DiscreteAlphaIndexColorModel.getNumComponents is giving the wrong information

Describe the bug According to my understanding, the ColorModel.getNumComponents should return the number of components/channels in the final image, not in the sample buffer. DiscreteAlphaIndexColorModel.getNumComponents’ implementation is giving sample size.

Version information

1. 3.8.2

2. More details:

If you look at the documentation for getNumComponents you can see that this is the number of channels in the final image. E.g. RGB would be 3. RGB with Alpha would be 4. This can be verified by looking at the implementation for an IndexColorModel (which defaults to ColorModels implementation). Usually, with an IndexColorModel, the sample buffer would contain an single array of 1 byte per pixel. That means the “number of samples” per pixel is 1. However, if I have an RGB image with an IndexColorModel, the .getNumComponents is 3.

DiscreteAlphaIndexColorModel is basically a 2 “sample” per pixel model (one sample is an RGB image with an IndexColorModel and the other is the Alpha channel). Therefore, under almost all circumstances, the getNumComponents should return 4 (unless it somehow got constructed for an image without an alpha channel, in which case it should return 3 since it should act just like an IndexColorModel without an alpha channel).

Instead, it always returns 2 (potentially 1 if it doesn’t have an alpha channel).

It’s possible there’s a misunderstanding on my part. If so, please let me know.

[jimfcarroll]

I have a project with a conversion from a BufferedImage to an OpenCV mat. In the test for the conversion I compare the pixel values from the original BufferedImage to the resulting Mat pixel-by-pixel. That compare fails because of the number of channels. Here is my workaround in the test code:

https://github.com/KognitionAI/pilecv4j/blob/master/lib-image/src/test/java/ai/kognition/pilecv4j/image/UtilsForTesting.java#L108

If I’m misinterpreting what that method means on the color model, then I can just use: colorModel.getNumColorComponents() + (hasAlpha ? 1 : 0) but I thought I’d just add the issue here as an FYI in case I’m not misinterpreting it.

[jimfcarroll]

Thanks. Sounds great.