GLKit Doesn't draw GL_POINTS or GL_LINES

Question

I am working hard on a new iOS game that is drawn only with procedurally generated lines. All is working well, except for a few strange hiccups with drawing some primitives.

I am at a point where I need to implement text, and the characters are set up to be a series of points in an array. When I go to draw the points (which are CGPoints) some of the drawing modes are working funny.

    effect.transform.modelviewMatrix = matrix;
[effect prepareToDraw];
glEnableVertexAttribArray(GLKVertexAttribPosition);
glVertexAttribPointer(GLKVertexAttribPosition, 2, GL_FLOAT, 0, 0, &points);
glDrawArrays(GL_POINTS, 0, ccc);

I am using this code to draw from the array, and when the mode is set to GL_LINE_LOOP or GL_LINE_STRIP all works well. But if I set it to GL_POINTS, I get a gpus_ReturnGuiltyForHardwareRestert error. And if I try GL_LINES it just doesn't draw anything.

What could possibly be going on?

Answer 1

When you draw with GL_POINTS in ES2 or ES3, you need to specify gl_PointSize in the vertex shader or you'll get undefined behavior (ugly rendering on device at best, the crash you're seeing at worst). The vertex shader GLKBaseEffect uses doesn't do gl_PointSize, so you can't use it with GL_POINTS. You'll need to implement your own shaders. (For a starting point, try the ones in the "OpenGL Game" template you get when creating a new Xcode project, or using the Xcode Frame Debugger to look at the GLSL that GLKBaseEffect generates.)

GL_LINES should work fine as long as you're setting an appropriate width with glLineWidth() in client code.