Draw UIImage From CIImage In drawRect:

Question

I'm learning about drawing UIImages and CGImages, using CIFilters etc. To test my knowledge I made a small test app with sliders that programmatically change the color of a potion sprite and display it on screen (using a CIHueBlendMode CIFilter). After I finished, I wanted to cleanup the relatively lengthy code and noticed that instead of going from the filter's outputted CIImage to a CGImage and then a UIImage, I could go directly from a CIImage to UIImage using UIImage's imageWithCIImage: method.

However, when I tried to draw the resultant UIImage using drawInRect:, nothing was drawn. Going through the CGImage stage rectifies this, of course. My understanding of this is that making a UIImage from a CIImage results in a NULL CGImage property in the UIImage, which is used in drawInRect:. Is this correct? If so, is there a better way to display a CIImage than to go through CGImage followed by UIImage? I could just draw a CGImage made with the CIImage, but that would flip the image, which leads to another question. Currently, I wrap anything I draw in a UIImage first to take care of flipping. Is there another, more efficient way?

Too Long; Didn't Read: Is there a better way to draw CIImages other than turning it into a CGImage, then a UIImage and drawing that? What's the best way to handle flipping when drawing CGImages?

Thanks to anyone who can answer some of my questions. :)

Answer 1

After doing some research into what a CIImage is, I realize now that you cannot skip the step of making a CGImage from the CIImage, and even if you could, it wouldn't really be any more efficient, since you'd still have to process the CIImage regardless. A CIImage is not really an image, as noted in Apple's documentation, which is processed when it's turned into a CGImage. That's also why if I use Time Profiler on my project I see that 99% of my time in my drawRect: method is spent on createCGImage:, and not using CIFilters.

As for the most efficient way to cope with the coordinate system change between Core Graphics and the iPhone, it seems that wrapping the object in a UIImage instance is the easiest (not sure about best) way to go. It's simple, and relatively efficient. Another option would be to transform the graphics context.

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If I don't get a better answer than my own within three days, I'll mark it as accepted.