ReadPixel command is self-explanatory.
Shl (shift left) and Shr (shift right) are there to shift the bits of the colors value 8 bits left and then 8 bits right. This is to erase the alpha transparency value in the color.
example:
If any value was in the alpha part, it will be lost with the Shl. Then I shift the data 8 bits right again to put it back where it should be.
Basically, it's just a very fast way to erase the alpha part without effecting the color parts.
Shl (shift left) and Shr (shift right) are there to shift the bits of the colors value 8 bits left and then 8 bits right. This is to erase the alpha transparency value in the color.
example:
01234567 01234567 01234567 01234567 alpha red green blue
If any value was in the alpha part, it will be lost with the Shl. Then I shift the data 8 bits right again to put it back where it should be.
Basically, it's just a very fast way to erase the alpha part without effecting the color parts.