How to flip images horizontally with HTML5

Question

In IE, I can use:

to implement an image flip horizontally.

Answer 1

I came across this page, and no-one had quite written a function to do what I wanted, so here's mine. It draws scaled, rotated, and flipped images (I used this for rending DOM elements to canvas that have these such transforms applied).

var myCanvas = document.getElementById("myCanvas");
var ctx = myCanvas.getContext("2d");
var img = document.getElementById("myimage.jpg"); //or whatever
var deg = 13; //13 degrees rotation, for example
var flip = "true";

function drawImage(img, x, y, width, height, deg, flip){
    //save current context before applying transformations
    ctx.save();
    //convert degrees to radians
    if(flip == "true"){ 
        var rad = deg * Math.PI / 180;
    }else{
        var rad = 2*Math.PI - deg * Math.PI / 180;
    }
    //set the origin to the center of the image
    ctx.translate(x + width/2, y + height/2);
    //rotate the canvas around the origin
    ctx.rotate(rad);
    if(flip == "true"){
        //flip the canvas
        ctx.scale(-1,1);
    }
    //draw the image    
    ctx.drawImage(img, -width/2, -height/2, width, height);
    //restore the canvas
    ctx.restore();
}

Answer 2

One option is to horizontally flip the pixels of images stored in ImageData objects directly, e.g.

function flip_image (canvas) {
   	var context   = canvas.getContext ('2d') ;
   	var imageData = context.getImageData (0, 0, canvas.width, canvas.height) ;
   	var imageFlip = new ImageData (canvas.width, canvas.height) ;
   	var Npel      = imageData.data.length / 4 ;

   	for ( var kPel = 0 ; kPel < Npel ; kPel++ ) {
   	   	var kFlip      = flip_index (kPel, canvas.width, canvas.height) ;
   	   	var offset     = 4 * kPel ;
   	   	var offsetFlip = 4 * kFlip ;
   	   	imageFlip.data[offsetFlip + 0] = imageData.data[offset + 0] ;
   	   	imageFlip.data[offsetFlip + 1] = imageData.data[offset + 1] ;
   	   	imageFlip.data[offsetFlip + 2] = imageData.data[offset + 2] ;
   	   	imageFlip.data[offsetFlip + 3] = imageData.data[offset + 3] ;
   	}

   	var canvasFlip = document.createElement('canvas') ;
   	canvasFlip.setAttribute('width', width) ;
   	canvasFlip.setAttribute('height', height) ;

   	canvasFlip.getContext('2d').putImageData(imageFlip, 0, 0) ;
   	return canvasFlip ;
}

function flip_index (kPel, width, height) {
   	var i     = Math.floor (kPel / width) ;
   	var j     = kPel % width ;
   	var jFlip = width - j - 1 ;
   	var kFlip = i * width + jFlip ;
   	return kFlip ;
}

Answer 3

I like Eschers function above. I have made it a little neater and better. I have added flop (vertically) besides flip. Also a possibility to draw/rotate around the center of the image instead of top left. Finally, the function does not require all arguments. img, x and y are required but the rest are not.

If you were using something like context.drawImage(...), you can now just use drawImage(...) and add the rotate/flip/flop functionality explained here:

function drawImage(img, x, y, width, height, deg, flip, flop, center) {

context.save();

if(typeof width === "undefined") width = img.width;
if(typeof height === "undefined") height = img.height;
if(typeof center === "undefined") center = false;

// Set rotation point to center of image, instead of top/left
if(center) {
    x -= width/2;
    y -= height/2;
}

// Set the origin to the center of the image
context.translate(x + width/2, y + height/2);

// Rotate the canvas around the origin
var rad = 2 * Math.PI - deg * Math.PI / 180;    
context.rotate(rad);

// Flip/flop the canvas
if(flip) flipScale = -1; else flipScale = 1;
if(flop) flopScale = -1; else flopScale = 1;
context.scale(flipScale, flopScale);

// Draw the image    
context.drawImage(img, -width/2, -height/2, width, height);

context.restore();
}

Examples:

var myCanvas = document.getElementById("myCanvas");
var context = myCanvas.getContext("2d"); // i use context instead of ctx

var img = document.getElementById("myImage"); // your img reference here!

drawImage(img, 100, 100); // just draw it 
drawImage(img, 100, 100, 200, 50); // draw it with width/height specified
drawImage(img, 100, 100, 200, 50, 45); // draw it at 45 degrees
drawImage(img, 100, 100, 200, 50, 0, true); // draw it flipped
drawImage(img, 100, 100, 200, 50, 0, false, true); // draw it flopped
drawImage(img, 100, 100, 200, 50, 0, true, true); // draw it flipflopped
drawImage(img, 100, 100, 200, 50, 45, true, true, true); // draw it flipflopped and 45 degrees rotated around the center of the image :-)

Answer 4

canvas = document.createElement('canvas');
canvasContext = canvas.getContext('2d');

canvasContext.translate(width, 0);
canvasContext.scale(-1, 1);
this.canvasContext.drawImage(image, 0, 0);

Here's a snippet from a sprite object being used for testing and it produces the results you seem to expect.

Here's another site with more details. http://andrew.hedges.name/widgets/dev/

Answer 5

Mirror an image or rendering using the canvas.

Note. This can be done via CSS as well.

---

Mirroring

Here is a simple utility function that will mirror an image horizontally, vertically or both.

function mirrorImage(ctx, image, x = 0, y = 0, horizontal = false, vertical = false){
    ctx.save();  // save the current canvas state
    ctx.setTransform(
        horizontal ? -1 : 1, 0, // set the direction of x axis
        0, vertical ? -1 : 1,   // set the direction of y axis
        x + (horizontal ? image.width : 0), // set the x origin
        y + (vertical ? image.height : 0)   // set the y origin
    );
    ctx.drawImage(image,0,0);
    ctx.restore(); // restore the state as it was when this function was called
}

Usage

mirrorImage(ctx, image, 0, 0, true, false); // horizontal mirror
mirrorImage(ctx, image, 0, 0, false, true); // vertical mirror
mirrorImage(ctx, image, 0, 0, true, true);  // horizontal and vertical mirror

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Drawable image.

Many times you will want to draw on images. I like to call them drawable images. To make an image drawable you convert it to a canvas

To convert an image to canvas.

function makeImageDrawable(image){
    if(image.complete){ // ensure the image has loaded
        var dImage = document.createElement("canvas"); // create a drawable image
        dImage.width = image.naturalWidth;      // set the resolution
        dImage.height = image.naturalHeight;
        dImage.style.width = image.style.width; // set the display size
        dImage.style.height = image.style.height; 
        dImage.ctx = dImage.getContext("2d");   // get drawing API
                                                // and add to image
                                                // for possible later use
        dImage.ctx.drawImage(image,0,0);
        return dImage;
    }
    throw new ReferenceError("Image is not complete.");
 }

---

Putting it all together

 var dImage = makeImageDrawable(image);  // convert DOM img to canvas
 mirrorImage(dImage.ctx, dImage, 0, 0, false, true); // vertical flip
 image.replaceWith(dImage);  // replace the DOM image with the flipped image
 

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More mirrors

If you wish to be able to mirror along an arbitrary line see the answer Mirror along line

Answer 6

You don't need HTML5, it can be done with CSS same as in IE:

-moz-transform: scale(-1, 1);
-webkit-transform: scale(-1, 1);
-o-transform: scale(-1, 1);
transform: scale(-1, 1);
filter: FlipH;