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Pixel Data

Applications that store pixel data in memory using ARGB format must take care in how they read data. If the code is not written correctly, it’s possible to misread the data; the result is colors or alpha that appear wrong.

If you see colors that appear wrong when your application runs on an Intel-based Macintosh computer, the following strategy may help you identify where pixel data is being read incorrectly.

Create a test image whose pixel data is easy to identify. For example, set each pixel so that alpha is ff, red is aa, green is bb, and blue is cc. Then read that image into your application. Figure 4-1 shows such an image.

It's also helpful to go through your code and cast pixel data to the unsigned char data type.

Start with the portion of your code that reads the image. Use the following GDB command to examine the pixel data as hexadecimal bytes:

x/<number_bytes>xb <address of first byte>

This command prints the specified number of bytes, starting with the first byte of the first pixel. You should easily be able to see whether what’s displayed onscreen matches the values of the pixels in the test image. If the values you see do not match the test image, then you've identified the misreading problem. If the values match, then you need to identify other portions of your code that modify or transform pixel data, and inspect the pixel data after each transformation.

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