getAlpha

Category:Canvas

getAlpha(imageData, x, y)

Category: Canvas

Returns the amount of alpha (opacity) (ranging from 0 to 255) in the color of the pixel located at the given x and y position in the given image data.

One advantage of using a canvas for apps containing images or drawing is that you can access the image data at the pixel level. This allows your app to process an image just like many image editing programs.

How pixel colors work: The color you see in a pixel on the screen is made up of 4 values. The red, green, blue, and alpha values of a pixel determine exactly the shade of color that appears on the screen. Each of these values ranges from a minimum of 0 up to a maximum of 255. They are usually listed in the order of Red, Green, Blue, then Alpha - or RGBA. A fully red (and only red) pixel would be written as (255, 0, 0, 255). A black pixel is (0, 0, 0, 255). So reducing a pixel's color values will cause it to be closer to black. The alpha value is special because it shows how opaque the pixel should be in comparison to other pixels on the same spot at the screen. So an alpha value of 0 would make a pixel fully transparent (regardless of the other color values) and 255 is fully visible.

Examples

// Print the alpha (opacity) value of a single pixel from a drawing.
createCanvas('canvas1');
setFillColor('red');
rect(0, 0, 100, 200);
var imageData = getImageData(0, 0, 100, 200);
var alphaValue = getAlpha(imageData, 50, 50);
console.log(alphaValue);

// Print the alpha (opacity) value of single pixels from an image.
createCanvas('canvas1');
drawImageURL("https://code.org/images/logo.png");
var imageData = getImageData(0, 0, 100, 100);
console.log(getAlpha(imageData, 1, 1));
console.log(getAlpha(imageData, 10, 10));

Example: How Transparent? Change the alpha value of a single pixel to zero.

// Change the alpha value of a single pixel to zero.
createCanvas('canvas1');
setFillColor('red');
rect(0, 0, 100, 200);
var imageData = getImageData(0, 0, 100, 200);
console.log(getAlpha(imageData, 50, 50));
setAlpha(imageData, 50, 50, 0);
putImageData(imageData, 0, 0);
console.log(getAlpha(imageData, 50, 50));

Example: It's Getting Clearer Halve all alpha values in the canvas.

// Halve all alpha values for an image and display it next to the original.
createCanvas('canvas1');
drawImageURL("https://studio.code.org/blockly/media/skins/bee/static_avatar.png");
button("id", "Transparent");
setPosition('id', 200, 0);
onEvent("id", "click", function() {
  var imageData = getImageData(0, 0, 175, 200);
  putImageData(halveAlpha(imageData), 0, 225);
});

function halveAlpha(thisImageData){
    for(var y=0; y < thisImageData.height; y++) {
        for(var x=0; x < thisImageData.width; x++) {
            var newAlpha = (getAlpha(thisImageData, x, y) / 2);
            setAlpha(thisImageData, x, y, newAlpha);
        }
    }
  return thisImageData;
}

Syntax

getAlpha(imageData, x, y);

Parameters

Name	Type	Required?	Description
imageData	object	Yes	The image data object that describes data captured from a canvas element (use getImageData())
x	number	Yes	The x position in pixels starting from the upper left corner of image.
y	number	Yes	The y position in pixels starting from the upper left corner of image.

Returns

Returns a number representing the alpha (opacity) value (between 0 and 255) of the pixel in the input image data at the input x and y coordinates.

Tips

Canvas and image data must exist before image color functions can be used. Create a canvas element in Design mode first or call createCanvas(), and then you can capture image data using getImageData() before calling getAlpha().
Use the output of this function with setAlpha() or setRGB().
You will have to use putImageData() to update the canvas with modified image data.

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