fix tile atlas stuff 2: fix harder

libcygnet/src/shaders.cc

@@ -12,7 +12,11 @@ const char *spriteVx = R"glsl(
 	varying vec2 v_texCoord;
 
 	void main() {
-		float pixoffset = (1.0 - vertex.y * 2.0) / (TILE_SIZE * frameInfo.x * 2.0);
+		// Here, I'm basically treating 1/(TILE_SIZE*16) as half the size of a "pixel".
+		// It's just an arbitrary small number, but it works as an offset to make sure
+		// neighbouring parts of the atlas don't bleed into the frame we actually
+		// want to draw due to (nearest neighbour) interpolation.
+		float pixoffset = (1.0 - vertex.y * 2.0) / (frameInfo.x * TILE_SIZE * 16.0);
 		v_texCoord = vec2(
 			vertex.x,
 			(frameInfo.x * frameInfo.z + (frameInfo.x * vertex.y)) /
@@ -62,8 +66,9 @@ const char *chunkFr = R"glsl(
 		vec4 tileColor = texture2D(tiles, tilePos / vec2(CHUNK_WIDTH, CHUNK_HEIGHT));
 		float tileID = floor((tileColor.r * 256.0 + tileColor.a) * 256.0);
 
+		// 1/(TILE_SIZE*16) plays the same role here as in the sprite vertex shader.
 		vec2 offset = v_tileCoord - tilePos;
-		vec2 pixoffset = (1.0 - offset * 2.0) / (TILE_SIZE * 2.0);
+		vec2 pixoffset = (1.0 - offset * 2.0) / (TILE_SIZE * 16.0);
 		vec2 atlasPos = vec2(
 			pixoffset.x + tileID + offset.x,
 			pixoffset.y + floor(tileID / tileAtlasSize.x) + offset.y);