trying to tile by shader

assets/tiling.frag

@@ -0,0 +1,46 @@
+#version 120
+#ifdef GL_ES
+    precision mediump float;
+#endif
+
+layout(origin_upper_left) in vec4 gl_FragCoord;
+
+
+varying vec4 v_color;
+varying vec2 v_texCoords;
+
+uniform sampler2D tilesAtlas;
+uniform sampler2D backgroundTexture;
+
+uniform vec2 tileInAtlas = vec2(256, 256);
+uniform vec2 atlasTexSize = vec2(4096, 4096);
+
+
+uniform vec2 tileInDim; // vec2(tiles_in_horizontal, tiles_in_vertical)
+uniform vec2 cameraTranslation = vec2(0, 0);
+uniform float tileSizeInPx = 16;
+
+
+uniform float tilemap[tileInDim.x * tileInDim.y]; // must be float array
+
+
+void main() {
+
+    vec2 pxCoord = gl_FragCoord.xy - cameraTranslation;
+    vec2 pxCoordModTilesize = mod(pxCoord, tileSizeInPx);
+    vec2 tileCoord = floor(pxCoord / tileCoord);
+
+    int absoluteTileCoord = int(tileCoord.x + tileCoord.y * tileInDim.x);
+
+
+    float tile = tilemap[absoluteTileCoord]; // sure it's integer at this point
+    vec2 fragCoordInAtlas = vec2(
+            tileSizeInPx * mod(tile, tileInAtlas.x)    + pxCoordModTilesize.x,
+            tileSizeInPx * floor(tile / tileInAtlas.x) + pxCoordModTilesize.y
+    );
+    vec2 fragCoordUV = vec2(fragCoordInAtlas.x / atlasTexSize.x, 1 - fragCoordInAtlas.y / atlasTexSize.y);
+    vec4 fragInAtlas = texture2D(tilesAtlas, fragCoordUV);
+
+
+	gl_FragColor = fragInAtlas;
+}