Question

I want to render 2D images. To do this I scale and transform everything into position with the centre being where I want it. I need to rotate the 2D coordinates around the centre but when I do this it is as if there was some x coordinate involved since the images are flattened out strangely. Here is the code:

attribute vec4 vPosition;
attribute vec2 a_texCoord;
varying vec2 v_texCoord;
uniform vec2 scale;
uniform vec2 trans;
uniform float move_down;
void main(){ 
    gl_Position = vPosition;
    gl_Position.x *= scale.x;
    gl_Position.y *= scale.y;
    gl_Position.x += trans.x;
    gl_Position.y += trans.y - move_down;
    gl_Position.x *= 2.0;
    gl_Position.y *= 2.0;
    gl_Position.x -= 1.0;
    gl_Position.y -= 1.0;
    gl_Position.x *= 0.2; //For visual testing only
    gl_Position.y *= 0.2; //Same
    gl_Position.x = cos(1.0)*gl_Position.x - sin(1.0)*gl_Position.y;
    gl_Position.y = sin(1.0)*gl_Position.x + cos(1.0)*gl_Position.y;
    v_texCoord = a_texCoord;
}

Here is the image without the rotation lines…

And here is the image with the rotation at 1.0 radians…

Clearly there is an issue, since it should be a simple 2D rotation. If I do 90 degrees the image disappears completely.

Done with OpenGL-ES 2 for Android.

I appreciate any answers to this.

Edit: I also tried this with the same problem:

mat4 RotationMatrix = mat4( cos(1.0), -sin(1.0), 0.0, 0.0,
                               sin(1.0),  cos(1.0), 0.0, 0.0,
                               0.0,           0.0, 1.0, 0.0,
                               0.0,           0.0, 0.0, 1.0 );
    gl_Position *= RotationMatrix;

Answer 1

I was multiplying the matrix after the vector… Wrong way around. The Matrix works when it is done as:

gl_Position = RotationMatrix * Position;