c ++，无符号字符之间的线性插值

Question

Unexpectedly for me I faced strange issue:

Here is example of LOGICAL implementation of trivial linear Lagrange interpolation :

unsigned char mix(unsigned char x0, unsigned char x1, float position){
    // LOGICALLY must be something like (real implementation should be 
    // with casts)...
    return x0 + (x1 - x0) * position;
}

Arguments x0 , x1 are always in range 0 - 255.
Argument position is always in range 0.0f - 1.0f .

Really I tried huge amount of implementations (with different casts and etc.) but it doesn't work in my case! It returns incorrect results (looks like variable overflow or something similar. After looking for solution in internet for a whole week i decided to ask. May be someone has faced similar issues.

I'm using MSVC 2017 compiler (most of parameters are default except language level).
OS - Windows 10x64, Little Endian.

What do i do wrong and what is possible source of the issue?

UPDATED: It looks like this issue is more deep than I expected (thanks for your responses).

Here is the link to tiny github project which demonstrates my issue: https://github.com/elRadiance/altitudeMapVisualiser

Output bmp-file should contain smooth altitude map. Instead of it, it contains garbage. If I use just x0 or x1 as result of interpolation function (without interpolation) it works. Without it - doesn't (produces garbage).

Desired result (as here, but in interpolated colors, smooth)

Actual result (updated, best result achieved)

Main class to run it:

#include "Visualiser.h"


int main() {

unsigned int width = 512;
unsigned int height = 512;


float* altitudes = new float[width * height];

float c;

for (int w = 0; w < width; w++) {
    c = (2.0f * w / width) - 1.0f;
    for (int h = 0; h < height; h++) {
        altitudes[w*height + h] = c;
    }
}

Visualiser::visualiseAltitudeMap("gggggggggg.bmp", width, height, altitudes);

delete(altitudes);
}

Thank you in advance!

Answer 1

SOLVED: Thankfully @wololo. Mistake in my project was not in calculations.

I should open file with option "binary":

file.open("test.bin", std::ios_base::out | std::ios_base::trunc | std::ios_base::binary);

Without it in some point in data can be faced byte with value 10 In Windows environment it can be processed like LineFeed and changed to 13 .