CODEWITHSUNDEEP
opengl
andandandand
andandandand

Reputation: 22270

Why do reshape codes end in glMatrixMode(GL_MODELVIEW)?

When looking at the reshape examples in the redbook, I usually find something like: 

void reshape(int w, int h)
{

    glViewport(0, 0, (GLsizei) w, (GLsizei) h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glFrustum(-1.0, 1.0, -1.0, 1.0, 1.5, 20.0);
    glMatrixMode(GL_MODELVIEW);

}

I understand that calling glMatrixMode(GL_PROJECTION); followed by glLoadIdentity(); resets the projection matrix but I don't understand why glMatrixMode(GL_MODELVIEW) is usually called at the end of reshape.

In this particular example glFrustum affects the projection matrix, right? Why is GL_MODELVIEW called later? Would it make a difference if the last call to glMatrixMode(GL_MODELVIEW) is omitted?

Upvotes: 2

Views: 1222

Answers (1)

li.davidm
li.davidm

Reputation: 12126

Most of your rendering code is going to affect GL_MODELVIEW, because it's what affects object translation and camera position. However, the resizing code works on GL_PROJECTION. The programmer probably assumes the current matrix is the modelview one in most of his code, and when a different one needs to be affected, he/she would change the matrix, update it, and then change the target back to GL_MODELVIEW so the rest of the code doesn't target the wrong matrix.

Generally, OpenGL functions affect whatever matrix is currently being targeted, which is why you change the matrix.

Upvotes: 3

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