2D Sprite batching in OpenGL: How to send transformation data to GPU

Question

OpenGL newbie here.

So I'm trying to implement sprite batching to draw ~100 sprites per draw call. I've created a VBO that contains texture coordinates, vertex coordinates, color data (for tinting), and a mat4 matrix for transforms.

So far, so good, right? Well, everything is working but the transform portion of my VBO. My theory is that I could push a mat4 with all of the rotation, scaling, flipping, and world location data applied to it, so that it gets multiplied in with viewports (see glsl code below).

#version 130

//Vertex position attribute
in vec2 vertexPos;

//Texture coordinate attribute
in vec2 texCoordIn;
out vec2 texCoord;

// Vertex color
in vec4 textureColorIn;
out vec4 textureColor;

// Matrix transformations for the sprite
in mat4 transformation;

void main()
{
    //Process texCoord
    texCoord = texCoordIn;

    // Process texture color
    textureColor = textureColorIn;



    //Process vertex
    gl_Position =  gl_ProjectionMatrix * gl_ModelViewMatrix * vec4( vertexPos.x, vertexPos.y, 0.0, 1.0 ) * transformation;
}

So here's my question, if you were to send transform data to the GPU, how would you do it? Sending a mat4 over the wire per vertex seems like overkill.

Answer 1

Transform them in software using the CPU and give the GPU only the final vertex coordinates.

The savings in bus bandwidth by reducing the VBO size will be greater than the cost of the transform on the CPU which can be greatly simplified by combining the sprite expansion and transform into a single step, no heavy matrix multiplication needed.

2D rotation, scaling, flipping only needs two 2D vectors: up and right.

vertex[0] = center + up - right;
vertex[1] = center - up - right;
vertex[2] = center - up + right;
vertex[3] = center + up + right;

right is equivalent to the first row of the 4x4 matrix you would have sent over to the GPU, up the 2nd row of the same matrix, center is the 3rd row (translation) of the 4x4 matrix

up and right are not normals, they are half-sizes vectors of the sprite's up direction (half height) and right direction (half width).