1 files changed, 203 insertions, 0 deletions
diff --git a/remote/shared/Capture.sys.mjs b/remote/shared/Capture.sys.mjs
new file mode 100644
index 0000000000..ec34d09aba
--- /dev/null
+++ b/remote/shared/Capture.sys.mjs
@@ -0,0 +1,203 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
+ChromeUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
+
+const CONTEXT_2D = "2d";
+const BG_COLOUR = "rgb(255,255,255)";
+const MAX_CANVAS_DIMENSION = 32767;
+const MAX_CANVAS_AREA = 472907776;
+const PNG_MIME = "image/png";
+const XHTML_NS = "http://www.w3.org/1999/xhtml";
+
+/**
+ * Provides primitives to capture screenshots.
+ *
+ * @namespace
+ */
+export const capture = {};
+
+capture.Format = {
+  Base64: 0,
+  Hash: 1,
+};
+
+/**
+ * Draw a rectangle off the framebuffer.
+ *
+ * @param {DOMWindow} win
+ *     The DOM window used for the framebuffer, and providing the interfaces
+ *     for creating an HTMLCanvasElement.
+ * @param {BrowsingContext} browsingContext
+ *     The BrowsingContext from which the snapshot should be taken.
+ * @param {number} left
+ *     The left, X axis offset of the rectangle.
+ * @param {number} top
+ *     The top, Y axis offset of the rectangle.
+ * @param {number} width
+ *     The width dimension of the rectangle to paint.
+ * @param {number} height
+ *     The height dimension of the rectangle to paint.
+ * @param {object=} options
+ * @param {HTMLCanvasElement=} options.canvas
+ *     Optional canvas to reuse for the screenshot.
+ * @param {number=} options.flags
+ *     Optional integer representing flags to pass to drawWindow; these
+ *     are defined on CanvasRenderingContext2D.
+ * @param {number=} options.dX
+ *     Horizontal offset between the browser window and content area. Defaults to 0.
+ * @param {number=} options.dY
+ *     Vertical offset between the browser window and content area. Defaults to 0.
+ * @param {boolean=} options.readback
+ *     If true, read back a snapshot of the pixel data currently in the
+ *     compositor/window. Defaults to false.
+ *
+ * @returns {HTMLCanvasElement}
+ *     The canvas on which the selection from the window's framebuffer
+ *     has been painted on.
+ */
+capture.canvas = async function (
+  win,
+  browsingContext,
+  left,
+  top,
+  width,
+  height,
+  { canvas = null, flags = null, dX = 0, dY = 0, readback = false } = {}
+) {
+  // FIXME(bug 1761032): This looks a bit sketchy, overrideDPPX doesn't
+  // influence rendering...
+  const scale = win.browsingContext.overrideDPPX || win.devicePixelRatio;
+
+  let canvasHeight = height * scale;
+  let canvasWidth = width * scale;
+
+  // Cap the screenshot size for width and height at 2^16 pixels,
+  // which is the maximum allowed canvas size. Higher dimensions will
+  // trigger exceptions in Gecko.
+  if (canvasWidth > MAX_CANVAS_DIMENSION) {
+    lazy.logger.warn(
+      "Limiting screen capture width to maximum allowed " +
+        MAX_CANVAS_DIMENSION +
+        " pixels"
+    );
+    width = Math.floor(MAX_CANVAS_DIMENSION / scale);
+    canvasWidth = width * scale;
+  }
+
+  if (canvasHeight > MAX_CANVAS_DIMENSION) {
+    lazy.logger.warn(
+      "Limiting screen capture height to maximum allowed " +
+        MAX_CANVAS_DIMENSION +
+        " pixels"
+    );
+    height = Math.floor(MAX_CANVAS_DIMENSION / scale);
+    canvasHeight = height * scale;
+  }
+
+  // If the area is larger, reduce the height to keep the full width.
+  if (canvasWidth * canvasHeight > MAX_CANVAS_AREA) {
+    lazy.logger.warn(
+      "Limiting screen capture area to maximum allowed " +
+        MAX_CANVAS_AREA +
+        " pixels"
+    );
+    height = Math.floor(MAX_CANVAS_AREA / (canvasWidth * scale));
+    canvasHeight = height * scale;
+  }
+
+  if (canvas === null) {
+    canvas = win.document.createElementNS(XHTML_NS, "canvas");
+    canvas.width = canvasWidth;
+    canvas.height = canvasHeight;
+  }
+
+  const ctx = canvas.getContext(CONTEXT_2D);
+
+  if (readback) {
+    if (flags === null) {
+      flags =
+        ctx.DRAWWINDOW_DRAW_CARET |
+        ctx.DRAWWINDOW_DRAW_VIEW |
+        ctx.DRAWWINDOW_USE_WIDGET_LAYERS;
+    }
+
+    // drawWindow doesn't take scaling into account.
+    ctx.scale(scale, scale);
+    ctx.drawWindow(win, left + dX, top + dY, width, height, BG_COLOUR, flags);
+  } else {
+    let rect = new DOMRect(left, top, width, height);
+    let snapshot = await browsingContext.currentWindowGlobal.drawSnapshot(
+      rect,
+      scale,
+      BG_COLOUR
+    );
+
+    ctx.drawImage(snapshot, 0, 0);
+
+    // Bug 1574935 - Huge dimensions can trigger an OOM because multiple copies
+    // of the bitmap will exist in memory. Force the removal of the snapshot
+    // because it is no longer needed.
+    snapshot.close();
+  }
+
+  return canvas;
+};
+
+/**
+ * Encode the contents of an HTMLCanvasElement to a Base64 encoded string.
+ *
+ * @param {HTMLCanvasElement} canvas
+ *     The canvas to encode.
+ *
+ * @returns {string}
+ *     A Base64 encoded string.
+ */
+capture.toBase64 = function (canvas) {
+  let u = canvas.toDataURL(PNG_MIME);
+  return u.substring(u.indexOf(",") + 1);
+};
+
+/**
+ * Hash the contents of an HTMLCanvasElement to a SHA-256 hex digest.
+ *
+ * @param {HTMLCanvasElement} canvas
+ *     The canvas to encode.
+ *
+ * @returns {string}
+ *     A hex digest of the SHA-256 hash of the base64 encoded string.
+ */
+capture.toHash = function (canvas) {
+  let u = capture.toBase64(canvas);
+  let buffer = new TextEncoder().encode(u);
+  return crypto.subtle.digest("SHA-256", buffer).then(hash => hex(hash));
+};
+
+/**
+ * Convert buffer into to hex.
+ *
+ * @param {ArrayBuffer} buffer
+ *     The buffer containing the data to convert to hex.
+ *
+ * @returns {string}
+ *     A hex digest of the input buffer.
+ */
+function hex(buffer) {
+  let hexCodes = [];
+  let view = new DataView(buffer);
+  for (let i = 0; i < view.byteLength; i += 4) {
+    let value = view.getUint32(i);
+    let stringValue = value.toString(16);
+    let padding = "00000000";
+    let paddedValue = (padding + stringValue).slice(-padding.length);
+    hexCodes.push(paddedValue);
+  }
+  return hexCodes.join("");
+}