Question

I am using this tutorial / sample to do some basic object tracking on the iPhone. Everything works fine, I even tweaked the performance a whole lot, but I am still stuck with one problem.

The basic problem is that I do not understand OpenGL well enough and I should be punished for shamefully taking sample code and turning it into something that works for me. As a matter of fact I am being punished;

The sample shows how to render (with shaders) the iPhone’s camera into an offscreen texture in order to be able to process it and show it on screen. Now I have figured out that it draws the texture/layer using an array of vertex attributes (a principle I barely understand, despite my Google searches).

The vertex array’s are as follows:

static const GLfloat squareVertices[] = {
        -1.0f, -1.0f,
        1.0f, -1.0f,
        -1.0f,  1.0f,
        1.0f,  1.0f,
};

static const GLfloat textureVertices[] = {
        1.0f, 1.0f,
        1.0f, 0.0f,
        0.0f,  1.0f,
        0.0f,  0.0f,
};

I have also figured out that these vertex attributes can alter the orientation of the drawn texture. The texture is now drawn in portrait, which means that if I keep my iPhone in landscape (which I desire), and let the views rotate along, everything I see on the screen is a camera with a 90 degree angle.

I think I narrowed the problem down enough to blame these vertexes and I have been messing around with their values somewhat but without any acceptable result.

Is there anybody out there who can help me draw the texture in landscape?

P.S: if I inverse the values of ‘squareVerticles’ I am able to get a 180 degree rotated picture. But I want the texture to be rotated 90 degrees, not 180.

Answer 1

Imagine that your screen has normalized coordinates that go from -1.0,-1.0 (left, bottom) to 1.0, 1.0. The first array specifies the coordinates of the vertices of a square (presumably as a triangle strip, because they are given in a “Z” fashion).

The second array specifies the texture coordinates. Same thing, except that they are in the 0.0, 1.0 range.

So, to rotate 90 degrees clockwise:

1.0f, 0.0f
0.0f, 0.0f
1.0f, 1.0f
0.0f, 1.0f

anticlockwise:

0.0f, 1.0f
1.0f, 1.0f
0.0f, 0.0f
1.0f, 0.0f

I hope that works!