How to Convert This RGB format in Python

Question

I am reading values from something in Python. The values come in a strange format. The documentation says:

Color is represented by a single value from 0 to 8355711. The RGB(r,g,b) function calculates
((R*127 x 2^16) + (G*127 x 2^8) + B*127), where R, G and B are values between 0.00 and 1.00

Som, a red color has the value of 16712965 I would love to know how to 'unpack' those values as a tuple or something but am struggling with that math. If this is not possible, a way to convert that value to an rgb value somehow would be great. Please help! Thanks

Yes, but I am not sure how I can split the one integer into three... — Startec, Sep 24 '14 at 05:05
All it is doing is converting it to hex (multiplying by powers of 2 is the same as bit shifts). Try googling converting back and forth between hex and rgb, it's covered all over the place including so, like here http://stackoverflow.com/questions/214359/converting-hex-color-to-rgb-and-vice-versa — en_Knight, Sep 24 '14 at 05:09

score 1 · Answer 1 · answered Sep 24 '14 at 05:12

1

Notice the 2^8 an 2^16 in the formula, this suggest that you can use something akin to right shift in C. Dividing the input number by 2^8=256 is equivalent to 8 bit right shift. Another point to note here is that your R,G,B output values are real numbers. So you would like to use the float function during your calculations.

c = 8355711
print 'input colour', c

# conversion to RGB format
B = float( c % 256 )/127.0
c = c / 256
G = float( c % 256)/127.0
c = c / 256
R = float( c % 256)/127.0

print R, G, B

# reverse calculation for verifying the result
colour = int((R*127 * 65536) + (G*127 * 256) + B*127)
print colour

answered Sep 24 '14 at 05:12

Raiyan

1,589
1
14
28

This seems very very close, however, I am not sure my math is correct: For instance, when I try your result with the following: `c = 8912743 B = float( c % 256 )/127.0 c = c / 256 G = float( c % 256)/127.0 c = c / 256 R = float( c % 256)/127.0 print(R, G, B) colour = int((R*127 * 65536) + (G*127 * 256) + B*127) print(colour)` The same value is not returned (it is however very close). similarly, the RGB values seem to be between something other than 0 - 1 (between 1 and three perhaps?) – Startec Sep 24 '14 at 06:09
@Startec with the information you've given `8912743` should be an invalid color, the only way to produce it is with `G=2.0079`. – Mark Ransom Sep 24 '14 at 11:31

score 1 · Accepted Answer · answered Sep 24 '14 at 10:22

you have mistake in text

Red=16712965=0xFF0505h
is out of range of your claimed limit 8355711=7F7F7Fh

So what exactly is your source and destination color format ???

the most common is 8bit per R,G,B channel packed into 32 bit value
yours looks like 8bit per channel (7bit used) packed into ?? bit value

What exactly you want ?

double R,G,B values in range <0.0,1.0> from int col?

not a python coder in C++ it looks like this:

// 8(7 used) bit per channel
double R=double(col>>16)/127.0;
double G=double(col>> 8)/127.0;
double B=double(col    )/127.0;

// 8 bit per channel
double R=double(col>>16)/255.0;
double G=double(col>> 8)/255.0;
double B=double(col    )/255.0;

x>>n means arithmetic bit shift right (padding zeros to MSB bit) of value x by n bits
it is the same as x/(2^n)

If you want different conversions then you must specify it.

Thanks for catching the error. Not sure how that happened. You explained it perfectly. — Startec, Sep 25 '14 at 01:48

How to Convert This RGB format in Python

2 Answers2