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package ppu
const (
ScreenWidth = 256
ScreenHeight = 240
preRenderLine = 261
)
// pixels of the last (or in progress) frame as ARGB8888
func (p *PPU) Framebuffer() []uint32 {
return p.framebuffer[:]
}
// runs after Tick has moved to the new dot
func (p *PPU) renderTick() {
line, dot := p.Scanline, p.Dot
visibleLine := line < ScreenHeight
if p.renderingEnabled() && (visibleLine || line == preRenderLine) {
p.backgroundFetch(dot)
if dot == 256 {
p.incrementY()
}
if dot == 257 {
p.loadBackgroundShifters()
p.copyHorizontal()
}
if line == preRenderLine && dot >= 280 && dot <= 304 {
p.copyVertical()
}
}
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
}
if p.Mask&maskShowBackground != 0 {
p.shiftBackground()
}
switch (dot - 1) % 8 {
case 0:
p.loadBackgroundShifters()
p.nextTileID = p.readVRAM(0x2000 | (p.v & 0x0FFF))
case 2:
// attribute byte holds 4 palettes (2 bits each) for a 4x4 tile area,
// coarse y/x bit 1 pick which quadrant is ours
addr := 0x23C0 | (p.v & 0x0C00) | ((p.v >> 4) & 0x38) | ((p.v >> 2) & 0x07)
attr := p.readVRAM(addr)
if p.v&0x40 != 0 {
attr >>= 4
}
if p.v&0x02 != 0 {
attr >>= 2
}
p.nextAttr = attr & 0x03
case 4:
p.nextPatternLo = p.readVRAM(p.backgroundPatternAddr())
case 6:
p.nextPatternHi = p.readVRAM(p.backgroundPatternAddr() + 8)
case 7:
p.incrementCoarseX()
}
}
// address of the current row of the next tiles pattern
func (p *PPU) backgroundPatternAddr() uint16 {
var base uint16
if p.Ctrl&ctrlBGPatternHigh != 0 {
base = 0x1000
}
fineY := (p.v >> 12) & 0x07
return base + uint16(p.nextTileID)*16 + fineY
}
func (p *PPU) shiftBackground() {
p.patternLo <<= 1
p.patternHi <<= 1
p.attrLo <<= 1
p.attrHi <<= 1
}
// puts fetched tile into the low byte of the shifters. the attribute bits
// are the same for all 8 pixels of the tile, so they get repeated
func (p *PPU) loadBackgroundShifters() {
p.patternLo = (p.patternLo & 0xFF00) | uint16(p.nextPatternLo)
p.patternHi = (p.patternHi & 0xFF00) | uint16(p.nextPatternHi)
p.attrLo = (p.attrLo & 0xFF00) | repeatBit(p.nextAttr&0x01)
p.attrHi = (p.attrHi & 0xFF00) | repeatBit(p.nextAttr&0x02)
}
func repeatBit(b uint8) uint16 {
if b != 0 {
return 0x00FF
}
return 0
}
// scroll helpers, v layout is yyy NN YYYYY XXXXX
// https://www.nesdev.org/wiki/PPU_scrolling
func (p *PPU) incrementCoarseX() {
if p.v&0x001F == 31 {
p.v &^= 0x001F
p.v ^= 0x0400 // wrap into the horizontally adjacent nametable
} else {
p.v++
}
}
func (p *PPU) incrementY() {
if p.v&0x7000 != 0x7000 {
p.v += 0x1000 // fine y
return
}
p.v &^= 0x7000
y := (p.v & 0x03E0) >> 5
switch y {
case 29:
y = 0
p.v ^= 0x0800 // wrap into the vertically adjacent nametable
case 31:
y = 0
default:
y++
}
p.v = (p.v &^ 0x03E0) | (y << 5)
}
func (p *PPU) copyHorizontal() {
p.v = (p.v &^ 0x041F) | (p.t & 0x041F)
}
func (p *PPU) copyVertical() {
p.v = (p.v &^ 0x7BE0) | (p.t & 0x7BE0)
}
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)
}
// pixel 0 of every palette is the shared backdrop color at $3F00
addr := uint16(0x3F00)
if pixel != 0 {
addr += uint16(palette)<<2 | uint16(pixel)
}
color := p.readPalette(addr)
if p.Mask&maskGreyscale != 0 {
color &= 0x30
}
p.framebuffer[y*ScreenWidth+x] = 0xFF000000 | systemPalette[color&0x3F]
}
func bitAt(v, bit uint16) uint8 {
if v&bit != 0 {
return 1
}
return 0
}
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