Feature request: Improve scaling API to simplify zoom implementation in embedders

[nicolo-ribaudo]

To properly implement zoom gestures (Ctrl+wheel, or pinch-to-zoom), applications that embed the viewer currently have to:

1. compute the scaling factor/steps based on the input method
2. call pdfViewer.decreaseScale or pdfViewer.increaseScale, depending on whether it's a zoom in or out
3. scroll the PDF to restore the center of the gesture, knowing that decreaseScale/increaseScale preserve the position of the top-left visible corner.

As an example, in the PDF.js app, (2) needs to be checked 4 times (one for each zoom input method):

pdf.js/web/app.js

Lines 2641 to 2646 in b676540

	if (scaleFactor < 1) {
	PDFViewerApplication.zoomOut(null, scaleFactor);
	} else if (scaleFactor > 1) {
	PDFViewerApplication.zoomIn(null, scaleFactor);
	} else {
	return;

pdf.js/web/app.js

Lines 2679 to 2685 in b676540

	if (ticks < 0) {
	PDFViewerApplication.zoomOut(-ticks);
	} else if (ticks > 0) {
	PDFViewerApplication.zoomIn(ticks);
	} else {
	return;
	}

pdf.js/web/app.js

Lines 2800 to 2806 in b676540

	if (newScaleFactor < 1) {
	PDFViewerApplication.zoomOut(null, newScaleFactor);
	} else if (newScaleFactor > 1) {
	PDFViewerApplication.zoomIn(null, newScaleFactor);
	} else {
	return;
	}

pdf.js/web/app.js

Lines 2813 to 2819 in b676540

	if (ticks < 0) {
	PDFViewerApplication.zoomOut(-ticks);
	} else if (ticks > 0) {
	PDFViewerApplication.zoomIn(ticks);
	} else {
	return;
	}

And for (3) it has a "post-zoom scroll fixup" implemented at

pdf.js/web/app.js

Lines 2101 to 2109 in b676540

	_centerAtPos(previousScale, x, y) {
	const { pdfViewer } = this;
	const scaleDiff = pdfViewer.currentScale / previousScale - 1;
	if (scaleDiff !== 0) {
	const [top, left] = pdfViewer.containerTopLeft;
	pdfViewer.container.scrollLeft += (x - left) * scaleDiff;
	pdfViewer.container.scrollTop += (y - top) * scaleDiff;
	}
	},

The same logic needs to be re-implemented in other embedders that implement gestures-based zoom, such as in ngx-extended-pdf-viewer for angular (or the internal one I'm helping with, which re-implements similar logic).

---

What I would like the viewer to provide is:

• a center: [x, y] parameter in increaseScale/decreaseScale that allows customizing the "zoom center" rather than always scrolling to _location.top, _location.left and then having to manually adjust it separately. Having to compute the "scroll adjustment" can be tricky if you don't know exactly how the various position/zoom data from the viewer fit together.
• an updateScale function, that zooms in if the scaleFactor > 1 || steps > 0, and zooms out if scaleFactor < 1 || steps < 0

The first one is more important, since it centralizes the "adjust scroll after zoom" logic in a single place (the viewer) rather than it being split across the viewer and the wrapper. The second one is just for convenience.