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I tested several Grayscale images downloaded from the Internet. Some of them are 8bpp indexed while others are 24bpp (true colored). Looks like different Grayscale images have different depths.

For instance, this online application converts a color image to Grayscale. But, when I check its bit depth, I see that bit depth is not changed.

I think, Grayscales with 24bpp are pseudo-grayscales. That is, they are the average of R, G and B components. I think, real Grayscales are always 8 bit per pixel indexed.

Is my notion correct?

If not, please, provide me some links that I can study.

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It really depends on the format. But from a hardware perspective, the sensor is the thing that determines the amount of data per pixel in the first place. See for example: EV76C560. It is a 10 bit sensor. You can of course store those 10 bits in 16 bits in memory, or interpolate etc., but originally it was 10 bits. So if you do not want to lose data, you must use more than 8 bits.

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I don't think that it is that easy. Grayscale images do have certain benefits when you think of imaging experiments. When the measurement is squeezed into eight bits, even though the measurement is much more precise, then there is a loss of information due to the quantization. See the DICOM format for example that is widely used in medical imaging. It is very common there that the information is encoded in 12 bit, or even more.

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Try looking at PGM images.

The PGM format is a lowest common denominator grayscale file format. It is designed to be extremely easy to learn and write programs for. (It's so simple that most people will simply reverse engineer it because it's easier than reading this specification).

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