I want to have an R8G8B8A8 texture where 2 channels hold IDs and i want the last 2 channels to hold a weight. How do i do that?

You can sort of think of it as two base-256 digits. So if you wanted to store the value 1000 there, you would have:

B: (1000 / 256 ) A: (1000 - 1000 / 256 * 256) // (integer math)

B: 3 A: 232

And then to reconstruct:

weight = B * (256 ^ 1) + A * (256 ^ 0)

weight = 3 * 256 + 232 * 1

weight = 1000

Of course, if it's a weight, you're probably expecting normalized [0-1] values. (Splitting it across 2 channels gives you 65536 separate weights between 0 and 1 instead of 256). In this case, multiply the weight by 256 and use the fractional and non fractional values.

So, in HLSL for instance:

float weightTemp = weight * 256;

float bValue = floor(weightTemp) / 256; // bring it back to [0-1] so it fits in B

float aValue = frac(weightTemp);

Then to reconstruct:

float4 weightTemp; // Assume this is the texture sample

float weight = weightTemp.b + weightTemp.a / 256;

Note that texture filtering doesn't make sense anymore if you're storing a value across two channels. So you'll have to use point sampling.

You an use 1000 % 256 to get the remainder instead of doing A = (1000 - 1000 / 256 * 256)

And if you want to encode values in the interval [0, 1] into a byte you multiply or divide by 255 instead of 256.

Or just use R16G16 texture.