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I was having this problem with OpenGL where I'd have all my textures being rendered upside down. I did the most logical thing I could think of and tried reversing the order of the mapping and that fixed the issue. But why? I know that OpenGL's coordinate system is cartesian and that it works by mapping each uv to a specific vertex. I had something like [0,0], [0,1], [1,1], [1,0] which would theoretically as far as I understand it go from:

top-left -> bottom-left -> bottom-right -> top-right.

I changed this to: [1,1], [1,0], [0,0], [0,1] which would represent:

bottom-right -> top-right -> top-left -> bottom left.

In my understanding it should be quite the opposite. Making a pretty quick sketch on paper it shows me that the initial order would theoretically render my texture correctly and not upside down. Am I correct and theres something else messing with my rendering? Like my perspective matrix?

This is my orthographic matrix:

        Matrix4f ortho = new Matrix4f();
        ortho.setIdentity();

        float zNear = 0.01f;
        float zFar = 100f;

        ortho.m00 = 2 / (float) RenderManager.getWindowWidth();
        ortho.m11 = 2 / -(float) RenderManager.getWindowHeight();
        ortho.m22 = -2 / (zFar - zNear);

        return ortho;

I can't really say I understand it though, I had quite a hard time with it. And looking through youtube tutorials and articles you can see most people don't really understand it either, they just use it. I do have a good linear algebra background but still can't wrap my head around how is this matrix normalizing my coordinates (screen coords to OpenGL coords (-1,1)). Anyway, I digress, any help is appreciated.

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In OpenGL, by convention, the bottom-left, not top-left, is the origin (0,0). See also Why is OpenGL point (0,0) not in top-left corner of the screen?

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  • \$\begingroup\$ So it turns out that initially I was operating it the wrong way, right? And when I reversed it, I respected OpenGL's coordinate system? \$\endgroup\$ Aug 28, 2020 at 19:25

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