Color normalization results in gray tones - c

Question

I use floating point numbers in my code (c code with openGL) to operate with colors, but the graphic I am using is in the rgb byte format. Loaded with the following code everything works correctly:
//... glfw window initialization...
for (int y = 0; y < 32; y++)
{
for (int x = 0; x < 32; x++)
{
int p = (y * 32 + x) * 3;
float r = rgb[p];
float g = rgb[p + 1];
float b = rgb[p + 2];
glColor3ub(r, g, b);
glPointSize(16);
glBegin(GL_POINTS);
glVertex2i(x * 16, y * 16);
glEnd();
}
}
Resulting in:
The error occurs if I try to normalize the bytes (one by one) with the following code:
//... glfw window initialization...
for (int y = 0; y < 32; y++)
{
for (int x = 0; x < 32; x++)
{
int p = (y * 32 + x) * 3;
float r = ((float)rgb[p]) / 255.0f;
float g = ((float)rgb[p + 1]) / 255.0f;
float b = ((float)rgb[p + 2]) / 255.0f;
glColor3f(r,g, b);
glPointSize(16);
glBegin(GL_POINTS);
glVertex2i(x * 16, y * 16);
glEnd();
}
}
The picture loaded this way has different colors:

Answer 1

Suppose the array rbg[] is signed, and consider a red component value 255.
In the first example the 255 is actually -1 and is converted to -1.000000 and this is then passed to the function after converting to type GLubyte which is unsigned 8-bit. The conversion rules will make this value 255 which is what you thought you had.
In the second example the 255 is again converted to -1.000000 and scaled to be -0.003922 instead of the expected 1.000000and so the result is very different from what you expected.
The solution is to define the array as unsigned char or uint8_t.