I know there is formula for going RGB -> Luminance, but I need given a brightness parameter to modify the RGB values of an image. How do I do that?
Thanks
Algorithm to modify brightness for RGB image
image processingrgb
Related Solutions
It's probably best to use the Python Image Library to do this which I'm afraid is a separate download.
The easiest way to do what you want is via the load() method on the Image object which returns a pixel access object which you can manipulate like an array:
from PIL import Image
im = Image.open('dead_parrot.jpg') # Can be many different formats.
pix = im.load()
print im.size # Get the width and hight of the image for iterating over
print pix[x,y] # Get the RGBA Value of the a pixel of an image
pix[x,y] = value # Set the RGBA Value of the image (tuple)
im.save('alive_parrot.png') # Save the modified pixels as .png
Alternatively, look at ImageDraw which gives a much richer API for creating images.
The method could vary depending on your needs. Here are 3 ways to calculate Luminance:
Luminance (standard for certain colour spaces):
(0.2126*R + 0.7152*G + 0.0722*B)source
Luminance (perceived option 1):
(0.299*R + 0.587*G + 0.114*B)source
Luminance (perceived option 2, slower to calculate):
sqrt( 0.241*R^2 + 0.691*G^2 + 0.068*B^2 )sqrt( 0.299*R^2 + 0.587*G^2 + 0.114*B^2 )(thanks to @MatthewHerbst) source
[Edit: added examples using named css colors sorted with each method.]
Best Answer
The easiest way is to multiply each of the R,G,B values by some constant - if the constant is >1 it will make it brighter, and if <1 it will be darker. If you're making it brighter then you must test each value to make sure it doesn't go over the maximum (usually 255).
Not only is this simpler than the translation from RGB to HSL and back again, but it more closely approximates what happens when you shine a different amount of light at a physical object.