Unexpected screen overlay blending when rendering CIImage that has been loaded DIRECTLY from a file instead of via a UIImage

Question

I'm trying to speed up my image processing code. One factor I was trying was to create a CIImage directly, like so:

CIImage* ciImageStrong;
if (_cachedData)
{
    _cachedData = [NSData dataWithContentsOfFile:pathForResource];
    ciImageStrong = [CIImage imageWithData:_cachedData];
}
else
{
    ciImageStrong = [CIImage imageWithContentsOfURL:[NSURL fileURLWithPath:pathForResource]];
}

my problem is, that when using it with a standard @"CISourceOverCompositing" filter, all images are drawn ADDITIVELY instead of normal alpha blend.

When I use the following code, it all works fine:

UIImage* uiImageStrong;
if (_cachedData)
{
    _cachedData = [NSData dataWithContentsOfFile:pathForResource];
    uiImageStrong = [UIImage imageWithData:_cachedData];
}
else
{
    uiImageStrong = [UIImage imageWithContentsOfFile:pathForResource];
}
CIImage* ciImageStrong = [CIImage imageWithCGImage:uiImageStrong.CGImage];

I've tried loading it with the kCGColorSpaceModelRGB colorspace to no avail. Questions:

• Does anybody know why this is going on?
• Maybe someone knows if there is something to be gained by loading a CIImage directly?

Answer 1

In the end I've leaned enough about CGImage to know the first solution wouldn't have speed up the loading of images. [UIImage imageWithContentsOfFile:pathForResource] is very fast and UIImage.CGImage has no discernible performance cost whatsoever.