What you want to do, I believe, is move the image you want offset to the correct location before applying the composite filter. You can create a CGAffineTransform
to do this like so:
CGAffineTransform transform = CGAffineTransformMakeTranslation(100, 100);
CIImage *adjustedImage = [inputImage imageByApplyingTransform:transform];
Alternatively there is a CIAffineTransform
filter which does the same job. Once you've got your transformed image, you can comp it with the other image normally and it should appear offset in the output.