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Viewschange the alpha for the background color of the canvas
Hi guy i am trying to clone colol wheel picker from adobe color wheel, i am having a problem with changing the alpha for the color wheel, the code i used to generate the color wheel is referenced from here.Below is the code i use to change the alpha for the background of the canvas(default alpha is 255), but it's not working, i still can't figure out why
useEffect(() => {
const canvas = canvasRef.current;
const ctx = canvas.getContext("2d");
let ImageData = ctx.getImageData(0, 0, canvas.width, canvas.height);
for (var i = 0; i < canvas.height; i++)
for (var j = 0; j < canvas.width; j++)
if ((i + 1) % 4 === 0) {
ImageData.data[i] = 150;
}
ctx.putImageData(ImageData, 0, 0); //put image data back
}, []);
you can find full code and demo in here, thank you
2 answers
Answer question0
I'm guessing that you want to change the alpha value of each color component contained in the ImageData array, are you?
In this case your for-loop is simply wrong. For illustrative purposes try something like this:
let canvas = {
width: 100,
height: 20
};
let values = [];
for (var i = 0; i < canvas.height; i++)
for (var j = 0; j < canvas.width; j++)
if ((i + 1) % 4 === 0) {
values.push(i);
}
document.getElementById("result").innerText = values.toString();#result {
width: 20em;
display: block;
word-break: break-all;
}<span id="result"></span>If you run the code above you'll notice that the only unique values we get are 3, 7 ,11, 15 and 19.
If you want to change the alpha of the whole ImageData array, you simply need to loop over it and change any 4th value. For example:
let ImageData = ctx.getImageData(0, 0, canvas.width, canvas.height);
for (let i = 0; i < ImageData.length; i += 4) {
ImageData.data[i] = 150;
}
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If changing the alpha is what you need, you should be using the canvas globalAlpha:
https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/globalAlpha
Here is an example drawing the same shape with different alpha values
canvas = document.getElementById("cv0");
ctx = canvas.getContext('2d')
ctx.fillStyle = "red"
function draw(x, y, alpha) {
ctx.globalAlpha = alpha
ctx.beginPath()
ctx.moveTo(x, y)
ctx.arc(x+20, y-10, 30, 0, 6)
ctx.fill()
ctx.globalAlpha = 1
}
draw(10, 40, 0.4)
draw(80, 40, 0.7)
draw(150, 40, 1)<canvas id='cv0' width=200 height=90></canvas>When you provide a sample snippet please remove all other functions, variables and anything else that is not relevant to the problem at hand, read this for more info:
https://stackoverflow.com/help/minimal-reproducible-example
that will save a lot of time to everyone reading your code for the first time, and you increase your chances of getting a fast response.
Here is my changes to your code snippet:
https://codesandbox.io/s/gracious-fog-q303zf?file=/src/Canvas.js
I'm using the globalAlpha to draw two circles (like yours just smaller) with different alpha
