On my project, I originally used CircleStyle to create points on a map and then used fill: new Fill({ with CanvasGradient to style the point with two colours.
I now want to use a custom icon (for example 'icon.png') instead of just a coloured dot for these points.
I have tried using image: new Icon for this which works for displaying the icon but I cannot apply a CanvasGradient to this.
I can apply a single colour to the icon with color which overlays the icon with that colour with transparency, ideally I would want this with 2 colours; half the icon being one colour and half the icon being the other colour.
I have uploaded an image showing how my points currently looked below. circleStyle point with 2 colours applied using ColourGradient
The documentation suggests that CanvasGradient cannot be applied to icons so my question is: How can I apply two colours/a gradient to an icon in OpenLayers?
The OpenLayers Icon style can take a canvas element as well as an icon url. So you could load the icon url, draw it to a canvas, apply the gradient using a multiply operation, and finally use a destination-in operation to restore transparency:
const img = document.createElement('img');
img.onload = function () {
const width = img.width;
const height = img.height;
const canvas = document.createElement('canvas');
canvas.width = width;
canvas.height = height;
const context = canvas.getContext('2d');
const gradient = context.createLinearGradient(0, 0, width, 0);
gradient.addColorStop(0, 'red');
gradient.addColorStop(0.5, 'red');
gradient.addColorStop(0.5, 'green');
gradient.addColorStop(1, 'green');
context.drawImage(img, 0, 0);
context.globalCompositeOperation = 'multiply';
context.fillStyle = gradient;
context.fillRect(0, 0, width, height);
context.globalCompositeOperation = 'destination-in';
context.drawImage(img, 0, 0);
feature.setStyle(
new Style({
image: new Icon({
img: canvas,
imgSize: [width, height],
})
})
);
};
img.src = icon_url;
Working example https://codesandbox.io/s/icon-color-forked-4b1e1?file=/main.js