pdf.js/src/display/text_layer.js

/* Copyright 2015 Mozilla Foundation
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import {
  AbortException,
  createPromiseCapability,
  Util,
} from "../shared/util.js";

/**
 * Text layer render parameters.
 *
 * @typedef {Object} TextLayerRenderParameters
 * @property {import("./api").TextContent} [textContent] - Text content to
 *   render (the object is returned by the page's `getTextContent` method).
 * @property {ReadableStream} [textContentStream] - Text content stream to
 *   render (the stream is returned by the page's `streamTextContent` method).
 * @property {DocumentFragment | HTMLElement} container - The DOM node that
 *   will contain the text runs.
 * @property {import("./display_utils").PageViewport} viewport - The target
 *   viewport to properly layout the text runs.
 * @property {Array<HTMLElement>} [textDivs] - HTML elements that correspond to
 *   the text items of the textContent input.
 *   This is output and shall initially be set to an empty array.
 * @property {Array<string>} [textContentItemsStr] - Strings that correspond to
 *   the `str` property of the text items of the textContent input.
 *   This is output and shall initially be set to an empty array.
 * @property {number} [timeout] - Delay in milliseconds before rendering of the
 *   text runs occurs.
 */

const MAX_TEXT_DIVS_TO_RENDER = 100000;
const DEFAULT_FONT_SIZE = 30;
const DEFAULT_FONT_ASCENT = 0.8;
const ascentCache = new Map();

function getAscent(fontFamily, ctx) {
  const cachedAscent = ascentCache.get(fontFamily);
  if (cachedAscent) {
    return cachedAscent;
  }

  ctx.save();
  ctx.font = `${DEFAULT_FONT_SIZE}px ${fontFamily}`;
  const metrics = ctx.measureText("");

  // Both properties aren't available by default in Firefox.
  let ascent = metrics.fontBoundingBoxAscent;
  let descent = Math.abs(metrics.fontBoundingBoxDescent);
  if (ascent) {
    ctx.restore();
    const ratio = ascent / (ascent + descent);
    ascentCache.set(fontFamily, ratio);
    return ratio;
  }

  // Try basic heuristic to guess ascent/descent.
  // Draw a g with baseline at 0,0 and then get the line
  // number where a pixel has non-null red component (starting
  // from bottom).
  ctx.strokeStyle = "red";
  ctx.clearRect(0, 0, DEFAULT_FONT_SIZE, DEFAULT_FONT_SIZE);
  ctx.strokeText("g", 0, 0);
  let pixels = ctx.getImageData(
    0,
    0,
    DEFAULT_FONT_SIZE,
    DEFAULT_FONT_SIZE
  ).data;
  descent = 0;
  for (let i = pixels.length - 1 - 3; i >= 0; i -= 4) {
    if (pixels[i] > 0) {
      descent = Math.ceil(i / 4 / DEFAULT_FONT_SIZE);
      break;
    }
  }

  // Draw an A with baseline at 0,DEFAULT_FONT_SIZE and then get the line
  // number where a pixel has non-null red component (starting
  // from top).
  ctx.clearRect(0, 0, DEFAULT_FONT_SIZE, DEFAULT_FONT_SIZE);
  ctx.strokeText("A", 0, DEFAULT_FONT_SIZE);
  pixels = ctx.getImageData(0, 0, DEFAULT_FONT_SIZE, DEFAULT_FONT_SIZE).data;
  ascent = 0;
  for (let i = 0, ii = pixels.length; i < ii; i += 4) {
    if (pixels[i] > 0) {
      ascent = DEFAULT_FONT_SIZE - Math.floor(i / 4 / DEFAULT_FONT_SIZE);
      break;
    }
  }

  ctx.restore();

  if (ascent) {
    const ratio = ascent / (ascent + descent);
    ascentCache.set(fontFamily, ratio);
    return ratio;
  }

  ascentCache.set(fontFamily, DEFAULT_FONT_ASCENT);
  return DEFAULT_FONT_ASCENT;
}

function appendText(task, geom, styles, ctx) {
  // Initialize all used properties to keep the caches monomorphic.
  const textDiv = document.createElement("span");
  const textDivProperties = {
    angle: 0,
    canvasWidth: 0,
    hasText: geom.str !== "",
    hasEOL: geom.hasEOL,
    fontSize: 0,
  };
  task._textDivs.push(textDiv);

  const tx = Util.transform(task._viewport.transform, geom.transform);
  let angle = Math.atan2(tx[1], tx[0]);
  const style = styles[geom.fontName];
  if (style.vertical) {
    angle += Math.PI / 2;
  }
  const fontHeight = Math.hypot(tx[2], tx[3]);
  const fontAscent = fontHeight * getAscent(style.fontFamily, ctx);

  let left, top;
  if (angle === 0) {
    left = tx[4];
    top = tx[5] - fontAscent;
  } else {
    left = tx[4] + fontAscent * Math.sin(angle);
    top = tx[5] - fontAscent * Math.cos(angle);
  }
  // Setting the style properties individually, rather than all at once,
  // should be OK since the `textDiv` isn't appended to the document yet.
  textDiv.style.left = `${left}px`;
  textDiv.style.top = `${top}px`;
  textDiv.style.fontSize = `${fontHeight}px`;
  textDiv.style.fontFamily = style.fontFamily;

  textDivProperties.fontSize = fontHeight;

  // Keeps screen readers from pausing on every new text span.
  textDiv.setAttribute("role", "presentation");

  textDiv.textContent = geom.str;
  // geom.dir may be 'ttb' for vertical texts.
  textDiv.dir = geom.dir;

  // `fontName` is only used by the FontInspector, and we only use `dataset`
  // here to make the font name available in the debugger.
  if (task._fontInspectorEnabled) {
    textDiv.dataset.fontName = geom.fontName;
  }
  if (angle !== 0) {
    textDivProperties.angle = angle * (180 / Math.PI);
  }
  // We don't bother scaling single-char text divs, because it has very
  // little effect on text highlighting. This makes scrolling on docs with
  // lots of such divs a lot faster.
  let shouldScaleText = false;
  if (geom.str.length > 1) {
    shouldScaleText = true;
  } else if (geom.str !== " " && geom.transform[0] !== geom.transform[3]) {
    const absScaleX = Math.abs(geom.transform[0]),
      absScaleY = Math.abs(geom.transform[3]);
    // When the horizontal/vertical scaling differs significantly, also scale
    // even single-char text to improve highlighting (fixes issue11713.pdf).
    if (
      absScaleX !== absScaleY &&
      Math.max(absScaleX, absScaleY) / Math.min(absScaleX, absScaleY) > 1.5
    ) {
      shouldScaleText = true;
    }
  }
  if (shouldScaleText) {
    if (style.vertical) {
      textDivProperties.canvasWidth = geom.height * task._viewport.scale;
    } else {
      textDivProperties.canvasWidth = geom.width * task._viewport.scale;
    }
  }
  task._textDivProperties.set(textDiv, textDivProperties);
  if (task._textContentStream) {
    task._layoutText(textDiv);
  }
}

function render(task) {
  if (task._canceled) {
    return;
  }
  const textDivs = task._textDivs;
  const capability = task._capability;
  const textDivsLength = textDivs.length;

  // No point in rendering many divs as it would make the browser
  // unusable even after the divs are rendered.
  if (textDivsLength > MAX_TEXT_DIVS_TO_RENDER) {
    task._renderingDone = true;
    capability.resolve();
    return;
  }

  if (!task._textContentStream) {
    for (const textDiv of textDivs) {
      task._layoutText(textDiv);
    }
  }

  task._renderingDone = true;
  capability.resolve();
}

class TextLayerRenderTask {
  #initialContainer = null;

  constructor({
    textContent,
    textContentStream,
    container,
    viewport,
    textDivs,
    textContentItemsStr,
  }) {
    this._textContent = textContent;
    this._textContentStream = textContentStream;
    this._container = container;
    this.#initialContainer = container;
    this._document = container.ownerDocument;
    this._viewport = viewport;
    this._textDivs = textDivs || [];
    this._textContentItemsStr = textContentItemsStr || [];
    this._fontInspectorEnabled = !!globalThis.FontInspector?.enabled;

    this._reader = null;
    this._layoutTextLastFontSize = null;
    this._layoutTextLastFontFamily = null;
    this._layoutTextCtx = null;
    this._textDivProperties = new WeakMap();
    this._renderingDone = false;
    this._canceled = false;
    this._capability = createPromiseCapability();
    this._renderTimer = null;
    this._bounds = [];
    this._devicePixelRatio = globalThis.devicePixelRatio || 1;

    // Always clean-up the temporary canvas once rendering is no longer pending.
    this._capability.promise
      .finally(() => {
        // The `textDiv` properties are no longer needed.
        this._textDivProperties = null;

        if (this._layoutTextCtx) {
          // Zeroing the width and height cause Firefox to release graphics
          // resources immediately, which can greatly reduce memory consumption.
          this._layoutTextCtx.canvas.width = 0;
          this._layoutTextCtx.canvas.height = 0;
          this._layoutTextCtx = null;
        }
      })
      .catch(() => {
        // Avoid "Uncaught promise" messages in the console.
      });
  }

  /**
   * Promise for textLayer rendering task completion.
   * @type {Promise<void>}
   */
  get promise() {
    return this._capability.promise;
  }

  /**
   * Cancel rendering of the textLayer.
   */
  cancel() {
    this._canceled = true;
    if (this._reader) {
      this._reader
        .cancel(new AbortException("TextLayer task cancelled."))
        .catch(() => {
          // Avoid "Uncaught promise" messages in the console.
        });
      this._reader = null;
    }
    if (this._renderTimer !== null) {
      clearTimeout(this._renderTimer);
      this._renderTimer = null;
    }
    this._capability.reject(new Error("TextLayer task cancelled."));
  }

  /**
   * @private
   */
  _processItems(items, styleCache) {
    for (const item of items) {
      if (item.str === undefined) {
        if (
          item.type === "beginMarkedContentProps" ||
          item.type === "beginMarkedContent"
        ) {
          const parent = this._container;
          this._container = document.createElement("span");
          this._container.classList.add("markedContent");
          if (item.id !== null) {
            this._container.setAttribute("id", `${item.id}`);
          }
          parent.append(this._container);
        } else if (item.type === "endMarkedContent") {
          const parent = this._container.parentNode;
          if (!parent || this._container === this.#initialContainer) {
            // Handle unbalanced beginMarkedContent/endMarkedContent operators
            // (fixes issue15629.pdf).
            continue;
          }
          this._container = parent;
        }
        continue;
      }
      this._textContentItemsStr.push(item.str);
      appendText(this, item, styleCache, this._layoutTextCtx);
    }
  }

  /**
   * @private
   */
  _layoutText(textDiv) {
    const textDivProperties = this._textDivProperties.get(textDiv);

    let transform = "";
    if (textDivProperties.canvasWidth !== 0 && textDivProperties.hasText) {
      const { fontFamily } = textDiv.style;
      const { fontSize } = textDivProperties;

      // Only build font string and set to context if different from last.
      if (
        fontSize !== this._layoutTextLastFontSize ||
        fontFamily !== this._layoutTextLastFontFamily
      ) {
        this._layoutTextCtx.font = `${
          fontSize * this._devicePixelRatio
        }px ${fontFamily}`;
        this._layoutTextLastFontSize = fontSize;
        this._layoutTextLastFontFamily = fontFamily;
      }
      // Only measure the width for multi-char text divs, see `appendText`.
      const { width } = this._layoutTextCtx.measureText(textDiv.textContent);

      if (width > 0) {
        transform = `scaleX(${
          (this._devicePixelRatio * textDivProperties.canvasWidth) / width
        })`;
      }
    }
    if (textDivProperties.angle !== 0) {
      transform = `rotate(${textDivProperties.angle}deg) ${transform}`;
    }
    if (transform.length > 0) {
      textDiv.style.transform = transform;
    }

    if (textDivProperties.hasText) {
      this._container.append(textDiv);
    }
    if (textDivProperties.hasEOL) {
      const br = document.createElement("br");
      br.setAttribute("role", "presentation");
      this._container.append(br);
    }
  }

  /**
   * @private
   */
  _render(timeout = 0) {
    const capability = createPromiseCapability();
    let styleCache = Object.create(null);

    // The temporary canvas is used to measure text length in the DOM.
    const canvas = this._document.createElement("canvas");
    canvas.height = canvas.width = DEFAULT_FONT_SIZE;

    this._layoutTextCtx = canvas.getContext("2d", { alpha: false });

    if (this._textContent) {
      const textItems = this._textContent.items;
      const textStyles = this._textContent.styles;
      this._processItems(textItems, textStyles);
      capability.resolve();
    } else if (this._textContentStream) {
      const pump = () => {
        this._reader.read().then(({ value, done }) => {
          if (done) {
            capability.resolve();
            return;
          }

          Object.assign(styleCache, value.styles);
          this._processItems(value.items, styleCache);
          pump();
        }, capability.reject);
      };

      this._reader = this._textContentStream.getReader();
      pump();
    } else {
      throw new Error(
        'Neither "textContent" nor "textContentStream" parameters specified.'
      );
    }

    capability.promise.then(() => {
      styleCache = null;
      if (!timeout) {
        // Render right away
        render(this);
      } else {
        // Schedule
        this._renderTimer = setTimeout(() => {
          render(this);
          this._renderTimer = null;
        }, timeout);
      }
    }, this._capability.reject);
  }
}

/**
 * @param {TextLayerRenderParameters} renderParameters
 * @returns {TextLayerRenderTask}
 */
function renderTextLayer(renderParameters) {
  const task = new TextLayerRenderTask({
    textContent: renderParameters.textContent,
    textContentStream: renderParameters.textContentStream,
    container: renderParameters.container,
    viewport: renderParameters.viewport,
    textDivs: renderParameters.textDivs,
    textContentItemsStr: renderParameters.textContentItemsStr,
  });
  task._render(renderParameters.timeout);
  return task;
}

export { renderTextLayer, TextLayerRenderTask };