2D Drawing In OpenGL ES 2.0 (iOS)

Question

I'm learning how to use OpenGL ES 2.0 on iOS. Right now I want to just do some basic 2D animation (e.g. move a rectangle around the screen and change its size). I've started out with the project template for OpenGL ES provided by Apple in Xcode. My drawing code looks like this:

static GLfloat squareVertices[] = {
    -0.5f, -0.33f,
    0.5f, -0.33f,
    -0.5f,  0.33f,
    0.5f,  0.33f
 };

// Update attribute values.
glVertexAttribPointer(VERTEX_ATTR, 2, GL_FLOAT, 0, 0, squareVertices);
glEnableVertexAttribArray(VERTEX_ATTR);

glVertexAttribPointer(COLOR_ATTR, 4, GL_UNSIGNED_BYTE, 1, 0, squareColors);
glEnableVertexAttribArray(COLOR_ATTR);

glDrawArrays(GL_TRIANGLE_STRIP, 0, 5);

Now this will draw a nice rectangle in the middle of the screen. But if I start to change the rectangle by adding the following code, it starts to look funky:

squareVertices[5] -= .001;
squareVertices[7] -= .001;

It is as if part of the rectangle is attached to the center of the screen. I am completely new to OpenGL ES so I'm sure my problem is obvious. I also assume this has something to do with OpenGL ES being a 3D graphics library and I'm trying to treat it as a 2D space. So my question is: What is the best way to draw and animate 2D objects in OpenGL ES 2.0? I've seen some stuff online for OpenGL ES 1.1, but that is not much help to me. Are their special techniques for 2D drawing in OpenGL ES 2.0, or is there some sort of 2D drawing mode?

Any guidance would be greatly appreciated.

Answer 1

@macinjosh: This is a response to your updated question for those who are interested in the answer. I'm guessing you've gained further knowledge since Dec '10 when you posted!

OpenGL vertices are in 3D, not 2D. To be completely truthful, they're actually in 4D since they include a 'w' component as well, but for now just consider that as value 1.0 like a homogenous vector.

Due to their 3D-ness, unless you add a 'z' component in a shader you must specify one as a vertex attribute.

Answer 2

Because you have a '3' as the third parameter of glVertexAttribPointer. I believe you can set it to 2 but I haven't tried this in GL. My guess is the missing z axis would be filled in internally as '0' (but again, try it and see!). Internally, it's probably all going to be 4 float vectors with the 4th ('w') parameter used for homogeneous matrix multiplication gubbins.

If you're doing this on a mobile device you may also wish to look into fixed point maths (faster on some devices that don't have a floating point co-pro) and also Vertex Buffer Objects, which are more efficient on many machines. Also, a fixed vertex format such as used by

glInterleavedArrays(format_code, stride, data)

may prove to be more efficient as the device may have optimized code paths for whatever 'format_code' you decide to go with.

Answer 3

I'm new with OpenGL, but vertex doesn't require 3 float, one every axe : X,Y,Z?

So, the first vertex array will be :

( -0.50f, -0.33f, 0.50f) 
( -0.33f, -0.50f, 0.33f)
(  0.50f,  0.33f, ?????)

the second will be :

( -0.50f, -0.33f, 0.00f )
(  0.50f, -0.33f, 0.00f )
( -0.50f,  0.33f, 0.00f )
(  0.50f,  0.33f, 0.00f )

I'm REALLY a newbie, so don't take me too seriously… :)

BTW, this is the best source of knowledge about OpenGL I found :

http://iphonedevelopment.blogspot.com/2010/10/opengl-es-20-for-ios-chapter-3.html

Hope this answer makes sense…

Answer 4

After some playing around I changed the drawing code to this:

static GLfloat squareVertices[12] = {
    -0.5f, -0.33f, 0.0,
    0.5f, -0.33f, 0.0,
    -0.5f, 0.33f, 0.0,
    0.5f, 0.33f, 0.0
};

// Update attribute values.
glVertexAttribPointer(VERTEX_ATTR, 3, GL_FLOAT, 0, 0, squareVertices);
glEnableVertexAttribArray(VERTEX_ATTR);

glVertexAttribPointer(COLOR_ATTR, 4, GL_UNSIGNED_BYTE, 1, 0, squareColors);
glEnableVertexAttribArray(COLOR_ATTR);

squareVertices[7] -= .001;
squareVertices[10] -= .001;

glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

Adding the third 0.0 float to each vertex seemed to do the trick. I am unclear on why this is so if any one could shed some light I would apreciate it.