for(int y1 = 0; y1 < height; y1++) {
for(int x1 = 0; x1 <width * 4; x1++) {
buffer2[(height - 1 - y1) * width * 4 + x1] = buffer[y1 * 4 * width + x1];
}
}
free(buffer);
CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, buffer2, myDataLength, NULL);
int bitsPerComponent = 8;
int bitsPerPixel = 32;
int bytesPerRow = 4 * width;
CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault;
CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;
CGImageRef imageRef = CGImageCreate(width, height, bitsPerComponent, bitsPerPixel, bytesPerRow, colorSpaceRef, bitmapInfo, provider, NULL, NO, renderingIntent);
CGColorSpaceRelease(colorSpaceRef);
CGDataProviderRelease(provider);
UIImage *image = [ UIImage imageWithCGImage:imageRef scale:s orientation:UIImageOrientationUp ];
NSData *thumbAsData = UIImageJPEGRepresentation(image, 5);
return thumbAsData;
}
To be honest, I have no idea how most of this works. I copied it from somewhere a while ago (I don't remember the source). It mostly works, however frequently it seems parts of the image are missing.
Can someone point me in the right direction? Most of the other posts I see regarding OpenGL screenshots are fairly old, and don't seem to apply.
Thanks.

Answer 1

I wrote a class to work around this problem.
Basically you take directly a screenshot from the screen, and then if you want you can take just a part of the image and also scale it.
Taking a screenshot from the screen, you take every stuff. UIKit , OpenGL, AVFoundation, etc.
Here the class: https://github.com/matteogobbi/MGScreenshotHelper/
Below useful functions, but i suggest you to download (and star :D) directly my helper class ;)
/* Get the screenshot of the screen (useful when you have UIKit elements and OpenGL or AVFoundation stuff */
+ (UIImage *)screenshotFromScreen
{
CGImageRef UIGetScreenImage(void);
CGImageRef screen = UIGetScreenImage();
UIImage* screenImage = [UIImage imageWithCGImage:screen];
CGImageRelease(screen);
return screenImage;
}
/* Get the screenshot of a determinate rect of the screen, and scale it to the size that you want. */
+ (UIImage *)getScreenshotFromScreenWithRect:(CGRect)captureRect andScaleToSize:(CGSize)newSize
{
UIImage *image = [[self class] screenshotFromScreen];
image = [[self class] cropImage:image withRect:captureRect];
image = [[self class] scaleImage:image toSize:newSize];
return image;
}
#pragma mark - Other methods
/* Get a portion of an image */
+ (UIImage *)cropImage:(UIImage *)image withRect:(CGRect)rect
{
CGImageRef imageRef = CGImageCreateWithImageInRect([image CGImage], rect);
UIImage *cropedImage = [UIImage imageWithCGImage:imageRef];
return cropedImage;
}
/* Scale an image */
+ (UIImage *)scaleImage:(UIImage *)image toSize:(CGSize)newSize
{
UIGraphicsBeginImageContextWithOptions(newSize, YES, 0.0);
[image drawInRect:CGRectMake(0, 0, newSize.width, newSize.height)];
UIImage *scaledImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return scaledImage;
}

Answer 2

Ok turns out this is a problem only in the Simulator. On a device it seems to work 98% of the time.