Modern opengl. (Flipping a texture)

OpenGL OpenGL: Advanced Coding
1

Hi!

In modern opengl we have to pass each matrix to a shader, I use a texture matrix for passing from [0,size] to [0,1] for uv coordinates.

It works fine except I can’t flip a texture, when I try to invert the y coordinates with this matrix :


math::Matrix4f Texture::getTextureMatrix() const {
            math::Matrix4f matrix(1.f, 0.f, 0.f, 0.f,
                       0.f, 1.f, 0.f, 0.f,
                       0.f, 0.f, 1.f, 0.f,
                       0.f, 0.f, 0.f, 1.f);
            matrix.m11 = 1.f / m_actualSize.x;
            matrix.m22 = 1.f / m_actualSize.y;
            if (m_pixelsFlipped)
            {
                matrix.m22 = -matrix.m22;
            }
            return matrix;
        }

Here is how I do in the vertex shader :

[code=cpp]
#version 330 core
layout (location = 0) in vec4 position;
layout (location = 1) in vec4 color;
layout (location = 2) in vec2 uv;
layout (location = 3) in vec3 normals;
uniform mat4 p;
uniform mat4 v;
uniform mat4 t;
uniform float haveTexture;
out vec4 f_color;
out vec2 texCoords;
void main() {
gl_Position = p * v * position;
texCoords = (haveTexture == 1) ? (t * vec4(uv.xy, 0, 0)).xy : vec2(0, 0);
f_color = color;
}



If I don't invert the v coordinate, the texture is well displayed but if I invert the v coordinates, the texture is not displayed. :/
2

Are you using GL_REPEAT as texture wrap mode?

3

Are you using GL_REPEAT as texture wrap mode?

No, I use GL_CAMP_TO_EDGE

4

If you have a number that goes from [0, size], and you multiply it by -1, then you get numbers on the range [-size, 0]. Therefore, if you divide this by size, you get [-1, 0]. Which is clearly not the same as [0, 1].

To “flip” a texture, you need to divide by the size, multiply by -1, and add 1 to the result. You’re using a 4x4 matrix, so you just need to use the translation part of the matrix.

Alternatively, you can flip the texture on the CPU and just load it into OpenGL correctly.

5

I’ve added a 1 to the translation part of the matrix, but, it doesn’t solve the problem.


 math::Matrix4f Texture::getTextureMatrix() const {
            math::Matrix4f matrix(1.f, 0.f, 0.f, 0.f,
                       0.f, 1.f, 0.f, 0.f,
                       0.f, 0.f, 1.f, 0.f,
                       0.f, 0.f, 0.f, 1.f);
            matrix.m11 = 1.f / m_actualSize.x;
            matrix.m22 = 1.f / m_actualSize.y;
            if (m_pixelsFlipped)
            {
                matrix.m22 = -matrix.m22;
                matrix.m42 = 1.f;
            }
            return matrix;
        }

so, I’ve just flipped the texture on the CPU and now it works, thanks.


frameBufferTile->setScale(math::Vec3f(1.f, -1.f, 1.f));