Refactor char atlas to enable generation with worker

This doesn't actually add workers but it separates the code nicely
such that the minimal amount of relevant code can be pulled in by
worker code.

Part of #955

src/shared/CharAtlasGenerator.ts

@@ -0,0 +1,133 @@
+/**
+ * Copyright (c) 2018 The xterm.js authors. All rights reserved.
+ * @license MIT
+ */
+
+import { isFirefox } from './utils/Browser';
+
+declare const Promise: any;
+
+export interface IOffscreenCanvas {
+  width: number;
+  height: number;
+  getContext(type: '2d', config?: Canvas2DContextAttributes): CanvasRenderingContext2D;
+  transferToImageBitmap(): ImageBitmap;
+}
+
+export interface ICharAtlasRequest {
+  scaledCharWidth: number;
+  scaledCharHeight: number;
+  fontSize: number;
+  fontFamily: string;
+  background: string;
+  foreground: string;
+  ansiColors: string[];
+  devicePixelRatio: number;
+}
+
+export const CHAR_ATLAS_CELL_SPACING = 1;
+
+/**
+ * Generates a char atlas.
+ * @param context The window or worker context.
+ * @param canvasFactory A function to generate a canvas with a width or height.
+ * @param request The config for the new char atlas.
+ */
+export function generateCharAtlas(context: Window, canvasFactory: (width: number, height: number) => HTMLCanvasElement | IOffscreenCanvas, request: ICharAtlasRequest): HTMLCanvasElement | Promise<ImageBitmap> {
+  const cellWidth = request.scaledCharWidth + CHAR_ATLAS_CELL_SPACING;
+  const cellHeight = request.scaledCharHeight + CHAR_ATLAS_CELL_SPACING;
+  const canvas = canvasFactory(
+    /*255 ascii chars*/255 * cellWidth,
+    (/*default+default bold*/2 + /*0-15*/16) * cellHeight
+  );
+  const ctx = canvas.getContext('2d', {alpha: false});
+
+  ctx.fillStyle = request.background;
+  ctx.fillRect(0, 0, canvas.width, canvas.height);
+
+  ctx.save();
+  ctx.fillStyle = request.foreground;
+  ctx.font = `${request.fontSize * request.devicePixelRatio}px ${request.fontFamily}`;
+  ctx.textBaseline = 'top';
+
+  // Default color
+  for (let i = 0; i < 256; i++) {
+    ctx.save();
+    ctx.beginPath();
+    ctx.rect(i * cellWidth, 0, cellWidth, cellHeight);
+    ctx.clip();
+    ctx.fillText(String.fromCharCode(i), i * cellWidth, 0);
+    ctx.restore();
+  }
+  // Default color bold
+  ctx.save();
+  ctx.font = `bold ${ctx.font}`;
+  for (let i = 0; i < 256; i++) {
+    ctx.save();
+    ctx.beginPath();
+    ctx.rect(i * cellWidth, cellHeight, cellWidth, cellHeight);
+    ctx.clip();
+    ctx.fillText(String.fromCharCode(i), i * cellWidth, cellHeight);
+    ctx.restore();
+  }
+  ctx.restore();
+
+  // Colors 0-15
+  ctx.font = `${request.fontSize * request.devicePixelRatio}px ${request.fontFamily}`;
+  for (let colorIndex = 0; colorIndex < 16; colorIndex++) {
+    // colors 8-15 are bold
+    if (colorIndex === 8) {
+      ctx.font = `bold ${ctx.font}`;
+    }
+    const y = (colorIndex + 2) * cellHeight;
+    // Draw ascii characters
+    for (let i = 0; i < 256; i++) {
+      ctx.save();
+      ctx.beginPath();
+      ctx.rect(i * cellWidth, y, cellWidth, cellHeight);
+      ctx.clip();
+      ctx.fillStyle = request.ansiColors[colorIndex];
+      ctx.fillText(String.fromCharCode(i), i * cellWidth, y);
+      ctx.restore();
+    }
+  }
+  ctx.restore();
+
+  // Support is patchy for createImageBitmap at the moment, pass a canvas back
+  // if support is lacking as drawImage works there too. Firefox is also
+  // included here as ImageBitmap appears both buggy and has horrible
+  // performance (tested on v55).
+  if (!('createImageBitmap' in context) || isFirefox) {
+    // Don't attempt to clear background colors if createImageBitmap is not supported
+    if (canvas instanceof HTMLCanvasElement) {
+      // Just return the HTMLCanvas if it's a HTMLCanvasElement
+      return canvas;
+    } else {
+      // Transfer to an ImageBitmap is this is an OffscreenCanvas
+      return new Promise(r => r(canvas.transferToImageBitmap()));
+    }
+  }
+
+  const charAtlasImageData = ctx.getImageData(0, 0, canvas.width, canvas.height);
+
+  // Remove the background color from the image so characters may overlap
+  const r = parseInt(request.background.substr(1, 2), 16);
+  const g = parseInt(request.background.substr(3, 2), 16);
+  const b = parseInt(request.background.substr(5, 2), 16);
+  clearColor(charAtlasImageData, r, g, b);
+
+  return context.createImageBitmap(charAtlasImageData);
+}
+
+/**
+ * Makes a partiicular rgb color in an ImageData completely transparent.
+ */
+function clearColor(imageData: ImageData, r: number, g: number, b: number): void {
+  for (let offset = 0; offset < imageData.data.length; offset += 4) {
+    if (imageData.data[offset] === r &&
+        imageData.data[offset + 1] === g &&
+        imageData.data[offset + 2] === b) {
+      imageData.data[offset + 3] = 0;
+    }
+  }
+}

src/shared/utils/Browser.ts

@@ -0,0 +1,29 @@
+/**
+ * Copyright (c) 2016 The xterm.js authors. All rights reserved.
+ * @license MIT
+ */
+
+const isNode = (typeof navigator === 'undefined') ? true : false;
+const userAgent = (isNode) ? 'node' : navigator.userAgent;
+const platform = (isNode) ? 'node' : navigator.platform;
+
+export const isFirefox = !!~userAgent.indexOf('Firefox');
+export const isMSIE = !!~userAgent.indexOf('MSIE') || !!~userAgent.indexOf('Trident');
+
+// Find the users platform. We use this to interpret the meta key
+// and ISO third level shifts.
+// http://stackoverflow.com/q/19877924/577598
+export const isMac = contains(['Macintosh', 'MacIntel', 'MacPPC', 'Mac68K'], platform);
+export const isIpad = platform === 'iPad';
+export const isIphone = platform === 'iPhone';
+export const isMSWindows = contains(['Windows', 'Win16', 'Win32', 'WinCE'], platform);
+export const isLinux = platform.indexOf('Linux') >= 0;
+
+/**
+ * Return if the given array contains the given element
+ * @param {Array} array The array to search for the given element.
+ * @param {Object} el The element to look for into the array
+ */
+export function contains(arr: any[], el: any): boolean {
+  return arr.indexOf(el) >= 0;
+};