frag.glsl

#version 150 core

uniform sampler2D textureAtlas;
in vec2 v_TexCoord;

out vec4 Target0;

void main() {
    Target0 = texture(textureAtlas, v_TexCoord);
}

vertex.glsl

#version 150 core

// Comes from "canonical quad"
in vec2 a_Pos;
in vec2 a_TexCoord;

// Comes from instance data. These are both in "tiles" units
in vec2 i_PosOffset;
in vec2 i_TexCoordOffset;

// Data gets sent to frag shader and interpolated
out vec2 v_TexCoord;

// Texture atlas size, screen pixel size, and tile size in pixels. These should probably be uniforms.
const vec2 atlasPixelSize = vec2(512.0, 512.0);
const vec2 atlasTilePixelSize = vec2(16.0, 16.0);

// Screen size and screen display tile size
const vec2 screenSize = vec2(640.0, 480.0);
const vec2 screenTileSize = vec2(32.0, 32.0);

// Size of one tile in texture coords
const vec2 tileTexSize = tilePixelSize/atlasTilePixelSize;
// Size of one tile in screen coords
const vec2 tileScreenSize = screenTileSize/screenSize * 2.0;

void main() {
    v_TexCoord = (a_TexCoord + i_TexCoordOffset) * tileTexSize;
    
    // Since OGL is +y = up, but we want top left tile = (0,0)
    i_PosOffset.y *= -1.0; 
    a_Pos.y *= -1.0;

    gl_Position = (a_Pos + i_PosOffset) * tileScreenSize - vec2(1.0);
}