Problem with saving screenshot

Hi guys,there is a problem when i saving a screenshot. when i set unity qualitysetting to good level,it can get the shot,but set to fantastic level cannot. please help me out. Here's my code.

 - (UIImage*)snapshot:(UIView*)eaglview{
 
 GLint backingWidth1, backingHeight1;
 // 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, _surface.renderbuffer);
 // Get the size of the backing CAEAGLLayer
 glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_WIDTH_OES, &backingWidth1);
 glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_HEIGHT_OES, &backingHeight1);
 //
 NSInteger x = 0, y = 0, width = backingWidth1, height = backingHeight1;
 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;

}