"use strict";

const screenEmu = (function () {

  // Classes

  const Point = class {
    constructor(x, y) {
      this.x = x;
      this.y = y;
    }
  }

  const Size = class {
    constructor(width, height) {
      this.width = width;
      this.height = height;
    }

    copy() {
      return new Size(this.width, this.height);
    }

    get ratio() {
      return this.width / this.height;
    }
  }

  const Rect = class {
    constructor(x, y, width, height) {
      this.origin = new Point(x, y);
      this.size = new Size(width, height);
    }

    get x() {
      return this.origin.x;
    }

    get y() {
      return this.origin.y;
    }

    get width() {
      return this.size.width;
    }

    get height() {
      return this.size.height;
    }

    get l() {
      return this.origin.x;
    }

    get r() {
      return this.origin.x + this.size.width;
    }

    get t() {
      return this.origin.y;
    }

    get b() {
      return this.origin.y + this.size.height;
    }
  }

  const Vector = class {
    constructor(n) {
      this.data = new Float32Array(n);
    }

    // Normalize the vector.
    normalize() {
      const vec = this.data;
      let sum = 0;
      for (const item of vec) {
        sum += item;
      }
      const gain = 1 / sum;
      for (const i in vec) {
        vec[i] *= gain;
      }
      return this;
    }

    // Multiply this Vector by another, or by a number.
    mul(other) {
      const w = new Vector(0);
      if ((typeof other != "number") && (this.data.length != other.data.length)) {
        return w;
      }

      w.data = new Float32Array(this.data);

      for (let i = 0; i < w.data.length; i++) {
        if (typeof other == "number") {
          w.data[i] *= other;
        } else {
          w.data[i] *= other.data[i];
        }
      }

      return w;
    }

    realIDFT() {
      const size = this.data.length;
      const w = new Vector(size);
      for (let i = 0; i < size; i++) {
        const omega = 2 * Math.PI * i / size;

        for (let j = 0; j < size; j++) {
          w.data[i] += this.data[j] * Math.cos(j * omega);
        }
      }

      for (let i = 0; i < size; i++) {
        w.data[i] /= size;
      }

      return w;
    }

    resize(n) {
      const newData = new Float32Array(n);
      for (let i = 0; i < Math.min(newData.length, this.data.length); i++) {
        newData[i] = this.data[i];
      }
      this.data = newData;
      return this;
    }

    // Chebyshev Window
    //
    // Based on ideas at:
    // http://www.dsprelated.com/showarticle/42.php
    //
    static chebyshevWindow(n, sidelobeDb) {
      const m = n - 1;
      let w = new Vector(m);

      const alpha = Math.cosh(Math.acosh(Math.pow(10, sidelobeDb / 20)) / m);
      for (let i = 0; i < m; i++) {
        const a = Math.abs(alpha * Math.cos(Math.PI * i / m));
        if (a > 1)
          w.data[i] = Math.pow(-1, i) * Math.cosh(m * Math.acosh(a));
        else
          w.data[i] = Math.pow(-1, i) * Math.cos(m * Math.acos(a));
      }

      w = w.realIDFT();

      w.resize(n);
      w.data[0] /= 2;
      w.data[n - 1] = w.data[0];

      const max = w.data.reduce((prev, cur) => Math.max(prev, Math.abs(cur)));
      for (const i in w.data) {
        w.data[i] /= max;
      }

      return w;
    }

    // Lanczos Window
    static lanczosWindow(n, fc) {
      let v = new Vector(n);
      fc = Math.min(fc, 0.5);
      const halfN = Math.floor(n / 2);

      for (let i = 0; i < n; i++) {
        const x = 2 * Math.PI * fc * (i - halfN);

        v.data[i] = (x == 0.0) ? 1.0 : Math.sin(x) / x;
      }

      return v;
    }

  };

  const Matrix3 = class {
    constructor(c00, c01, c02,
      c10, c11, c12,
      c20, c21, c22) {
      this.data = new Float32Array([c00, c01, c02, c10, c11, c12, c20, c21, c22]);
    }

    at(i, j) {
      return this.data[3 * i + j];
    }

    mul(val) {
      const m = new Matrix3(0, 0, 0, 0, 0, 0, 0, 0, 0);
      if (typeof val == "number") {
        m.data = this.data.map(x => x * val);
      } else {
        for (let i = 0; i < 3; i++) {
          for (let j = 0; j < 3; j++) {
            for (let k = 0; k < 3; k++) {
              m.data[i * 3 + j] += val.data[i * 3 + k] * this.data[k * 3 + j];
            }
          }
        }
      }
      return m;
    }
  }


  // From AppleIIVideo.cpp

  const HORIZ_START = 16;
  const HORIZ_BLANK = (9 + HORIZ_START) // 25;
  const HORIZ_DISPLAY = 40;
  const HORIZ_TOTAL = (HORIZ_BLANK + HORIZ_DISPLAY) // 65;
  const CELL_WIDTH = 14;
  const VERT_NTSC_START = 38;
  const VERT_DISPLAY = 192;

  // From CanvasInterface.h

  const NTSC_FSC = 315 / 88 * 1e6;         // 3579545 = 3.5 Mhz: Color Subcarrier
  const NTSC_4FSC = 4 * NTSC_FSC;         // 14318180 = 14.3 Mhz
  const NTSC_HLENGTH = (52 + 8 / 9) * 1e-6;
  const NTSC_HHALF = (35 + 2 / 3) * 1e-6;
  const NTSC_HSTART = NTSC_HHALF - NTSC_HLENGTH / 2;
  const NTSC_VLENGTH = 240;
  const NTSC_VSTART = 19;

  // From AppleIIVideo::updateTiming
  const ntscClockFrequency = NTSC_4FSC * HORIZ_TOTAL / 912;
  const ntscVisibleRect = new Rect(ntscClockFrequency * NTSC_HSTART, NTSC_VSTART,
    ntscClockFrequency * NTSC_HLENGTH, NTSC_VLENGTH);
  const ntscDisplayRect = new Rect(HORIZ_START, VERT_NTSC_START,
    HORIZ_DISPLAY, VERT_DISPLAY);

  const VERTEX_RENDER_SHADER = `
// an attribute will receive data from a buffer
attribute vec4 a_position;
attribute vec2 a_texCoord;
varying vec2 v_texCoord;

// all shaders have a main function
void main() {
  gl_Position = a_position;
  v_texCoord = a_texCoord;
}
`;

  const VERTEX_DISPLAY_SHADER = `
// an attribute will receive data from a buffer
attribute vec4 a_position;
attribute vec2 a_texCoord;
attribute vec2 a_texCoord2;
varying vec2 v_texCoord;
varying vec2 v_texCoord2;

// all shaders have a main function
void main() {
  gl_Position = vec4(a_position.x, -a_position.y, a_position.z, a_position.w);
  v_texCoord = a_texCoord;
  v_texCoord2 = a_texCoord2;
}
`;

  const COMPOSITE_SHADER = `
precision mediump float;

varying vec2 v_texCoord;

uniform sampler2D texture;
uniform vec2 textureSize;
uniform float subcarrier;
uniform sampler2D phaseInfo;
uniform vec3 c0, c1, c2, c3, c4, c5, c6, c7, c8;
uniform mat3 decoderMatrix;

float PI = 3.14159265358979323846264;

vec3 pixel(in vec2 q)
{
  vec3 c = texture2D(texture, q).rgb;
  vec2 p = texture2D(phaseInfo, vec2(0, q.y)).rg;
  float phase = 2.0 * PI * (subcarrier * textureSize.x * q.x + p.x);
  return c * vec3(1.0, sin(phase), (1.0 - 2.0 * p.y) * cos(phase));
}

vec3 pixels(vec2 q, float i)
{
  return pixel(vec2(q.x + i, q.y)) + pixel(vec2(q.x - i, q.y));
}

void main(void)
{
  vec2 q = v_texCoord;
  vec3 c = pixel(q) * c0;
  c += pixels(q, 1.0 / textureSize.x) * c1;
  c += pixels(q, 2.0 / textureSize.x) * c2;
  c += pixels(q, 3.0 / textureSize.x) * c3;
  c += pixels(q, 4.0 / textureSize.x) * c4;
  c += pixels(q, 5.0 / textureSize.x) * c5;
  c += pixels(q, 6.0 / textureSize.x) * c6;
  c += pixels(q, 7.0 / textureSize.x) * c7;
  c += pixels(q, 8.0 / textureSize.x) * c8;
  gl_FragColor = vec4(decoderMatrix * c, 1.0);
}
`;

  const DISPLAY_SHADER = `
precision mediump float;

varying vec2 v_texCoord;
varying vec2 v_texCoord2;

uniform sampler2D texture;
uniform vec2 textureSize;

float PI = 3.14159265358979323846264;

void main(void)
{
  vec2 q = v_texCoord;
  vec3 c = texture2D(texture, q).rgb;
  gl_FragColor = vec4(c, 1.0);
}
`;

  function buildTiming(displayRect, visibleRect) {
    const vertStart = displayRect.y;
    // first displayed column.
    const horizStart = Math.floor(displayRect.x);
    // imageSize is [14 * visible rect width in cells, visible lines]
    const imageSize = new Size(Math.floor(CELL_WIDTH * visibleRect.width),
      Math.floor(visibleRect.height));
    // imageLeft is # of pixels from first visible point to first displayed point.
    const imageLeft = Math.floor((horizStart - visibleRect.x) * CELL_WIDTH);
    const colorBurst = [2 * Math.PI * (-33 / 360 + (imageLeft % 4) / 4)];

    // First pixel that OpenEmulator draws when painting normally.
    const topLeft = new Point(imageLeft, vertStart - visibleRect.y);
    // First pixel that OpenEmulator draws when painting 80-column mode.
    const topLeft80Col = new Point(imageLeft - CELL_WIDTH / 2, vertStart - visibleRect.y);

    return {
      imageSize: imageSize,
      colorBurst: colorBurst,
      topLeft: topLeft,
      topLeft80Col: topLeft80Col,
    };
  }

  const NTSC_DETAILS = buildTiming(ntscDisplayRect, ntscVisibleRect);
  // https://codereview.stackexchange.com/a/128619
  const loadImage = path =>
    new Promise((resolve, reject) => {
      const img = new Image();
      img.onload = () => resolve(img);
      img.onerror = () => reject(`error loading '${path}'`);
      img.src = path;
    });

  // Given an image that's 560x192, render it into the larger space
  // required for NTSC.
  // image: a 560x192 image, from the same domain (hence readable).
  // returns: a canvas
  const screenData = (image, dhgr = true) => {
    if ((image.naturalWidth != 560) || (image.naturalHeight != 192)) {
      throw new Error('screenData expects an image 560x192;' +
        ` got ${image.naturalWidth}x${image.naturalHeight}`);
    }
    const canvas = document.createElement('canvas');
    const context = canvas.getContext('2d');
    const width = NTSC_DETAILS.imageSize.width;
    const height = NTSC_DETAILS.imageSize.height;
    canvas.width = width;
    canvas.height = height;
    context.fillStyle = 'rgba(0,0,0,1)';
    context.fillRect(0, 0, width, height);
    const topLeft = dhgr ? NTSC_DETAILS.topLeft80Col : NTSC_DETAILS.topLeft;
    context.drawImage(image, topLeft.x, topLeft.y);
    const imageData = context.getImageData(0, 0, width, height);
    return [canvas, imageData];
  };

  // Given an ImageData (RGBA), convert to luminance by taking the max
  // of (R,G,B) for each pixel. Return a Uint8Array.
  const luminanceData = (imageData) => {
    const width = imageData.width;
    const height = imageData.height;
    const data = imageData.data;
    const size = width * height;
    const ary = new Uint8Array(size);
    for (let i = 0; i < size; i++) {
      ary[i] = Math.max(data[i * 4], data[i * 4 + 1], data[i * 4 + 2]);
    }
    return ary;
  };

  const TEXTURE_NAMES = [
    "IMAGE_PHASEINFO",
    "IMAGE_IN",
    "IMAGE_DECODED",
  ];

  const BUFFER_COUNT = 3;

  const resizeCanvas = (canvas) => {
    // Lookup the size the browser is displaying the canvas.
    const displayWidth = canvas.clientWidth;
    const displayHeight = canvas.clientHeight;

    // Check if the canvas is not the same size.
    if (canvas.width != displayWidth ||
      canvas.height != displayHeight) {
      canvas.width = displayWidth;
      canvas.height = displayHeight;
    }
  };

  // Code from:
  // https://webglfundamentals.org/webgl/lessons/webgl-fundamentals.html
  const createShader = (gl, name, type, source) => {
    const shader = gl.createShader(type);
    gl.shaderSource(shader, source);
    gl.compileShader(shader);
    const success = gl.getShaderParameter(shader, gl.COMPILE_STATUS);
    if (success) {
      return shader;
    }

    const log = gl.getShaderInfoLog(shader);
    gl.deleteShader(shader);
    throw new Error(`unable to compile shader ${name}: \n${log}`);
  };

  // Code from:
  // https://webglfundamentals.org/webgl/lessons/webgl-fundamentals.html
  const createProgram = (gl, name, ...shaders) => {
    const program = gl.createProgram();
    for (let shader of shaders) {
      gl.attachShader(program, shader);
    }
    gl.linkProgram(program);
    const success = gl.getProgramParameter(program, gl.LINK_STATUS);
    if (success) {
      return program;
    }
    const log = gl.getProgramInfoLog(program);
    gl.deleteProgram(program);
    throw new Error(`unable to compile program ${name}: \n${log}`);
  };

  const TextureInfo = class {
    constructor(width, height, glTexture) {
      this.width = width;
      this.height = height;
      this.glTexture = glTexture;
    }

    get size() {
      return new Size(this.width, this.height);
    }
  };

  const DisplayConfiguration = class {
    constructor() {
      this.videoContrast = 1;
      this.videoSaturation = 1;
      this.videoHue = 0;
      this.videoCenter = new Point(0, 0);
      this.videoSize = new Size(1.05, 1.05);
      this.videoLumaBandwidth = 2000000; // 600000;
      this.videoChromaBandwidth = 600000; // 2000000;
      this.displayResolution = new Size(640, 480);
    }
  };

  // Corresponds to OEImage. Contains the data on an NTSC/PAL/whatever
  // image. The `data` field is an ImageData object with the actual
  // image data.
  const ImageInfo = class {
    constructor(data) {
      if (typeof data != "object") {
        throw new Error(`want typeof data == 'object'; got '${typeof data}'`);
      }
      if (!(data instanceof ImageData)) {
        throw new Error(`want data instanceof ImageData; got '${data.constructor.name}'`);
      }

      this.sampleRate = NTSC_4FSC;
      this.interlace = 0;
      this.subCarrier = NTSC_FSC;
      this.colorBurst = NTSC_DETAILS.colorBurst;
      this.phaseAlternation = [false];
      this.data = data;
    }

    get width() {
      return this.data.width;
    }

    get height() {
      return this.data.height;
    }

    get size() {
      return new Size(this.data.width, this.data.height);
    }
  };

  const ScreenView = class {
    constructor(canvas) {
      const gl = canvas.getContext("webgl");
      const float_texture_ext = gl.getExtension('OES_texture_float');
      if (float_texture_ext == null) {
        throw new Error("WebGL extension 'OES_texture_float' unavailable");
      }

      this.canvas = canvas;
      this.gl = gl;
      this.textures = {};
      this.shaders = {};
      this.buffers = [];
      this.image = null;
      this.display = null;
      this.imageSampleRate = null;
      this.imageSubcarrier = null;
      this.viewportSize = new Size(0, 0);

      this.configurationChanged = true;
      this.imageChanged = true;
    }

    get image() {
      return this._image
    }

    set image(image) {
      this._image = image;
      this.imageChanged = true;
    }

    set displayConfiguration(displayConfiguration) {
      this.display = displayConfiguration;
      this.configurationChanged = true;
    }

    async initOpenGL() {
      const gl = this.gl;

      gl.enable(gl.BLEND);
      gl.blendFunc(gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA);

      this.textures = {};

      for (let name of TEXTURE_NAMES) {
        this.textures[name] = new TextureInfo(0, 0, gl.createTexture());
      }

      for (let i = 0; i < BUFFER_COUNT; i++) {
        this.buffers.push(gl.createBuffer());
      }

      gl.pixelStorei(gl.PACK_ALIGNMENT, 1);
      gl.pixelStorei(gl.UNPACK_ALIGNMENT, 1);

      this.loadShaders();
    }

    freeOpenGL() {
      const gl = this.gl;

      for (let name of TEXTURE_NAMES) {
        gl.deleteTexture(this.textures[name].glTexture);
      }

      for (let buffer of this.buffers) {
        gl.deleteBuffer(buffer);
      }

      this.deleteShaders();
    }

    loadShaders() {
      this.loadShader("COMPOSITE", COMPOSITE_SHADER, VERTEX_RENDER_SHADER);
      this.loadShader("DISPLAY", DISPLAY_SHADER, VERTEX_DISPLAY_SHADER);
    }

    loadShader(name, fragmentSource, vertexSource) {
      const glVertexShader = createShader(this.gl, name, this.gl.VERTEX_SHADER, vertexSource);
      const glFragmentShader = createShader(this.gl, name, this.gl.FRAGMENT_SHADER,
        fragmentSource);
      const glProgram = createProgram(this.gl, name, glVertexShader, glFragmentShader);
      this.gl.deleteShader(glVertexShader);
      this.gl.deleteShader(glFragmentShader);
      this.shaders[name] = glProgram;
    }

    deleteShaders() {
      for (let name of ["COMPOSITE", "DISPLAY"]) {
        if (this.shaders[name]) {
          this.gl.deleteProgram(this.shaders[name]);
          this.shaders[name] = false;
        }
      }
    }

    vsync() {
      const gl = this.gl;
      resizeCanvas(this.canvas);
      const canvasWidth = this.canvas.width;
      const canvasHeight = this.canvas.height;

      // if viewport size has changed:
      if ((this.viewportSize.width != canvasWidth)
        || (this.viewportSize.height != this.canvasHeight)) {
        this.viewportSize = new Size(canvasWidth, canvasHeight);
        gl.viewport(0, 0, canvasWidth, canvasHeight);
        this.configurationChanged = true;
      }

      if (this.imageChanged) {
        this.uploadImage();
      }

      if (this.configurationChanged) {
        this.configureShaders();
      }

      if (this.imageChanged || this.configurationChanged) {
        this.renderImage();
      }

      if (this.imageChanged || this.configurationChanged) {
        this.drawDisplayCanvas();
      }
    }

    uploadImage() {
      const gl = this.gl;
      const image = this.image;

      this.resizeTexture("IMAGE_IN", image.width, image.height, true);
      const texInfoImage = this.textures["IMAGE_IN"];
      gl.bindTexture(gl.TEXTURE_2D, texInfoImage.glTexture);
      const format = gl.LUMINANCE;
      const type = gl.UNSIGNED_BYTE;
      const luminance = luminanceData(image.data);
      gl.texSubImage2D(gl.TEXTURE_2D, 0,
        0, 0, // xoffset, yoffset
        image.data.width,
        image.data.height,
        format, type, luminance);

      // Update configuration
      if ((image.sampleRate != this.imageSampleRate) ||
        (image.subCarrier != this.imageSubcarrier)) {
        this.imageSampleRate = image.sampleRate;
        this.imageSubcarrier = image.subCarrier;

        this.configurationChanged = true;
      }

      // Upload phase info
      const texHeight = 2 ** Math.ceil(Math.log2(image.height));
      const colorBurst = image.colorBurst
      const phaseAlternation = image.phaseAlternation;

      const phaseInfo = new Float32Array(3 * texHeight);

      for (let x = 0; x < image.height; x++) {
        const c = colorBurst[x % colorBurst.length] / 2 / Math.PI;
        phaseInfo[3 * x + 0] = c - Math.floor(c);
        phaseInfo[3 * x + 1] = phaseAlternation[x % phaseAlternation.length];
      }

      const texInfoPhase = this.textures["IMAGE_PHASEINFO"];
      gl.bindTexture(gl.TEXTURE_2D, texInfoPhase.glTexture);
      gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGB, 1, texHeight, 0,
        gl.RGB, gl.FLOAT, phaseInfo);
    }

    configureShaders() {
      const gl = this.gl;

      const renderShader = this.shaders["COMPOSITE"];
      const displayShader = this.shaders["DISPLAY"];

      // Render shader
      gl.useProgram(renderShader);

      // Subcarrier
      gl.uniform1f(gl.getUniformLocation(renderShader, "subcarrier"),
        this.imageSubcarrier / this.imageSampleRate);

      // Filters
      const w = Vector.chebyshevWindow(17, 50).normalize();

      let wy, wu, wv;

      let yBandwidth = this.display.videoLumaBandwidth / this.imageSampleRate;
      let uBandwidth = this.display.videoChromaBandwidth / this.imageSampleRate;
      let vBandwidth = uBandwidth;

      wy = w.mul(Vector.lanczosWindow(17, yBandwidth));
      wy = wy.normalize();

      wu = w.mul(Vector.lanczosWindow(17, uBandwidth));
      wu = wu.normalize().mul(2);

      wv = w.mul(Vector.lanczosWindow(17, vBandwidth));
      wv = wv.normalize().mul(2);

      gl.uniform3f(gl.getUniformLocation(renderShader, "c0"),
        wy.data[8], wu.data[8], wv.data[8]);
      gl.uniform3f(gl.getUniformLocation(renderShader, "c1"),
        wy.data[7], wu.data[7], wv.data[7]);
      gl.uniform3f(gl.getUniformLocation(renderShader, "c2"),
        wy.data[6], wu.data[6], wv.data[6]);
      gl.uniform3f(gl.getUniformLocation(renderShader, "c3"),
        wy.data[5], wu.data[5], wv.data[5]);
      gl.uniform3f(gl.getUniformLocation(renderShader, "c4"),
        wy.data[4], wu.data[4], wv.data[4]);
      gl.uniform3f(gl.getUniformLocation(renderShader, "c5"),
        wy.data[3], wu.data[3], wv.data[3]);
      gl.uniform3f(gl.getUniformLocation(renderShader, "c6"),
        wy.data[2], wu.data[2], wv.data[2]);
      gl.uniform3f(gl.getUniformLocation(renderShader, "c7"),
        wy.data[1], wu.data[1], wv.data[1]);
      gl.uniform3f(gl.getUniformLocation(renderShader, "c8"),
        wy.data[0], wu.data[0], wv.data[0]);

      // Decoder matrix
      let decoderMatrix = new Matrix3(1, 0, 0,
        0, 1, 0,
        0, 0, 1);

      // Saturation
      decoderMatrix = new Matrix3(1, 0, 0,
        0, this.display.videoSaturation, 0,
        0, 0, this.display.videoSaturation).mul(decoderMatrix);

      // Hue
      let hue = 2 * Math.PI * this.display.videoHue;

      decoderMatrix = new Matrix3(1, 0, 0,
        0, Math.cos(hue), -Math.sin(hue),
        0, Math.sin(hue), Math.cos(hue)).mul(decoderMatrix);

      // Decode
      // Y'UV decoder matrix
      decoderMatrix = new Matrix3(1, 1, 1,
        0, -0.394642, 2.032062,
        1.139883, -0.580622, 0).mul(decoderMatrix);

      // Contrast
      let contrast = this.display.videoContrast;

      decoderMatrix = decoderMatrix.mul(Math.max(contrast, 0));

      gl.uniformMatrix3fv(gl.getUniformLocation(renderShader, "decoderMatrix"),
        false, decoderMatrix.data);

      // Display shader
      gl.useProgram(displayShader);
    }

    renderImage() {
      const gl = this.gl;
      const renderShader = this.shaders["COMPOSITE"];

      gl.useProgram(renderShader);

      const texSize = this.textures["IMAGE_IN"].size;
      this.resizeTexture("IMAGE_DECODED", texSize.width, texSize.height);

      gl.uniform1i(gl.getUniformLocation(renderShader, "texture"), 0);
      gl.uniform2f(gl.getUniformLocation(renderShader, "textureSize"),
        texSize.width, texSize.height);

      gl.uniform1i(gl.getUniformLocation(renderShader, "phaseInfo"), 1);

      gl.activeTexture(gl.TEXTURE1);

      gl.bindTexture(gl.TEXTURE_2D, this.textures["IMAGE_PHASEINFO"].glTexture);

      gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.NEAREST);
      gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.NEAREST);

      gl.activeTexture(gl.TEXTURE0);

      // Render to the back buffer, to avoid using FBOs
      // (support for vanilla OpenGL 2.0 cards)

      // I think webgl is rendering to the back buffer anyway, until
      // we stop executing javascript statements and give control
      // back, at which point it flips. So we might not need
      // this. Although truly, I'm not certain what it's doing. If we
      // *do* end up needing it, we'll have to go full webgl2.

      // glReadBuffer(GL_BACK);

      const imageSize = this.image.size;

      for (let y = 0; y < this.image.height; y += this.viewportSize.height) {
        for (let x = 0; x < this.image.width; x += this.viewportSize.width) {
          // Calculate rects
          const clipSize = this.viewportSize.copy();

          if ((x + clipSize.width) > imageSize.width)
            clipSize.width = imageSize.width - x;
          if ((y + clipSize.height) > imageSize.height)
            clipSize.height = imageSize.height - y;
          const textureRect = new Rect(x / texSize.width,
            y / texSize.height,
            clipSize.width / texSize.width,
            clipSize.height / texSize.height);
          const canvasRect = new Rect(-1,
            -1,
            2 * clipSize.width / this.viewportSize.width,
            2 * clipSize.height / this.viewportSize.height);

          // Render
          gl.bindTexture(gl.TEXTURE_2D, this.textures["IMAGE_IN"].glTexture);

          gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.NEAREST);
          gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.NEAREST);

          // What is this, some ancient fixed pipeline nonsense? No way.
          // glLoadIdentity();

          this.drawRectangle(renderShader, canvasRect, textureRect);

          // Copy framebuffer
          gl.bindTexture(gl.TEXTURE_2D, this.textures["IMAGE_DECODED"].glTexture);

          gl.copyTexSubImage2D(gl.TEXTURE_2D, 0,
            x, y, 0, 0,
            clipSize.width, clipSize.height);
        }
      }
    }

    drawRectangle(shader, posRect, texRect, texRect2) {
      const gl = this.gl;

      gl.clearColor(0, 0, 0, 0);
      gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);

      const positionLocation = gl.getAttribLocation(shader, "a_position");
      const texcoordLocations = [gl.getAttribLocation(shader, "a_texCoord")];
      const texRects = [texRect];
      if (texRect2) {
        texcoordLocations.push(gl.getAttribLocation(shader, "a_texCoord2"));
        texRects.push(texRect2);
      }

      const positionBuffer = this.buffers[0];
      gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);
      const p_x1 = posRect.l;
      const p_x2 = posRect.r;
      const p_y1 = posRect.t;
      const p_y2 = posRect.b;
      gl.bufferData(gl.ARRAY_BUFFER, new Float32Array([
        p_x1, p_y1,
        p_x2, p_y1,
        p_x1, p_y2,
        p_x1, p_y2,
        p_x2, p_y1,
        p_x2, p_y2,
      ]), gl.STATIC_DRAW);

      gl.enableVertexAttribArray(positionLocation);
      gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);
      gl.vertexAttribPointer(positionLocation, 2, gl.FLOAT, false, 0, 0);

      for (let i = 0; i < texRects.length; i++) {
        const texcoordBuffer = this.buffers[i + 1];
        gl.bindBuffer(gl.ARRAY_BUFFER, texcoordBuffer);
        const t_x1 = texRects[i].l;
        const t_x2 = texRects[i].r;
        const t_y1 = texRects[i].t;
        const t_y2 = texRects[i].b;
        gl.bufferData(gl.ARRAY_BUFFER, new Float32Array([
          t_x1, t_y1,
          t_x2, t_y1,
          t_x1, t_y2,
          t_x1, t_y2,
          t_x2, t_y1,
          t_x2, t_y2,
        ]), gl.STATIC_DRAW);

        gl.enableVertexAttribArray(texcoordLocations[i]);
        gl.bindBuffer(gl.ARRAY_BUFFER, texcoordBuffer);
        gl.vertexAttribPointer(texcoordLocations[i], 2, gl.FLOAT, false, 0, 0);
      }

      gl.drawArrays(gl.TRIANGLES, 0, 6);
    }

    drawDisplayCanvas() {
      const gl = this.gl;

      const displayShader = this.shaders["DISPLAY"];

      // Clear
      // (Moved inside drawRectangle)
      // gl.clearColor(0, 0, 0, 1);
      // gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);

      if (this.image.width == 0 || this.image.height == 0) {
        return;
      }

      // Grab common variables
      const displayResolution = this.display.displayResolution;

      // Vertex rect
      const vertexRect = new Rect(-1, -1, 2, 2);

      const viewportAspectRatio = this.viewportSize.ratio;
      const displayAspectRatio = displayResolution.ratio;

      const ratio = viewportAspectRatio / displayAspectRatio;

      if (ratio > 1) {
        vertexRect.origin.x /= ratio;
        vertexRect.size.width /= ratio;
      } else {
        vertexRect.origin.y *= ratio;
        vertexRect.size.height *= ratio;
      }

      // Base texture rect
      const baseTexRect = new Rect(0, 0, 1, 1);

      // Canvas texture rect
      const interlaceShift = this.image.interlace / this.image.height;

      const canvasTexLowerLeft = this.getDisplayCanvasTexPoint(
        new Point(-1, -1 + 2 * interlaceShift));
      const canvasTexUpperRight = this.getDisplayCanvasTexPoint(
        new Point(1, 1 + 2 * interlaceShift));

      const canvasTexRect = new Rect(canvasTexLowerLeft.x,
        canvasTexLowerLeft.y,
        canvasTexUpperRight.x - canvasTexLowerLeft.x,
        canvasTexUpperRight.y - canvasTexLowerLeft.y);

      // Render
      const texture = this.textures["IMAGE_DECODED"];

      // Set uniforms
      gl.useProgram(displayShader);

      // Texture
      const texSize = texture.size;

      gl.uniform1i(gl.getUniformLocation(displayShader, "texture"), 0);
      gl.uniform2f(gl.getUniformLocation(displayShader, "textureSize"),
        texSize.width, texSize.height);

      gl.activeTexture(gl.TEXTURE1);

      gl.bindTexture(gl.TEXTURE_2D, texture.glTexture);

      gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
      gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR);

      gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE);
      gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE);


      // Render
      this.drawRectangle(displayShader, vertexRect, canvasTexRect, baseTexRect);
    }

    getDisplayCanvasTexPoint(p) {
      const videoCenter = this.display.videoCenter;
      const videoSize = this.display.videoSize;

      p = new Point((p.x - 2 * videoCenter.x) / videoSize.width,
        (p.y - 2 * videoCenter.y) / videoSize.height);

      const imageSize = this.image.size;
      const texSize = this.textures["IMAGE_IN"].size;

      p.x = (p.x + 1) * 0.5 * imageSize.width / texSize.width;
      p.y = (p.y + 1) * 0.5 * imageSize.height / texSize.height;

      return p;
    }

    // Resize the texture with the given name to the next
    // highest power of two width and height. Wouldn't be
    // necessary with webgl2.
    resizeTexture(name, width, height, luminance = false) {
      const gl = this.gl;
      const texInfo = this.textures[name];
      if (!texInfo) {
        throw new Error(`Cannot find texture named ${name}`);
      }
      if (width < 4) width = 4;
      if (height < 4) height = 4;
      width = 2 ** Math.ceil(Math.log2(width));
      height = 2 ** Math.ceil(Math.log2(height));
      if (texInfo.width != width || texInfo.height != height) {
        texInfo.width = width;
        texInfo.height = height;
        gl.bindTexture(gl.TEXTURE_2D, texInfo.glTexture);
        const length = width * height * (luminance ? 1 : 4);
        const dummy = new Uint8Array(length);
        const type = luminance ? gl.LUMINANCE : gl.RGBA;
        gl.texImage2D(gl.TEXTURE_2D, 0, type, width, height, 0,
          type, gl.UNSIGNED_BYTE, dummy);
      }
    }
  }

  return {
    loadImage: loadImage,
    screenData: screenData,

    // Classes.
    ScreenView: ScreenView,
    DisplayConfiguration: DisplayConfiguration,
    ImageInfo: ImageInfo,
    Size: Size,
    Point: Point,
  };
})();