By default, OpenGL uses normalized TexCoords, which mean the value must be between 0 and 1 ([0..1]). the common implementation of Vertex must be look something like this:
// Common Implementation of Vertex
Vertex v = new Vertex(
// Position
new Vector2(0f, 500f),
// 'Normalized' TexCoords [0..1]
new Vector2(0f, 1f),
// Color of Vertex
Color.White
);
// Or in immediate mode..
// It's the same, we still need to specify between 0 and 1 ([0..1])
GL.TexCoord2(1f, 0f);
However, is it possible to use non normalized texcoord (so it specified in pixel format, which mean the value must be between 0 and size ([0..size])) in OpenGL?
To set up the texture transformation for this case, use:
glMatrixMode(GL_TEXTURE);
glScalef(1.0f / textureWidth, 1.0f / textureHeight, 1.0f);
It looks like you made attempts to do the same thing with glOrtho(). While that's possible, the following will not give the desired result:
glOrtho(0.0, textureWidth, 0.0, textureHeight, ...);
glOrtho() builds a matrix that transforms the given ranges to the range [-1.0, 1.0] in both coordinate directions. This makes sense because it is mostly used for vertex coordinates that need to be transformed into NDC space, which is [-1.0, 1.0].
However, for texture coordinates, you want to transform the [0.0, textureWidth] and [0.0, textureHeight] ranges to the range [0.0, 1.0]. If you were completely set on using glOrtho(), the correct call would be:
glOrtho(-textureWidth, textureWidth, -textureHeight, textureHeight, 1.0, -1.0);
Since this now maps [-textureWidth, textureWidth] to [-1.0, 1.0], this means that it maps [0.0, textureWidth] to [0.0, 1.0], which is the desired transformation.
But it's much easier and cleaner to use the glScalef() call shown at the start of my answer.
I still can't figure out why GL.Ortho (while in MatrixMode.Texture) isn't give me intended result..
So finally, I ended up with calculating matrix manually:
// Initialize identity matrix
Matrix4 matrix = new Matrix4
(1f, 0f, 0f, 0f,
0f, 1f, 0f, 0f,
0f, 0f, 1f, 0f,
0f, 0f, 0f, 1f);
// Or use Matrix4.Identity (in OpenTK case), that should be equal(?)
// Convert to normalized
matrix[0, 0] = 1f / textureWidth;
matrix[1, 1] = 1f / textureHeight;
// Apply the matrix to texture matrix
GL.MatrixMode(MatrixMode.Texture);
GL.LoadMatrix(ref matrix);
