[api-minor] Improve pdf reading in high contrast mode

- Use Canvas & CanvasText color when they don't have their default value
  as background and foreground colors.
- The colors used to draw (stroke/fill) in a pdf are replaced by the bg/fg
  ones according to their luminance.

extensions/chromium/preferences_schema.json

@@ -208,6 +208,16 @@
         2
       ],
       "default": -1
+    },
+    "pageBackgroundColor": {
+      "description": "The color is a string as defined in CSS. Its goal is to help improve readability in high contrast mode",
+      "type": "string",
+      "default": "Canvas"
+    },
+    "pageForegroundColor": {
+      "description": "The color is a string as defined in CSS. Its goal is to help improve readability in high contrast mode",
+      "type": "string",
+      "default": "CanvasText"
     }
   }
 }

src/display/api.js

@@ -1169,6 +1169,12 @@ class PDFDocumentProxy {
  *   <color> value, a `CanvasGradient` object (a linear or radial gradient) or
  *   a `CanvasPattern` object (a repetitive image). The default value is
  *   'rgb(255,255,255)'.
+ *
+ *   NOTE: This option may be partially, or completely, ignored when the
+ *   `pageColors`-option is used.
+ * @property {Object} [pageColors] - Overwrites background and foreground colors
+ *   with user defined ones in order to improve readability in high contrast
+ *   mode.
  * @property {Promise<OptionalContentConfig>} [optionalContentConfigPromise] -
  *   A promise that should resolve with an {@link OptionalContentConfig}
  *   created from `PDFDocumentProxy.getOptionalContentConfig`. If `null`,
@@ -1393,6 +1399,7 @@ class PDFPageProxy {
     background = null,
     optionalContentConfigPromise = null,
     annotationCanvasMap = null,
+    pageColors = null,
   }) {
     if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("GENERIC")) {
       if (arguments[0]?.renderInteractiveForms !== undefined) {
@@ -1516,6 +1523,7 @@ class PDFPageProxy {
       canvasFactory: canvasFactoryInstance,
       useRequestAnimationFrame: !intentPrint,
       pdfBug: this._pdfBug,
+      pageColors,
     });
 
     (intentState.renderTasks ||= new Set()).add(internalRenderTask);
@@ -3219,6 +3227,7 @@ class InternalRenderTask {
     canvasFactory,
     useRequestAnimationFrame = false,
     pdfBug = false,
+    pageColors = null,
   }) {
     this.callback = callback;
     this.params = params;
@@ -3230,6 +3239,7 @@ class InternalRenderTask {
     this._pageIndex = pageIndex;
     this.canvasFactory = canvasFactory;
     this._pdfBug = pdfBug;
+    this.pageColors = pageColors;
 
     this.running = false;
     this.graphicsReadyCallback = null;
@@ -3284,7 +3294,8 @@ class InternalRenderTask {
       this.canvasFactory,
       imageLayer,
       optionalContentConfig,
-      this.annotationCanvasMap
+      this.annotationCanvasMap,
+      this.pageColors
     );
     this.gfx.beginDrawing({
       transform,

src/display/canvas.js

@@ -1042,9 +1042,8 @@ function copyCtxState(sourceCtx, destCtx) {
   }
 }
 
-function resetCtxToDefault(ctx) {
+function resetCtxToDefault(ctx, foregroundColor) {
-  ctx.strokeStyle = "#000000";
+  ctx.strokeStyle = ctx.fillStyle = foregroundColor || "#000000";
-  ctx.fillStyle = "#000000";
   ctx.fillRule = "nonzero";
   ctx.globalAlpha = 1;
   ctx.lineWidth = 1;
@@ -1212,7 +1211,8 @@ class CanvasGraphics {
     canvasFactory,
     imageLayer,
     optionalContentConfig,
-    annotationCanvasMap
+    annotationCanvasMap,
+    pageColors
   ) {
     this.ctx = canvasCtx;
     this.current = new CanvasExtraState(
@@ -1248,6 +1248,8 @@ class CanvasGraphics {
     this.viewportScale = 1;
     this.outputScaleX = 1;
     this.outputScaleY = 1;
+    this.backgroundColor = pageColors?.background || null;
+    this.foregroundColor = pageColors?.foreground || null;
     if (canvasCtx) {
       // NOTE: if mozCurrentTransform is polyfilled, then the current state of
       // the transformation must already be set in canvasCtx._transformMatrix.
@@ -1280,9 +1282,58 @@ class CanvasGraphics {
     // transparent canvas when we have blend modes.
     const width = this.ctx.canvas.width;
     const height = this.ctx.canvas.height;
-
+    const defaultBackgroundColor = background || "#ffffff";
     this.ctx.save();
-    this.ctx.fillStyle = background || "rgb(255, 255, 255)";
+
+    if (this.foregroundColor && this.backgroundColor) {
+      // Get the #RRGGBB value of the color. If it's a name (e.g. CanvasText)
+      // then it'll be converted to its rgb value.
+      this.ctx.fillStyle = this.foregroundColor;
+      const fg = (this.foregroundColor = this.ctx.fillStyle);
+      this.ctx.fillStyle = this.backgroundColor;
+      const bg = (this.backgroundColor = this.ctx.fillStyle);
+      let isValidDefaultBg = true;
+      let defaultBg = defaultBackgroundColor;
+
+      if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("MOZCENTRAL")) {
+        this.ctx.fillStyle = defaultBackgroundColor;
+        defaultBg = this.ctx.fillStyle;
+        isValidDefaultBg =
+          typeof defaultBg === "string" && /^#[0-9A-Fa-f]{6}$/.test(defaultBg);
+      }
+
+      if ((fg === "#000000" && bg === "#ffffff") || !isValidDefaultBg) {
+        this.foregroundColor = this.backgroundColor = null;
+      } else {
+        // https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_Colors_and_Luminance
+        //
+        // Relative luminance:
+        // https://www.w3.org/TR/WCAG20/#relativeluminancedef
+        //
+        // We compute the rounded luminance of the default background color.
+        // Then for every color in the pdf, if its rounded luminance is the
+        // same as the background one then it's replaced by the new
+        // background color else by the foreground one.
+        const cB = parseInt(defaultBg.slice(1), 16);
+        const rB = (cB && 0xff0000) >> 16;
+        const gB = (cB && 0x00ff00) >> 8;
+        const bB = cB && 0x0000ff;
+        const newComp = x => {
+          x /= 255;
+          return x <= 0.03928 ? x / 12.92 : ((x + 0.055) / 1.055) ** 2.4;
+        };
+        const lumB = Math.round(
+          0.2126 * newComp(rB) + 0.7152 * newComp(gB) + 0.0722 * newComp(bB)
+        );
+        this.selectColor = (r, g, b) => {
+          const lumC =
+            0.2126 * newComp(r) + 0.7152 * newComp(g) + 0.0722 * newComp(b);
+          return Math.round(lumC) === lumB ? bg : fg;
+        };
+      }
+    }
+
+    this.ctx.fillStyle = this.backgroundColor || defaultBackgroundColor;
     this.ctx.fillRect(0, 0, width, height);
     this.ctx.restore();
 
@@ -1303,7 +1354,7 @@ class CanvasGraphics {
     }
 
     this.ctx.save();
-    resetCtxToDefault(this.ctx);
+    resetCtxToDefault(this.ctx, this.foregroundColor);
     if (transform) {
       this.ctx.transform.apply(this.ctx, transform);
       this.outputScaleX = transform[0];
@@ -2636,13 +2687,13 @@ class CanvasGraphics {
   }
 
   setStrokeRGBColor(r, g, b) {
-    const color = Util.makeHexColor(r, g, b);
+    const color = this.selectColor?.(r, g, b) || Util.makeHexColor(r, g, b);
     this.ctx.strokeStyle = color;
     this.current.strokeColor = color;
   }
 
   setFillRGBColor(r, g, b) {
-    const color = Util.makeHexColor(r, g, b);
+    const color = this.selectColor?.(r, g, b) || Util.makeHexColor(r, g, b);
     this.ctx.fillStyle = color;
     this.current.fillColor = color;
     this.current.patternFill = false;
@@ -2964,9 +3015,9 @@ class CanvasGraphics {
         this.ctx.setTransform(scaleX, 0, 0, -scaleY, 0, height * scaleY);
         addContextCurrentTransform(this.ctx);
 
-        resetCtxToDefault(this.ctx);
+        resetCtxToDefault(this.ctx, this.foregroundColor);
       } else {
-        resetCtxToDefault(this.ctx);
+        resetCtxToDefault(this.ctx, this.foregroundColor);
 
         this.ctx.rect(rect[0], rect[1], width, height);
         this.ctx.clip();

test/driver.js

@@ -648,7 +648,8 @@ class Driver {
               renderForms = false,
               renderPrint = false,
               renderXfa = false,
-              annotationCanvasMap = null;
+              annotationCanvasMap = null,
+              pageColors = null;
 
             if (task.annotationStorage) {
               const entries = Object.entries(task.annotationStorage),
@@ -699,6 +700,7 @@ class Driver {
               renderForms = !!task.forms;
               renderPrint = !!task.print;
               renderXfa = !!task.enableXfa;
+              pageColors = task.pageColors || null;
 
               // Render the annotation layer if necessary.
               if (renderAnnotations || renderForms || renderXfa) {
@@ -746,6 +748,7 @@ class Driver {
               viewport,
               optionalContentConfigPromise: task.optionalContentConfigPromise,
               annotationCanvasMap,
+              pageColors,
               transform,
             };
             if (renderForms) {

test/test_manifest.json

@@ -60,6 +60,38 @@
        "enhance": true,
        "type": "text"
     },
+    {  "id": "tracemonkey-forced-colors-eq",
+      "file": "pdfs/tracemonkey.pdf",
+      "md5": "9a192d8b1a7dc652a19835f6f08098bd",
+      "rounds": 1,
+      "pageColors": {
+         "background": "black",
+         "foreground": "#00FF00"
+      },
+      "type": "eq"
+   },
+   {  "id": "tracemonkey-viewer-colors-eq",
+      "file": "pdfs/tracemonkey.pdf",
+      "md5": "9a192d8b1a7dc652a19835f6f08098bd",
+      "rounds": 1,
+      "pageColors": {
+         "background": "Canvas",
+         "foreground": "CanvasText"
+      },
+      "type": "eq",
+      "about": "Uses the same pageColors as the viewer."
+   },
+   {  "id": "tracemonkey-default-colors-eq",
+      "file": "pdfs/tracemonkey.pdf",
+      "md5": "9a192d8b1a7dc652a19835f6f08098bd",
+      "rounds": 1,
+      "pageColors": {
+         "background": "white",
+         "foreground": "black"
+      },
+      "type": "eq",
+      "about": "Uses the default colors."
+   },
     {  "id": "issue3925",
        "file": "pdfs/issue3925.pdf",
        "md5": "c5c895deecf7a7565393587e0d61be2b",

web/app.js

@@ -525,6 +525,10 @@ const PDFViewerApplication = {
       useOnlyCssZoom: AppOptions.get("useOnlyCssZoom"),
       maxCanvasPixels: AppOptions.get("maxCanvasPixels"),
       enablePermissions: AppOptions.get("enablePermissions"),
+      pageColors: {
+        background: AppOptions.get("pageBackgroundColor"),
+        foreground: AppOptions.get("pageForegroundColor"),
+      },
     });
     pdfRenderingQueue.setViewer(this.pdfViewer);
     pdfLinkService.setViewer(this.pdfViewer);

web/app_options.js

@@ -129,6 +129,16 @@ const defaultOptions = {
     compatibility: compatibilityParams.maxCanvasPixels,
     kind: OptionKind.VIEWER,
   },
+  pageBackgroundColor: {
+    /** @type {string} */
+    value: "Canvas",
+    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
+  },
+  pageForegroundColor: {
+    /** @type {string} */
+    value: "CanvasText",
+    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
+  },
   pdfBugEnabled: {
     /** @type {boolean} */
     value: typeof PDFJSDev === "undefined" || !PDFJSDev.test("PRODUCTION"),

web/base_viewer.js

@@ -116,6 +116,9 @@ const PagesCountLimit = {
  * @property {IL10n} l10n - Localization service.
  * @property {boolean} [enablePermissions] - Enables PDF document permissions,
  *   when they exist. The default value is `false`.
+ * @property {Object} [pageColors] - Overwrites background and foreground colors
+ *   with user defined ones in order to improve readability in high contrast
+ *   mode.
  */
 
 class PDFPageViewBuffer {
@@ -262,6 +265,22 @@ class BaseViewer {
     this.maxCanvasPixels = options.maxCanvasPixels;
     this.l10n = options.l10n || NullL10n;
     this.#enablePermissions = options.enablePermissions || false;
+    this.pageColors = options.pageColors || null;
+
+    if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("MOZCENTRAL")) {
+      if (
+        options.pageColors &&
+        (!CSS.supports("color", options.pageColors.background) ||
+          !CSS.supports("color", options.pageColors.foreground))
+      ) {
+        if (options.pageColors.background || options.pageColors.foreground) {
+          console.warn(
+            "Ignoring `pageColors`-option, since the browser doesn't support the values used."
+          );
+        }
+        this.pageColors = null;
+      }
+    }
 
     this.defaultRenderingQueue = !options.renderingQueue;
     if (this.defaultRenderingQueue) {
@@ -698,6 +717,7 @@ class BaseViewer {
             renderer: this.renderer,
             useOnlyCssZoom: this.useOnlyCssZoom,
             maxCanvasPixels: this.maxCanvasPixels,
+            pageColors: this.pageColors,
             l10n: this.l10n,
           });
           this._pages.push(pageView);

web/pdf_page_view.js

@@ -82,6 +82,9 @@ import { NullL10n } from "./l10n_utils.js";
  * @property {number} [maxCanvasPixels] - The maximum supported canvas size in
  *   total pixels, i.e. width * height. Use -1 for no limit. The default value
  *   is 4096 * 4096 (16 mega-pixels).
+ * @property {Object} [pageColors] - Overwrites background and foreground colors
+ *   with user defined ones in order to improve readability in high contrast
+ *   mode.
  * @property {IL10n} l10n - Localization service.
  */
 
@@ -118,6 +121,7 @@ class PDFPageView {
     this.imageResourcesPath = options.imageResourcesPath || "";
     this.useOnlyCssZoom = options.useOnlyCssZoom || false;
     this.maxCanvasPixels = options.maxCanvasPixels || MAX_CANVAS_PIXELS;
+    this.pageColors = options.pageColors || null;
 
     this.eventBus = options.eventBus;
     this.renderingQueue = options.renderingQueue;
@@ -832,6 +836,7 @@ class PDFPageView {
       annotationMode: this.#annotationMode,
       optionalContentConfigPromise: this._optionalContentConfigPromise,
       annotationCanvasMap: this._annotationCanvasMap,
+      pageColors: this.pageColors,
     };
     const renderTask = this.pdfPage.render(renderContext);
     renderTask.onContinue = function (cont) {