Programmatically use RGBa values in fillStyle in <canvas>?

html canvas
canvas fillstyle transparent
html rectangle fill color
html rgba
canvas fillstyle using css
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fillstyle and strokestyle

Using <canvas>, I want to set the RGBa value of the rectangle using a variable.

for example:

ctx.fillStyle = "rgba(32, 45, 21, 0.3)";

works fine, but using it with a variable:

var r_a =  0.3;
ctx.fillStyle = "rgba(32, 45, 21, r_a)";

doesn't work.

Apparently fillStyle only accepts a string. So how do I set a value of the rgba value using some variable instead of explicitly defining the values?

You just need to concatenate the r_a variable to build the string correctly:

var r_a = 0.3; 
ctx.fillStyle = "rgba(32, 45, 21, " + r_a + ")"; 

Applying styles and colors, By default, the stroke and fill color are set to black (CSS color value #000000 ). and fillStyle properties accept CSS rgba color values, we can use the Here's a little demo in which you can set miterLimit dynamically and see� Definition and Usage. The rgba() function define colors using the Red-green-blue-alpha (RGBA) model. RGBA color values are an extension of RGB color values with an alpha channel - which specifies the opacity of the color.

ctx.fillStyle = "rgba(32, 45, 21, "+r_a+")"; 

It is string concatenation.

Applying styles and colors � WebPlatform Docs, If you set the strokeStyle and/or fillStyle property, the new value becomes the // set the fill style for each rectangle drawn //the rgb() function only takes integers // use I've made a little demo in which you can set miterLimit dynamically and see � The fillStyle attribute is useful for specifying fill color for any closed path/figure/text on canvas. If you are filling a closed path, it is important to use the fill() method after drawing the path. The fillStyle attribute is very similar in usage to the strokeStyle attribute. You could directly specify the color as follows: fillStyle="green";

I'm very late to the party but I had a similar problem and solved it in a slightly different way that may be useful.

Since I needed to reuse the fill colours at different alpha levels, I used the JavaScript replace method:

colurfill = "rgba(255, 255, 0, [[opacity]])";
ctx.fillStyle = colurfill.replace("[[opacity]]", "0.5");
ctx.fillStyle = colurfill.replace("[[opacity]]", "0.75");

Obviously it doesn't have to be the alpha level that varies, it could be one of the other channels.

HTML canvas globalAlpha Property, var c = document.getElementById("myCanvas"); var ctx = c.getContext("2d"); ctx. fillStyle = "red"; ctx.fillRect(20, 20, 75, 50); // Turn transparency on ctx. = "rgba(x,x,x,0.1)"; // I have to know the rgb values to update the alpha value Is there a way to set the a value of rgba() without changing/knowing the rgb values? Maybe I can do something like this?

var r_a = 0.3
ctx.fillStyle = `rgba(32, 45, 21, ${r_a})`;

These are called template literals

The back-ticks `` are often found left of the number 1 on the keyboard on US keyboards.

HTML canvas fillStyle Property, Well organized and easy to understand Web building tutorials with lots of examples of how to use HTML, CSS, JavaScript, SQL, PHP, Python, Bootstrap, Java� Programmatically use RGBa values in fillStyle in <canvas>? 941. How to clear the canvas for redrawing. 309. Resizing an image in an HTML5 canvas. 881.

I was looking for something similar and ran into your post, after reading it, I came up with a solution for myself, I'm working with the audio API and wanted a dynamic color based on a variable coming from the frequency in the sound file. Here's what I came up with and it woks for now, thought I'd post it in case it helps since I got the inspiration from your question:

function frameLooper(){ 
    fbc_array = new Uint8Array(analyser.frequencyBinCount); 
    ctx.clearRect(0, 0, canvas.width, canvas.height); 
// Clear the canvas 

    bars = 100; 
        for (var i = 0; i < bars; i++) { 
            bar_x = i * 3; 
            bar_width = 2; 
            bar_height = -(fbc_array[i] / 2); 
            bar_color = i * 3;

    //fillRect( x, y, width, height ) 
    // Explanation of the parameters below 
        ctx.fillRect(bar_x, canvas.height, bar_width, bar_height); 
            ctx.fillStyle = "rgb(100," + bar_color + "," + bar_color + ")" ; 
// Color of the bars

Converting Color Spaces in JavaScript, Now we can use hexToRGB("#ff0",true) to get rgb(100%,100%,0%) ! Hex (# rrggbbaa) to RGBA. The procedure for hex values with alpha should� RGBa is a way that color gets declared in CSS3 and it includes the red, green, and blue channels of the color model, along with the alpha channel for transparency settings which enables alpha

.fillStyle isn't working, is their something wrong with my code? or any , Programmatically use RGBa values in fillStyle in <canvas>?, You just need to concatenate the r_a variable to build the string correctly: var r_a = 0.3; ctx.fillStyle � As written, I found the rgbToHex function returns a string with elements after the point and it requires that the r, g, b values fall within the range 0-255. I'm sure this may seem obvious to most, but it took two hours for me to figure out and by then the original method had ballooned to 7 lines before I realised my problem was elsewhere.

styles and colors on canvas, fillStyle="rgb(255,100,100)"; If you are specifying the text color using rgb value, you can also specify the transparency of the color by passing a fourth parameter. Save Your Code. If you click the save button, your code will be saved, and you get an URL you can share with others.

Gradients with Canvas, The canvas used in the first example is declared in the html like this The beginning will be black, rgb(0,0,0), and the end will be white, So all that's left to do is to tell the context that we're going to use the gradient for our fillStyle, and then to Those buttons are drawn dynamically on the canvas and respond to user input. For every pixel in an ImageData object there are four pieces of information, the RGBA values: R - The color red (from 0-255) G - The color green (from 0-255) B - The color blue (from 0-255) A - The alpha channel (from 0-255; 0 is transparent and 255 is fully visible)