Question

I have a bunch of image files in a proprietary binary format that I want to load into NSImages. The format is not a simple bitmap, but rather a kind of an RLE representation mixed with transparency and miscellaneous additional information.

In order to display one of these images in a Cocoa app, I need a way to parse the image file byte by byte and “calculate” a bitmap from it which I will then put into an NSImage.

What is a good approach for doing this in Objective-C/Cocoa?

Answer 1

The tasks of interpreting image data are handled by the image’s representation object(s). To use a proprietary format, you have a few options: (a) create a custom representation class, (b) use NSCustomImageRep with a custom delegate, or (c) use a custom object to translate your image to a supported format, such as a raw bitmap.

If you choose to create a custom representation class, you will create a subclass of NSImageRep as described in Creating New Image Representation Classes. This basically requires that your class register itself and be able to draw the image data. In addition to this, you can override methods to return information about the image, and you will be able to instantiate your images using the normal NSImage methods. This method requires the most work.

Using NSCustomImageRep requires less work than creating a custom implementation. Your delegate object only needs to be able to draw the image at a fixed location. However, you cannot return other information about the image, and you will need to create the NSCustomImageRep object manually before creating the NSImage.

Translating the image into a different format is also simpler than creating a custom representation. It could be as simple as creating a blank NSImage of the proper size and drawing into it. Creating the image is still more complicated since you need to call your translation method, and this will affect efficiency (both future drawing time and memory usage) since you are changing formats, which could be good or bad. You will also lose any association between the image object and its source.