Draw GL_TRIANGLE_STRIP based on centre point and size

Question

I am rendering TRIANGLE_STRIPS in OpenGL ES 2.0. I was wondering, would it be possible to modify the vertex shader such that instead of feeding it 4 texture vertices, you give it only one vertex that represents the centre of the TRIANGLE_STRIP, with a parameter for texture width and a height?

Assuming my texture vertex is:

GLfloat textureVertices[] = {
    x, y
};

Can the vertex shader be modified to work with texSize uniform, which would represent the width/height of the TRIANGLE_STRIP? :

attribute highp vec4 position;
attribute lowp vec4 inputPointCoordinate;
uniform mat4 MVP;
uniform lowp vec4 vertexColor;
uniform float texSize;

varying lowp vec2 textureCoordinate;
varying lowp vec4 color;

void main()
{
    gl_Position = MVP*position;
    textureCoordinate = inputPointCoordinate.xy;
    color = vertexColor;
}

Matic Oblak Matic Oblak · Accepted Answer · 2014-10-24T10:59:17

No, at least not in the vertex shader. You need to get the 3 different points in the vertex shader with different attribute values so you can receive the coordinate in the fragment shader which is interpolated.

What you actually can do is pass a center into the vertex shader which is the multiplied with the same matrix as the vertex coordinates. Beside that you would need some kind of radius (or the texture dimensions vector) which will probably need to be scaled if the matrix contains the scale as well. Then you can take both of these values and pass them to the fragment shader (using varying). In the fragment shader you then need to compute the texture coordinates from those 2 parameters and the fragment position.

A simular procedure is used to draw a very nice circle or sphere using only 2 triangles (a square) but I do not suggest you do this as you will only lose on performance plus it is quite a lot of work...

Draw GL_TRIANGLE_STRIP based on centre point and size

1 Answers