Draw Arcs in Google Slides with Apps Script and the JavaScript Canvas API

Published: 12/04/2021

In Google Slides, you can manually create circle segments, or arcs, but there doesn’t seem to be a corresponding way to create them programatically with the Slides API or with a ready-made Apps Script method.

I answered a StackOverflow question about this topic, and this article is adapted from the answer I gave.

Searching the Slides API

You could start by manually drawing an arc and then examining all the potential attribues of the Shape class from SlidesApp. None seem to refer to the arc sweep, though.

The next step would be getting a full JSON representation of the arc and the slide with the Slides API page GET request. Which at most yielded these attributes:

{
	"objectId": "p",
	"pageElements": [
		{
			"objectId": "SLIDES_API17000000589_3",
			"size": {...}
			},
			"transform": {
				"scaleX": -0.1652,
				"scaleY": -0.1636,
				"shearX": -0.1057,
				"shearY": 0.1067,
				"translateX": 6532089.87,
				"translateY": 1296513.29,
				"unit": "EMU"
			},
			"shape": {
				"shapeType": "ARC",
				"shapeProperties": {
					"shapeBackgroundFill": {...},
					"outline": {...},
					"shadow": {...}
				}
			}
		}
	],
	"slideProperties": {...},
	"revisionId": "_7MTqW3NeaZ8yQ",
	"pageProperties": {...}
}

Even changing the sweep of the arc manually and putting the two versions through a diff will yield no changes except for the revisionId.

If more native circle and arc drawing methods is something you are interested in seeing, there is an existing feature request. Go and add a ⭐ to it.

Workarounds

The simplest way to create custom progress bars would be to keep static images of them in Google Drive and then insert them as images.

Alternatively, there is a slightly more complex method using the canvas API. Since you can load HTML and JavaScript in a sidebar with getUi, that means you can use the Canvas API. Since you can use the Canvas API, that means you can create any shape you want, and create an image from it.

// code.gs

function test() {
	// This creates the HTML output from the file arc-creator.html
	let html = HtmlService.createHtmlOutputFromFile('arc-creator');
	// This uses the html to load the sidebar
	SlidesApp.getUi().showSidebar(html);
}

// This function will be called from the HTML
// once the canvas has finished drawing.
function addToPresentation(dataURL) {
	let slide = SlidesApp.getActivePresentation().getSlides()[0];
	// Convert the data Url to png and add to Presentation
	var type = dataURL.split(';')[0].replace('data:', '');
	var img = Utilities.base64Decode(dataURL.split(',')[1]);
	var blob = Utilities.newBlob(img, type, 'image.png');
	slide.insertImage(blob);
}
<!-- arc-creator.html -->

<!DOCTYPE html>
<html>
	<head>
		<base target="_top" />
	</head>
	<body>
		<canvas id="canvas" width="200" height="200"></canvas>
	</body>
	<script>
		// This is the function that creates a data URL image
		// The argument is the percentage complete of the progress bar

		function createProgressArc(number) {
			// ID the canvas element and initialize the context
			var canvas = document.getElementById('canvas');
			var context = canvas.getContext('2d');

			// Some utility variables
			var cw = context.canvas.width / 2;
			var ch = context.canvas.height / 2;

			// Drawing background
			context.clearRect(0, 0, 200, 200);

			// Drawing first circle
			context.beginPath();
			context.arc(cw, ch, 50, 0, 2 * Math.PI);
			context.fillStyle = '#FFF';
			context.fill();
			context.strokeStyle = '#e7f2ba';
			context.lineWidth = 10;
			context.stroke();

			// Drawing arc
			context.fillStyle = '#000';
			context.strokeStyle = '#b3cf3c';
			context.lineWidth = 10;
			context.beginPath();
			let progress = 2 * Math.PI * (number / 100);
			context.arc(cw, ch, 50, 0, progress);
			context.stroke();

			// Converting to data URL
			var dataURL = canvas.toDataURL('image/png');
			return dataURL;
		}

		let dataURL = createProgressArc(75);

		// Here is where the resulting image is sent back
		// to the Presentation as a data URL
		google.script.run.addToPresentation(dataURL);
	</script>
</html>

Running this will open a sidebar in the UI, draw the image and then add the image to the first slide.

enter image description here

The drawback of this method is that you need to have the UI open, or else it won’t run the JavaScript that is required to draw the arc.

Originally posted in Stack Overflow