CODEWITHSUNDEEP
c#openglopentk
chris
chris

Reputation: 83

OpenTK - Top Left Origin and pixel co-ordinates

I'm trying to learn how to use OpenGL in a 2D application by using OpenTK and have read that using the inbuilt calls glMatrixMode are not modern. I want to use top left origin and pixel co-ordinates in my shader inputs and assumed I could define a matrix to do these translations.

I am trying to do this using my own matrix using the OpenTK matrix clases. However I think I have made a mistake in setting up the projection matrix and want to verify what I should be doing:-

TranslationMatrix = Matrix4.Identity * Matrix4.CreateScale(1, -1, 1);
TranslationMatrix = TranslationMatrix * Matrix4.CreateOrthographicOffCenter(0, bounds.Width, 0, bounds.Height, -1, 1);
var TranslatedPoint = TranslationMatrix * new Vector4(new Vector3(1024, 768, 0), 1); // bounds = {0, 0, 1024, 768 }

This results in x.Xyz == { 2, -2, 0 }. I thought that the x and y co-ordinates used in gl_position in the vertex shader should range from -1 to 1.

I guess I've got a major misunderstanding somewhere, what should I be looking at?

Upvotes: 1

Views: 450

Answers (1)

BDL
BDL

Reputation: 22177

OpenTK stores the matrices in transposed form. This means you have to write everything in reversed order.

var TranslationMatrix = Matrix4.CreateOrthographicOffCenter(0, bounds.Width, 0, bounds.Height, -1, 1);
TranslationMatrix = TranslationMatrix * Matrix4.CreateScale(1, -1, 1);
var TranslatedPoint = new Vector4(1024, 768, 0, 1) * TranslationMatrix;

The result should now be [1, -1, 0, 1].

Upvotes: 1

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