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Account for the Firefox font size quantization in the text layer #21810

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Account for the Firefox font size quantization in the text layer #21810

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32 changes: 26 additions & 6 deletions src/display/text_layer.js
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Original file line number Diff line number Diff line change
Expand Up @@ -69,6 +69,8 @@ class TextLayer {

#pageWidth = 0;

#pixelRatio = OutputScale.pixelRatio;

#reader = null;

#rootContainer = null;
Expand Down Expand Up @@ -122,7 +124,7 @@ class TextLayer {

this.#imagesHandler = images;

this.#scale = viewport.scale * OutputScale.pixelRatio;
this.#scale = viewport.scale * this.#pixelRatio;
this.#rotation = viewport.rotation;
this.#layoutTextParams = {
div: null,
Expand Down Expand Up @@ -229,6 +231,7 @@ class TextLayer {
if (scale !== this.#scale) {
onBefore?.();
this.#scale = scale;
this.#pixelRatio = OutputScale.pixelRatio;
const params = {
div: null,
properties: null,
Expand Down Expand Up @@ -424,17 +427,26 @@ class TextLayer {
#layout(params) {
const { div, properties, ctx } = params;
const { style } = div;
const { canvasWidth, fontSize } = properties;

if (properties.canvasWidth !== 0 && properties.hasText) {
if (canvasWidth !== 0 && fontSize !== 0 && properties.hasText) {
const { fontFamily } = style;
const { canvasWidth, fontSize } = properties;

TextLayer.#ensureCtxFont(ctx, fontSize * this.#scale, fontFamily);
// Firefox quantizes canvas font sizes after dividing by the device-pixel
// ratio. Measure at a quantized size, then rescale the width.
const pixelRatio = this.#pixelRatio;
const measuredSize =
TextLayer.#quantizeFontSize((fontSize * this.#scale) / pixelRatio) *
pixelRatio;

TextLayer.#ensureCtxFont(ctx, measuredSize, fontFamily);
// Only measure the width for multi-char text divs, see `appendText`.
const { width } = ctx.measureText(div.textContent);

if (width > 0) {
style.setProperty("--scale-x", (canvasWidth * this.#scale) / width);
style.setProperty(
"--scale-x",
(canvasWidth * measuredSize) / (width * fontSize)
);
}
}
if (properties.angle !== 0) {
Expand Down Expand Up @@ -489,6 +501,14 @@ class TextLayer {
return ctx;
}

// Match Firefox's 7-bit `QuantizeFontSize` implementation:
// https://searchfox.org/firefox-main/rev/04b29f9c2d2dbf5639c3f45ea812bb4c21dc81c6/dom/canvas/CanvasRenderingContext2D.cpp#4205-4215
static #quantizeFontSize(size) {
size = Math.fround(size);
const d = Math.fround(size * ((1 << 17) + 1));
return Math.fround(d - Math.fround(d - size));
}

static #ensureCtxFont(ctx, size, family) {
const cached = this.#canvasCtxFonts.get(ctx);
if (size === cached.size && family === cached.family) {
Expand Down
4 changes: 3 additions & 1 deletion test/text_layer_test.css
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Original file line number Diff line number Diff line change
Expand Up @@ -26,7 +26,9 @@
position: absolute;
white-space: pre;
transform-origin: 0% 0%;
border: solid 1px rgb(255 0 0 / 0.5);

/* Gecko may drop transformed 1px borders during rasterization. */
border: solid 2px rgb(255 0 0 / 0.5);
background-color: rgb(255 255 32 / 0.1);
box-sizing: border-box;
}
Expand Down