Question

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How to represent 18bit color depth to 16bit color depth?

I’m porting a software that build from 16-bit color depth device to 18-bit color depth device? How can I represent the 18-bit color depth? Thanks.

Answer 1

NOTE: This is just an edit of my answer to your previous question, since the two are so similar – you just have to reverse the process.

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You will first of all have to access each of the colour components (i.e. extract the R value, the G value, and the B value). The way to do this will depend totally on the way that the colour is stored in memory. If it is stored as RGB, with 5-6-5 bits for the components, you could use something like this:

blueComponent  = (colour & 0x3F);
greenComponent = (colour >>  6) & 0x3F;
redComponent   = (colour >> 12) & 0x3F;

This will extract the colour components for you, then you can use the method outlined above to convert each of the components (I presume 18-bit will use 6 bits per component):

blueComponent  = (blueComponent  / 64.0) * 32;
//greenComponent = (greenComponent / 64.0) * 64;    //not needed
redComponent   = (redComponent   / 64.0) * 32;

Note that with the above code, it is important that you use 64.0, or some other method that will ensure that your program represents the number as a floating point number. Then to combine the components into your 18-bit colour, use:

colour = (redComponent << 11) | (greenComponent << 5) | blueComponent;