Convert BYTE buffer (0-255) to float buffer (0.0-1.0)

Question

How can I convert a BYTE buffer (from 0 to 255) to a float buffer (from 0.0 to 1.0)? Of course there should be a relation between the two values, eg: 0 in byte buffer will be .0.f in float buffer, 128 in byte buffer will be .5f in float buffer, 255 in byte buffer will be 1.f in float buffer.

Actually this is the code that I have:

for (int y=0;y<height;y++) {
    for (int x=0;x<width;x++) {
        float* floatpixel = floatbuffer + (y * width + x) * 4;
        BYTE* bytepixel = (bytebuffer + (y * width + x) * 4);
        floatpixel[0] = bytepixel[0]/255.f;
        floatpixel[1] = bytepixel[1]/255.f;
        floatpixel[2] = bytepixel[2]/255.f;
        floatpixel[3] = 1.0f; // A
    }
}

This runs very slow. A friend of mine suggested me to use a conversion table, but I wanted to know if someone else can give me another approach.

Answer 1

Whether you choose to use a lookup table or not, your code is doing a lot of work each loop iteration that it really does not need to - likely enough to overshadow the cost of the convert and multiply.

Declare your pointers restrict, and pointers you only read from const. Multiply by 1/255th instead of dividing by 255. Don't calculate the pointers in each iteration of the inner loop, just calculate initial values and increment them. Unroll the inner loop a few times. Use vector SIMD operations if your target supports it. Don't increment and compare with maximum, decrement and compare with zero instead.

Something like

float* restrict floatpixel = floatbuffer;
BYTE const* restrict bytepixel = bytebuffer;
for( int size = width*height; size > 0; --size )
{
    floatpixel[0] = bytepixel[0]*(1.f/255.f);
    floatpixel[1] = bytepixel[1]*(1.f/255.f);
    floatpixel[2] = bytepixel[2]*(1.f/255.f);
    floatpixel[3] = 1.0f; // A
    floatpixel += 4;
    bytepixel += 4;
}

would be a start.