1 files changed, 26 insertions, 10 deletions

diff --git a/ppu/render.go b/ppu/render.go
index bfde9e4..ef3ad9a 100644
--- a/ppu/render.go
+++ b/ppu/render.go
@@ -31,14 +31,15 @@ func (p *PPU) renderTick() {
 		}
 	}
 
+	if dot == 257 && (visibleLine || line == preRenderLine) {
+		p.evaluateSprites()
+	}
+
 	if visibleLine && dot >= 1 && dot <= ScreenWidth {
 		p.drawPixel(dot-1, line)
 	}
 }
 
-// the fetch cycle, every tile takes 8 dots to fetch (nametable byte, attribute
-// byte, pattern low, pattern high), 2 dots each. dots 321-336 fetch the first two
-// tiles of the next line ahead of time
 func (p *PPU) backgroundFetch(dot int) {
 	if !((dot >= 2 && dot <= 257) || (dot >= 321 && dot <= 337)) {
 		return
@@ -144,18 +145,33 @@ func (p *PPU) copyVertical() {
 	p.v = (p.v &^ 0x7BE0) | (p.t & 0x7BE0)
 }
 
+func (p *PPU) backgroundPixel(x int) (pixel, palette uint8) {
+	if p.Mask&maskShowBackground == 0 || (x < 8 && p.Mask&maskShowBackgroundLeft == 0) {
+		return 0, 0
+	}
+	bit := uint16(0x8000) >> p.x
+	pixel = bitAt(p.patternHi, bit)<<1 | bitAt(p.patternLo, bit)
+	palette = bitAt(p.attrHi, bit)<<1 | bitAt(p.attrLo, bit)
+	return pixel, palette
+}
+
+// mixes the background and sprite pixel at (x, y) into the framebuffer
 func (p *PPU) drawPixel(x, y int) {
-	var pixel, palette uint8
-	if p.Mask&maskShowBackground != 0 && (x >= 8 || p.Mask&maskShowBackgroundLeft != 0) {
-		bit := uint16(0x8000) >> p.x
-		pixel = bitAt(p.patternHi, bit)<<1 | bitAt(p.patternLo, bit)
-		palette = bitAt(p.attrHi, bit)<<1 | bitAt(p.attrLo, bit)
+	bgPixel, bgPalette := p.backgroundPixel(x)
+	spritePixel, spritePalette, behind, isSprite0 := p.spritePixel(x)
+
+	// opaque sprite 0 pixel over an opaque background pixel, never at x = 255
+	if isSprite0 && bgPixel != 0 && x != 255 {
+		p.sprite0Hit = true
 	}
 
 	// pixel 0 of every palette is the shared backdrop color at $3F00
 	addr := uint16(0x3F00)
-	if pixel != 0 {
-		addr += uint16(palette)<<2 | uint16(pixel)
+	switch {
+	case spritePixel != 0 && (bgPixel == 0 || !behind):
+		addr = 0x3F10 | uint16(spritePalette)<<2 | uint16(spritePixel)
+	case bgPixel != 0:
+		addr |= uint16(bgPalette)<<2 | uint16(bgPixel)
 	}
 	color := p.readPalette(addr)
 	if p.Mask&maskGreyscale != 0 {