Question

I’ve created a basic orthographic shader that displays sprites from textures. It works great.

I’ve added a “zoom” factor to it to allow the sprite to scale to become larger or smaller. Assuming that the texture is anchored with its origin in the “lower left”, what it does is shrink towards that origin point, or expand from it towards the upper right. What I actually want is to shrink or expand “in place” to stay centered.

So, one way of achieving that would be to figure out how many pixels I’ll shrink or expand, and compensate. I’m not quite sure how I’d do that, and I also know that’s not the best way. I fooled with order of my translates and scales, thinking I can scale first and then place, but I just get various bad results. I can’t wrap my head around a way to solve the issue.

Here’s my shader:

// Set up orthographic projection (960 x 640)
mat4 projectionMatrix = mat4( 2.0/960.0, 0.0, 0.0, -1.0,
                             0.0, 2.0/640.0, 0.0, -1.0,
                             0.0, 0.0, -1.0, 0.0,
                             0.0, 0.0, 0.0, 1.0);                        

void main()
{
    // Set position
    gl_Position = a_position;

    // Translate by the uniforms for offsetting
    gl_Position.x += translateX;
    gl_Position.y += translateY;

    // Apply our (pre-computed) zoom factor to the X and Y of our matrix
    projectionMatrix[0][0] *= zoomFactorX;
    projectionMatrix[1][1] *= zoomFactorY;

    // Translate
    gl_Position *= projectionMatrix;

    // Pass right along to the frag shader
    v_texCoord = a_texCoord;
}

Answer 1

mat4 projectionMatrix =

Matrices in GLSL are constructed column-wise. For a mat4, the first 4 values are the first column, then the next 4 values are the second column and so on.

You transposed your matrix.

Also, what are those -1’s for?

For the rest of your question, scaling is not something the projection matrix should be dealing with. Not the kind of scaling you’re talking about. Scales should be applied to the positions before you multiply them with the projection matrix. Just like for 3D objects.

You didn’t post what your sprite’s vertex data is, so there’s no way to know for sure. But the way it ought to work is that the vertex positions for the sprite should be centered at the sprite’s center (which is wherever you define it to be).

So if you have a 16×24 sprite, and you want the center of the sprite to be offset 8 pixels right and 8 pixels up, then your sprite rectangle should be (-8, -8) to (8, 16) (from a bottom-left coordinate system).

Then, if you scale it, it will scale around the center of the sprite’s coordinate system.