OpenGL convert RGBA byte pixels to image integers array - c++

Question

I read pixels from OpenGL texture 2D into byte array (unsigned char) as it is usually done.But now I need to convert it into image array (of Integers I suppose) to have the layout and pixel range of the images loaded from CPU for reverse process.
My question is - is it enough just to to do :
glGetTexImage(GL_TEXTURE_2D,0,GL_RGBA,GL_UNSIGNED_INT,bytes);
instead of :
glGetTexImage(GL_TEXTURE_2D,0,GL_RGBA,GL_UNSIGNED_BYTE,bytes);
and then iterate over each integer and convert it from 0-1 range to 0-255?
I haven't really found any example doing such a conversion without using 3 party image libs.
If I do this :
size_t lenght=_viewWidth * _viewHeight ;
GLubyte *bytes=(GLubyte*)malloc(lenght);
/////////////// read pixels from tex /////////////////////
glBindTexture(GL_TEXTURE_2D,tex);
glGetTexImage(GL_TEXTURE_2D,0,GL_BGR,GL_UNSIGNED_BYTE,bytes);
uint8_t Rc, Gc, Bc;
for(x = 0; x < lenght; x+=3)
{
Bc = *bytes + x;
Gc = *bytes + x + 1;
Rc = *bytes + x + 2;
}
Is Rc , Gc and Bc going to be in the 0-255 range ?

Answer 1

When OpenGL loads a texture, it will convert the incoming pixels into the format provided as the internal format as specified in the glTexImage*() call. This operation may include a mapping step from the pixel format (glTexImage*()'s third parameter) to the internal format, and often includes mapping into one of the ranges [0,1] or [-1,1], and then onto the range for the internal format for each component. For example, a pixel format of GL_FLOAT, and an internal format of GL_RGBA8, will cause the input values to be mapped from the range [0,1] into the range [0,255].
When you retrieve the texels using glGetTexImage(), the process is done in reverse, and so the output pixel values (per component) will be in the range of the specified output type (e.g., GL_UNSIGNED_INT in your case). The range for unsigned ints is [0,232-1], so that will be the range of values returned in your integer image array. If you need those values in a different range (e.g., GL_UNSIGNED_BYTES), then you would need to manually convert values into the range you need.
Personally, if one of the data types OpenGL can return matches the range of values you need, try to use that type.