gamma.py

# Lookup table generator
#
# Really it's correcting for the eye's luminance response, it's not actually gamma correction (it's misnamed!)
#
# We're generating a set of 256-entry lookup tables which you use to (separately) convert the 8-bit R, G and B components of your color
# into something you output, typically with an intelligent RGB LED such as a WS2812B; but a simple CPU-based PWM will give the same results (and benefit from this correction)
# 
# We generate several sets  (2,4,8...) of 256-entry tables so you can cycle through them, using a successive 8-bit lookup table each successive output frame.
# This is 'temporal dithering', which aims to give you better color resolution at the dark end of the scale.
# If you update at e.g. 100hz, try using two or three 'ditherBits' (i.e. 2^^2 or 2^^3=8 tables. The faster your update the more tables you can use
# and you get better color resolution. More than 5 ditherbits is probably excessive, even 2 will help somewhat.
#
# If you get objectionable flickering when displaying low-intensity pixels, you should either update your leds faster or reduce ditherBits
# 
# 
#
fout=open("gamma.h","wt")

#adjust me! Each extra bit doubles the table size 
ditherBits=4

ditherMSB=1<<(ditherBits-1)

res="/* Dithered luminance correction table - autogenerated by gamma.py */\n#define DITHER_BITS %d\nconst unsigned char gammaTable[]={" % ditherBits

useRealLuminanceCalcuation=True # CIE 1931 formula

finalAdjust=0  #set this to 1 if you want your values to start at one (1..255). This is a quirky request for FastLED users only

for dither in range(1<<ditherBits):
    out=[]

    #reverse the low order bits so the dithering is less flickery
    ditherValue=0
    dread=1<<ditherBits
    dout=1
    for d in range(ditherBits):
        dread>>=1
        if dither & dread:
            ditherValue|=dout
        dout<<=1;
    
    ditherValue=(ditherValue<<(8-ditherBits))

    for n in range(256):
        if useRealLuminanceCalcuation:
            # CIE 1931 
            brightness=n/2.56
            if brightness > 8:
                pwmValue= pow( ((brightness + 16) / 116) , 3  )
            else:
                pwmValue = brightness / 903.3      
            pwmValue*=256
        else:
            #use simple power 
            pwmValue=pow(255,(n/256.0))-1
        pwmValue=int(pwmValue*256)
        pwmValue+=ditherValue
        pwmValue=min(255, (pwmValue>>8)+finalAdjust)
        out.append( pwmValue )
    if dither:
        res+=","
    res+="\n\t"+(",".join([ "0x%x"%n for n in out]))  
    
res+="\n\t};\n"

print >>fout,res
fout.close()