FromArgb vs FromScRgb

Question

Should these two expressions result in colors which are roughly the same?

Color.FromArgb(255, 255, 255, (byte)0.25 * 255))

Color.FromScRgb(1.0f, 1.0f, 1.0f,         


        

Answer 1

Same answer as already mentioned - they do not map linearly ( they are different colour spaces - ScRGB is not SRGB with higher precision )

To mzzzzb "@FurqanSafdar the solution you have given may be correct. but thats not the point here, the thing is that -- intuitively we assume them to be linearly related. btw how did you get to that solution"

( We all know what assume does. ) Perhaps it would have been better to link to the definitions of sRGB and ScRGB ! Also, given that they are different color spaces with different ranges, why would you assume ?

You can see the source code and what is being performed:

FromScRGB

FromArgb

In particular the conversion functions:

ScRgbTosRgb

sRgbToScRgb

The ScRgbTosRgb function is the ColorFloatToByte mentioned by Furqan Safdar.

Answer 2

from the docs i can see that the last argument is actually the blue component that you are modifying, alpha is same; fully opaque for both colors

(byte) 0.25 * 255 will give you 0 blue for 0.25 is cast to byte before multiplication, so first one is effectively RGB(255, 255, 0) which is pure yellow. you ought to do (byte) (0.25 * 255) which will be RGB(255, 255, 63)

when took the second color to paint i found out that it is actually RGB(255, 255, 140). so it seems that the scale 0.0 - 1.0 does not map linearly to 0 - 255.

Answer 3

Try to use ColorFloatToByte method to have your desired results:

private static byte ColorFloatToByte(float val)
{
    if (!(val > 0.0)) // Handles NaN case too
        return (0);
    else if (val <= 0.0031308)
        return ((byte)((255.0f * val * 12.92f) + 0.5f));
    else if (val < 1.0)
        return ((byte)((255.0f * ((1.055f * 
               (float)Math.Pow((double)val, (1.0 / 2.4))) - 0.055f)) + 0.5f));
    else
        return (255);
}

Usage:

Color.FromArgb(255, 255, 255, ColorFloatToByte(0.25f)))

Color.FromScRgb(1.0f, 1.0f, 1.0f, 0.25f))

Answer 4

The basic reason behind this is mentioned by mzzzzb above:

the scale 0.0 - 1.0 does not map linearly to 0 - 255

I.e. the ScRgb components do not map linearly to the Argb components.