nes-emu/ppu/renderers/renderBackground.js

A NES emulator

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.default = renderBackground;
var _constants = _interopRequireDefault(require("../../constants"));
function _interopRequireDefault(e) { return e && e.__esModule ? e : { default: e }; }
const NAME_TABLE_OFFSETS = [1, -1, 1, -1]; // (for `scrolledX` overflow)
const FULL_ALPHA = 0xff000000;

/** Renders the background from the Name tables. */
function renderBackground(_ref) {
  let {
    ppu
  } = _ref;
  const {
    registers,
    loopy
  } = ppu;
  const y = ppu.scanline;
  const scrolledY = registers.ppuScroll.scrolledY();
  const transparentColor = ppu.framePalette.getColorOf(0, 0);
  for (let x = 0; x < _constants.default.SCREEN_WIDTH; x++) {
    const cycle = x + 1;
    const scrolledX = registers.ppuScroll.scrolledX(x);

    // skip masked pixels
    if (!registers.ppuMask.showBackgroundInLeftmost8PixelsOfScreen && x < 8) {
      ppu.frameBuffer[y * _constants.default.SCREEN_WIDTH + x] = FULL_ALPHA | registers.ppuMask.transform(transparentColor);
      ppu.paletteIndexes[y * _constants.default.SCREEN_WIDTH + x] = 0;
      loopy.onVisibleLine(cycle);
      continue;
    }

    // background coordinates based on scroll
    const baseNameTableId = loopy.vAddress.nameTableId;
    const nameTableOffset =
    // (switch horizontal Name table if scrolledX has overflowed)
    scrolledX >= _constants.default.SCREEN_WIDTH ? NAME_TABLE_OFFSETS[baseNameTableId] : 0;
    const nameTableId = baseNameTableId + nameTableOffset;
    const nameTableX = scrolledX % _constants.default.SCREEN_WIDTH;
    const nameTableY = scrolledY % _constants.default.SCREEN_HEIGHT;

    // tile id and palette fetch
    const tileId = ppu.nameTable.getTileIdOf(nameTableId, nameTableX, nameTableY);
    const paletteId = ppu.attributeTable.getPaletteIdOf(nameTableId, nameTableX, nameTableY);
    const paletteColors = [ppu.framePalette.getColorOf(paletteId, 0), ppu.framePalette.getColorOf(paletteId, 1), ppu.framePalette.getColorOf(paletteId, 2), ppu.framePalette.getColorOf(paletteId, 3)];

    // tile row fetch
    const patternTableId = registers.ppuCtrl.patternTableAddressIdForBackground;
    const tileStartX = nameTableX % _constants.default.TILE_LENGTH;
    const tileStartY = nameTableY % _constants.default.TILE_LENGTH;
    const patternLowByte = ppu.patternTable.getLowByteOf(patternTableId, tileId, tileStartY);
    const patternHighByte = ppu.patternTable.getHighByteOf(patternTableId, tileId, tileStartY);

    // partially draw tile (from `tileStartX` until its end or the end of the Name table)
    const remainingNameTablePixels = _constants.default.SCREEN_WIDTH - nameTableX;
    const tilePixels = Math.min(_constants.default.TILE_LENGTH - tileStartX, remainingNameTablePixels);
    for (let i = 0; i < tilePixels; i++) {
      const paletteIndex = ppu.patternTable.getPaletteIndexFromBytes(patternLowByte, patternHighByte, tileStartX + i);
      const color = paletteIndex !== _constants.default.COLOR_TRANSPARENT ? paletteColors[paletteIndex] : transparentColor;
      ppu.frameBuffer[y * _constants.default.SCREEN_WIDTH + x + i] = FULL_ALPHA | registers.ppuMask.transform(color);
      ppu.paletteIndexes[y * _constants.default.SCREEN_WIDTH + x + i] = paletteIndex;
      loopy.onVisibleLine(cycle + i);
    }

    // (the x++ of the for loop will do the last increment)
    x += tilePixels - 1;
  }
}