CODEWITHSUNDEEP
c++opengl-esraspberry-pi
brAzzi64
brAzzi64

Reputation: 1104

Adjusting the distortion caused by a projector in OpenGL ES

I'm getting started with a OpenGL ES app (C++/SDL) running on a Raspberry PI, that I'll want to output through a pocket projector.

I want to give the user the ability to correct the distortion caused by aiming the projector from a direction that is non-normal to the surface. The surface will also be smaller than the projected area. To do this, the user will be able to "move" the 4 corners of the projection window independently to match the surface.

I was planning to do this by solving a simple linear system of equations that transforms the original corners (in the current projection matrix coordinates) to the corners set by the user, and just push this resulting matrix on top of the GL_PROJECTION matrix stack.

However... I found out that I can't "read" the projection matrix in OpenGL ES:

float matrix[16];
glGetFloatv(GL_PROJECTION_MATRIX, matrix);

In particular, the GL_PROJECTION_MATRIX symbol doesn't exist... and I've even read that there's no such thing as a projection matrix in OpenGL ES (something I find hard to believe, but I'm totally new to it, so...).

Any idea of a workaround for reading this matrix, or maybe an alternative approach to do what I'm trying to do?

Thanks!

Upvotes: 1

Views: 132

Answers (1)

Reto Koradi
Reto Koradi

Reputation: 54592

OpenGL ES 1.x has a projection matrix, and you should be able to get the current value with exactly the code you have in your question. See http://www.khronos.org/opengles/sdk/1.1/docs/man/glGet.xml for the documentation confirming this, or the header file at http://www.khronos.org/registry/gles/api/GLES/gl.h.

ES 2.0 and higher are a different story. ES 2.0 eliminates most of the fixed pipeline that was present in ES 1.x, and replaces it with shader based rendering.

Therefore, concepts like a built-in projection matrix don't exist anymore in ES 2.0 and are replaced with the much more flexible concept of shader programs written in GLSL. If you want to use a projection matrix in ES 2.0, you have a matrix variable in your vertex shader code, pass a value for the matrix into the shader program (using the glUniformMatrix4fv API call), and the GLSL code you provide for the vertex shader performs the multiplication of vectors with the projection matrix.

So in the case of ES 2.0, there is no need for a glGet*() call for the projection matrix. If you have one, you calculated it, and passed it into the shader program. Which means that you already have the matrix in your code. Well, you could get any matrix value back with the glGetUniformfv() API call, but it's much easier and cleaner to just keep it around in your code.

Upvotes: 1

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