setGreen

Category:Canvas

setGreen(imageData, x, y, greenValue)

Category: Canvas

Sets the amount of green (ranging from 0 to 255) in the color of the pixel located at the given x and y position in the given image data to the greenValue input amount.

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

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

Trading Colors Swap green to red, blue to green, red to blue.

// Swap green to red, blue to green, red to blue.
createCanvas('canvas1');
drawImageURL("https://studio.code.org/blockly/media/skins/bee/static_avatar.png");
button("id", "Swap Colors");
setPosition('id', 200, 0);
onEvent("id", "click", function() {
  var imageData = getImageData(0, 0, 175, 200);
  putImageData(swapRedGreenBlue(imageData), 0, 225);
});
function swapRedGreenBlue(thisImageData){
    for(var y=0; y < thisImageData.height; y++) {
        for(var x=0; x < thisImageData.width; x++) {
            var newRed = getGreen(thisImageData, x, y);
            var newGreen = getBlue(thisImageData, x, y);
            var newBlue = getRed(thisImageData, x, y);
            setRed(thisImageData, x, y, newRed);
            setGreen(thisImageData, x, y, newGreen);
            setBlue(thisImageData, x, y, newBlue);            
        }
    }
  return thisImageData;
}

Syntax

setGreen(imageData, x, y, greenValue);

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.
greenValue number Yes The amount of green (from 0 to 255) to set in the pixel.

Returns

No return value. Only modifies the input image data object. setGreen() will not automatically update what you see on the canvas on screen.

Tips

Found a bug in the documentation? Let us know at documentation@code.org

Found a bug in the documentation? Let us know at documentation@code.org