Question

This might sound lazy at first, but I ve been researching it for 2 days.

I have an SDL+GLEW app that draws a primitive. I wanted to make a few viewports in different perspectives. I see the four viewports but I can t change the the perspectives.

Assume you have

draw();
swapbuffers();

What is the simplest way - in OpenGL3+ spec - to create a perspective viewport (or ideally several ones)?

Answer 1

The simplest is the only way. Calculate yourself (with some lib, plain math, etc) a projection matrix, and upload it to a uniform in your vertex shader, and do the transformation there.

Answer 2

Here s GLSL 1.50 code for doing the same thing as gluPerspective. You can easily convert it to your language of choice, and upload it via glUniformMatrix4fv instead.

mat4 CreatePerspectiveMatrix(in float fov, in float aspect,
    in float near, in float far)
{
    mat4 m = mat4(0.0);

    float angle = (fov / 180.0f) * PI;
    float f = 1.0f / tan( angle * 0.5f );

    /* Note, matrices are accessed like 2D arrays in C.
       They are column major, i.e m[y][x] */

    m[0][0] = f / aspect;
    m[1][1] = f;
    m[2][2] = (far + near) / (near - far);
    m[2][3] = -1.0f;
    m[3][2] = (2.0f * far*near) / (near - far);

    return m;
}

Then you do:

mat4 worldMatrix = CreateSomeWorldMatrix();
mat4 clipMatrix = CreatePerspectiveMatrix(90.0, 4.0/3.0, 1.0, 128.0);

gl_Position = worldMatrix * clipMatrix * myvertex;
some_varying1 = worldMatrix * myvertex; 
some_varying2 = worldMatrix * vec4(mynormal.xyz, 0.0);

Answer 3

For perspective matrix: gluPerspective, then glGetFLoatv(GL_PROJECTION_MATRIX, ...), then glUniform or glUniformMatrix4fv

For viewports: glViewport and glScissor. Or use framebuffer object, render to texture, then render texture.

P.S. If you don t want to use deprecated features, grab matrix formula from gluPerspective documentation, calculate matrix yourself, then use glUniformMatrix4fv. Or work in compatibility profile.

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