Autotiles - HTML5 Game Development Insights - page 93

for (var dx = -1; dx<=1; dx++)
for (var dy = -1; dy<=1; dy++) {
var v = test[dx+ 3*dy+4] = map.getSurface(i+dx, j+dy) != this
&& !map.getObject(i+dx, j+dy).noSurface
if (v) noEdges = false;
}
if (noEdges) return 0;
var res = 0;
for (var i=0;i<4; i++) { // THIS IS THE MAGIC WITH INDICES
var dx = i&1, dy = i>>1;
var dx2 = dx*2-1, dy2 = dy*2-1;
var r;
if (test[4+dx2]) {
if (test[4+dy2*3]) {
r = 2; // position 5
} else r = 13 - dx2*2; // position 2
} else if (test[4+dy2*3]) {
r = 13 - dy2*8; // position 4
} else if (test[4 + dx2 + dy2*3]) {
r = 13 - dx2*2 - dy2*8; // position 3
} else r = 0; // position 1
r+= dx + dy*4;
res |= (r<<(8+i*6));
}
return res;
}
SurfaceTile.prototype.render = function(renderer, mask, x, y) {
var sprite = this.sprite
var context = renderer.context
if (!sprite) return
var dx, dy;
mask>>>=8;
var T2 = TILE_SIZE/2; // 16 pixels
if (mask!=0) {
dx = (mask&3)*T2; // 2 bits - X coordinate of quarter
dy = ((mask&63)>>2)*T2; // then 4 bits - Y coordinate of quarter
context.drawImage(sprite.source, dx+sprite.x, dy+sprite.y, T2, T2, x, y, T2, T2);
mask>>>=6;
dx = (mask&3)*T2;
dy = ((mask&63)>>2)*T2;
context.drawImage(sprite.source, dx+sprite.x, dy+sprite.y, T2, T2, x+T2, y, T2, T2);
mask>>>=6;
dx = (mask&3)*T2;
dy = ((mask&63)>>2)*T2;
context.drawImage(sprite.source, dx+sprite.x, dy+sprite.y, T2, T2, x, y+T2, T2, T2);
mask>>>=6;
dx = (mask&3)*T2;