HTML5 canvas overlay transparent gradients
I'm creating a color picker in HTML5 like the gradient below
It consists of three elements:
- A solid red background color (must be changeable)
- A black – transparent gradient from bottom to top
- A white – transparent gradient from the left to right
I have managed to create a single gradient and a single color, but I can't figure out how to overlay the solid color and two gradients together. How can I accomplish this?
(I could provide my code but it's pretty generic and wouldn't be useful)
You can overlay two gradients on top of each other:
Going from white to the color (HUE degree) you want to use with 100% saturation and 50% lightness:
var grH = ctx.createLinearGradient(0, 0, ctx.canvas.width, 0); grH.addColorStop(0, '#fff'); grH.addColorStop(1, 'hsl(' + hue + ', 100%, 50%)'); ctx.fillStyle = grH; ctx.fillRect(0, 0, ctx.canvas.width, ctx.canvas.height);
Going from black bottom to transparent at top.
var grV = ctx.createLinearGradient(0, 0, 0, ctx.canvas.height); grV.addColorStop(0, 'rgba(0,0,0,0)'); grV.addColorStop(1, '#000'); ctx.fillStyle = grV; ctx.fillRect(0, 0, ctx.canvas.width, ctx.canvas.height);
Drawing the horizontal first and then the vertical on top will result in this:
As in the demo it's easy to create a slider to update the hue palette. You don't need to re-create the black to transparent gradient as in the demo - just cache it to an off-screen canvas and re-use it for each update as that gives you a bit more performance.
Hope this helps.