john-bright - 2 years ago 188

Python Question

So I was looking how to get a list of RGB colors depending on a desired total of colors to retrieve and I found this piece of code. And there's a part that I can't understand, I already read the notes where the ">>" and "&" operators are bitwise operators but I can't fully understand what they are doing.

Can anybody help me on understanding the part where the colors values

are been assigned?

`def getDinstinctRGBColorsList(desiredColors)`

availableColors = 16000000

inc = availableColors/desiredColors

colorsList = {}

RGB = 0

count = 0

while count <= desiredColors:

RGB = RGB+inc

colorBlue = RGB & 255

colorGreen = (RGB >> 8) & 255

colorRed = (RGB >> 16) & 255

colorsList[count] = str(colorRed) + "," + str(colorGreen) + "," + str(colorBlue)

count += 1

return colorsList

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Answer Source

See BitwiseOperators and What are bitwise shift (bit-shift) operators and how do they work?.

From the code you posted, it looks like `RGB`

contains 24 bits of color information: 8 bits for red, 8 bits for green, and 8 bits for blue with the red data in the left most 8 bits, the green data in the middle 8 bits, and the blue data in the right most 8 bits.

Imagine the bits of `RGB`

look like `0brrrrrrrrggggggggbbbbbbbb`

where `r`

is a bit for the red value, `g`

is a bit for the green value, and `b`

is a bit for the blue value.

Note that `255`

in binary is `0b11111111`

(8 set bits).

`colorGreen = (RGB >> 8) & 255`

is extracting the middle 8 bits that represent green using `>>`

(right shift) and `&`

(bitwise and):

`0brrrrrrrrggggggggbbbbbbbb >> 8`

yields `0b00000000rrrrrrrrgggggggg`

Note how the bits for green are now the left-most 8 bits. However, the bits for red are still present.

`0b00000000rrrrrrrrgggggggg & 0b00000000000000001111111`

yields `0b0000000000000000gggggggg`

Note how only the bits for green remain.

Edit: This is a simplification. In Python, `>>`

is an arithmetic shift, not a logical shift. An arithmetic shift preserves sign. See [What are bitwise shift (bit-shift) operators and how do they work?] for a more detailed explanation.

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