Javascript画布垂直偏斜图像

Question

Is it possible to skew an image on a canvas to make it appear as a reflection of another image?

Here is an illustration:

I need to flip and skew the image to get the desired effect, but I have not been able to get it to look like that. How can this be achieved?

I have found this example:

http://jsfiddle.net/jpnrzc9y/

ctx.save();
ctx.transform(1,0,0.3,-1,0,0);
ctx.drawImage(tree1,74,-117,20,40);
ctx.restore();

They seem to set the transform based on some random values.

But I cannot make sense of it. The values seem very random to me. Im trying to create a dynamic function that allows me to determine the amount of skew and that works for all images.

Answer 1

You can use context.scale to flip an image vertically:

// flip the canvas vertically
context.scale(1,-1);

Here are the steps to create a skewed reflection of an image:

1. Move (move==translate) the 0,0 origin to the desired x,y: ctx.translate(x,y);

2. Draw the original image image: ctx.drawImage(img,0,0);

3. Move to the bottom of the original image: ctx.translate(0,img.height);

4. Scale to vertically flip the image: ctx.scale(1,-1);

5. Apply a skew: ctx.transform(1,0,skewAngle,1,0,0);

6. Shrink the reflected image (just for aesthetics): ctx.scale(1,0.50);

7. Make the reflected image 50% opacity: ctx.globalAlpha=0.50;

8. Draw the reflected image: ctx.drawImage(img,0,-img.height);

9. Clean up by setting all transforms to default: ctx.setTransform(1,0,0,1,0,0);

Here's example code and a demo:

 var canvas=document.getElementById("canvas"); var ctx=canvas.getContext("2d"); var cw=canvas.width; var ch=canvas.height; var img=new Image(); img.onload=start; img.src="https://dl.dropboxusercontent.com/u/139992952/multple/character1.png"; function start(){ // 60x110 skewedReflection(img,25,25) } function skewedReflection(img,x,y){ // calc the 45 degree skew angle needed by ctx.transform var skewAngle=-Math.tan(Math.PI/4); // move the 0,0 origin to x,y ctx.translate(x,y); // draw the original image image ctx.drawImage(img,0,0); // move to the bottom of the original image ctx.translate(0,img.height); // use scale to flip the image ctx.scale(1,-1); // apply a skew ctx.transform(1,0,skewAngle,1,0,0); // shrink the reflected image ctx.scale(1,0.50); // make the reflected image 50% opacity ctx.globalAlpha=0.50; // draw the reflected image ctx.drawImage(img,0,-img.height); // clean up by setting all transforms to default ctx.setTransform(1,0,0,1,0,0); } 

 body{ background-color: ivory; } #canvas{border:1px solid red; margin:0 auto; } 

 <canvas id="canvas" width=300 height=300></canvas>