putImageData(), how to keep old pixels if new pixels are transparent?
In html5, when you draw to a canvas using putImageData(), if some of the pixels you are drawing are transparent (or semi-transparent), how do you keep old pixels in the canvas unaffected?
example:
var imgData = context.createImageData(30,30);
for(var i=0; i<imgData.data.length; i+=4)
imgData.data[i]=255;
imgData.data[i+1]=0;
imgData.data[i+2]=0;
imgData.data[i+3]=255;
if((i/4)%30 > 15)imgData.data[i+3] = 0;
context.putImageData(imgData,0,0);
The right half of the 30x30 rect is transparent.
If this is drawn over something on the canvas, pixels behind the right half are removed (or become thransparent). How do I keep them?
javascript html5 canvas transparency pixel
asked Mar 5 '14 at 9:56
Mohsin
15429
add a comment |
In html5, when you draw to a canvas using putImageData(), if some of the pixels you are drawing are transparent (or semi-transparent), how do you keep old pixels in the canvas unaffected?
example:
var imgData = context.createImageData(30,30);
for(var i=0; i<imgData.data.length; i+=4)
imgData.data[i]=255;
imgData.data[i+1]=0;
imgData.data[i+2]=0;
imgData.data[i+3]=255;
if((i/4)%30 > 15)imgData.data[i+3] = 0;
context.putImageData(imgData,0,0);
The right half of the 30x30 rect is transparent.
If this is drawn over something on the canvas, pixels behind the right half are removed (or become thransparent). How do I keep them?
javascript html5 canvas transparency pixel
asked Mar 5 '14 at 9:56
Mohsin
15429
add a comment |
In html5, when you draw to a canvas using putImageData(), if some of the pixels you are drawing are transparent (or semi-transparent), how do you keep old pixels in the canvas unaffected?
example:
var imgData = context.createImageData(30,30);
for(var i=0; i<imgData.data.length; i+=4)
imgData.data[i]=255;
imgData.data[i+1]=0;
imgData.data[i+2]=0;
imgData.data[i+3]=255;
if((i/4)%30 > 15)imgData.data[i+3] = 0;
context.putImageData(imgData,0,0);
The right half of the 30x30 rect is transparent.
If this is drawn over something on the canvas, pixels behind the right half are removed (or become thransparent). How do I keep them?
javascript html5 canvas transparency pixel
asked Mar 5 '14 at 9:56
Mohsin
15429
In html5, when you draw to a canvas using putImageData(), if some of the pixels you are drawing are transparent (or semi-transparent), how do you keep old pixels in the canvas unaffected?
example:
var imgData = context.createImageData(30,30);
for(var i=0; i<imgData.data.length; i+=4)
imgData.data[i]=255;
imgData.data[i+1]=0;
imgData.data[i+2]=0;
imgData.data[i+3]=255;
if((i/4)%30 > 15)imgData.data[i+3] = 0;
context.putImageData(imgData,0,0);
The right half of the 30x30 rect is transparent.
If this is drawn over something on the canvas, pixels behind the right half are removed (or become thransparent). How do I keep them?
javascript html5 canvas transparency pixel
javascript html5 canvas transparency pixel
asked Mar 5 '14 at 9:56
Mohsin
15429
asked Mar 5 '14 at 9:56
Mohsin
15429
edited Mar 5 '14 at 10:52
asked Mar 5 '14 at 9:56
Mohsin
15429
asked Mar 5 '14 at 9:56
Mohsin
15429
asked Mar 5 '14 at 9:56
Mohsin
15429
15429
add a comment |
add a comment |
4 Answers
4
active
oldest
votes
You can use getImageData to create a semi-transparent overlay:
- create a temporary offscreen canvas
- getImageData to get the pixel data from the offscreen canvas
- modify the pixels as you desire
- putImageData the pixels back on the offscreen canvas
- use drawImage to draw the offscreen canvas to the onscreen canvas
Here's example code and a Demo: http://jsfiddle.net/m1erickson/CM7uY/
<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<style>
body background-color: ivory;
canvasborder:1px solid red;
</style>
<script>
$(function()
var canvas=document.getElementById("canvas");
var context=canvas.getContext("2d");
// draw an image on the canvas
var img=new Image();
img.onload=start;
img.src="https://dl.dropboxusercontent.com/u/139992952/stack1/landscape1.jpg";
function start()
canvas.width=img.width;
canvas.height=img.height;
context.drawImage(img,0,0);
// overlay a red gradient
drawSemiTransparentOverlay(canvas.width/2,canvas.height)
function drawSemiTransparentOverlay(w,h)
// create a temporary canvas to hold the gradient overlay
var canvas2=document.createElement("canvas");
canvas2.width=w;
canvas2.height=h
var ctx2=canvas2.getContext("2d");
// make gradient using ImageData
var imgData = ctx2.getImageData(0,0,w,h);
var data=imgData.data;
for(var y=0; y<h; y++)
for(var x=0; x<w; x++)
var n=((w*y)+x)*4;
data[n]=255;
data[n+1]=0;
data[n+2]=0;
data[n+3]=255;
if(x>w/2)
data[n+3]=255*(1-((x-w/2)/(w/2)));
// put the modified pixels on the temporary canvas
ctx2.putImageData(imgData,0,0);
// draw the temporary gradient canvas on the visible canvas
context.drawImage(canvas2,0,0);
); // end $(function());
</script>
</head>
<body>
<canvas id="canvas" width=200 height=200></canvas>
</body>
</html>
Alternatively, you might check out using a linear gradient to do your effect more directly.
http://jsfiddle.net/m1erickson/j6wLR/
answered Mar 5 '14 at 16:20
markE
81.3k9108126
Thanks, drawing a canvas onto another using drawImage() is exactly what I need.
– Mohsin
Mar 6 '14 at 17:18
add a comment |
Problem
As you know, your statement
if((i/4)%30 > 15)imgData.data[i+3] = 0;
will make pixels on the right half of the image be transparent, so that any other object on the page behind the canvas can be seen through the canvas at that pixel position. However, you are still overwriting the pixel of the canvas itself with context.putImageData, which replaces all of its previous pixels. The transparency that you add will not cause the previous pixels of to show through, because the result of putImageData is not a second set of pixels on top of the previous pixels in the canvas, but rather the replacement of existing pixels.
Solution
I suggest that you begin your code not with createImageData which will begin with a blank set of data, but rather with getImageData which will give you a copy of the existing data to work with. You can then use your conditional statement to avoid overwriting the portion of the image that you wish to preserve. This will also make your function more efficient.
var imgData = context.getImageData(30,30);
for(var i=0; i<imgData.data.length; i+=4)
if((i/4)%30 > 15) continue;
imgData.data[i]=255;
imgData.data[i+1]=0;
imgData.data[i+2]=0;
imgData.data[i+3]=255;
context.putImageData(imgData,0,0);
answered Mar 5 '14 at 16:44
Joseph Myers
5,2291934
Thanks. You are right, using getImageData() you can do whatever you want with old pixels, but you need to have a formula to blend new pixels with old ones. I've tried one and it was perfect, but soon when I began to use this in a for loop (for drawing), I got a strange result when I draw something that is transparent.
– Mohsin
Mar 6 '14 at 17:31
add a comment |
I wanted to copy a CRISP, un modified version of the canvas on top of itself. I eventually came up with this solution, which applies.
https://jsfiddle.net/4Le454ak/1/
The copy portion is in this code:
var imageData = canvas.toDataURL(0, 0, w, h);
var tmp = document.createElement('img');
tmp.style.display = 'none'
tmp.src = imageData;
document.body.appendChild(tmp);
ctx.drawImage(tmp, 30, 30);
What's happening:
- copy image data from canvas
- set image data to an unseen ( has to be in dom though)
- draw that image back onto the canvas
- you can delete or reuse the at this point
answered Apr 4 '17 at 14:24
Jacksonkr
17.7k34137234
add a comment |
Something that tripped me up that may be of use... I had problems with this because I assumed that putImageData() and drawImage() would work in the same way but it seems they don't. putImageData() will overwrite existing pixels with its own transparent data while drawImage() will leave them untouched.
When looking into this I just glanced at the docs for CanvasRenderingContext2D.globalCompositeOperation (should have read more closely), saw that source-over is the default and didn't realise this would not apply to putImageData()
Drawing into a temporary canvas then and using drawImage() to add the temp canvas to the main context was the solution I needed so cheers for that.
answered Nov 10 at 13:03
popClingwrap
1,30921431
add a comment |
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4 Answers
4
active
oldest
votes
4 Answers
4
active
oldest
votes
active
oldest
votes
active
oldest
votes
You can use getImageData to create a semi-transparent overlay:
- create a temporary offscreen canvas
- getImageData to get the pixel data from the offscreen canvas
- modify the pixels as you desire
- putImageData the pixels back on the offscreen canvas
- use drawImage to draw the offscreen canvas to the onscreen canvas
Here's example code and a Demo: http://jsfiddle.net/m1erickson/CM7uY/
<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<style>
body background-color: ivory;
canvasborder:1px solid red;
</style>
<script>
$(function()
var canvas=document.getElementById("canvas");
var context=canvas.getContext("2d");
// draw an image on the canvas
var img=new Image();
img.onload=start;
img.src="https://dl.dropboxusercontent.com/u/139992952/stack1/landscape1.jpg";
function start()
canvas.width=img.width;
canvas.height=img.height;
context.drawImage(img,0,0);
// overlay a red gradient
drawSemiTransparentOverlay(canvas.width/2,canvas.height)
function drawSemiTransparentOverlay(w,h)
// create a temporary canvas to hold the gradient overlay
var canvas2=document.createElement("canvas");
canvas2.width=w;
canvas2.height=h
var ctx2=canvas2.getContext("2d");
// make gradient using ImageData
var imgData = ctx2.getImageData(0,0,w,h);
var data=imgData.data;
for(var y=0; y<h; y++)
for(var x=0; x<w; x++)
var n=((w*y)+x)*4;
data[n]=255;
data[n+1]=0;
data[n+2]=0;
data[n+3]=255;
if(x>w/2)
data[n+3]=255*(1-((x-w/2)/(w/2)));
// put the modified pixels on the temporary canvas
ctx2.putImageData(imgData,0,0);
// draw the temporary gradient canvas on the visible canvas
context.drawImage(canvas2,0,0);
); // end $(function());
</script>
</head>
<body>
<canvas id="canvas" width=200 height=200></canvas>
</body>
</html>
Alternatively, you might check out using a linear gradient to do your effect more directly.
http://jsfiddle.net/m1erickson/j6wLR/
answered Mar 5 '14 at 16:20
markE
81.3k9108126
Thanks, drawing a canvas onto another using drawImage() is exactly what I need.
– Mohsin
Mar 6 '14 at 17:18
add a comment |
You can use getImageData to create a semi-transparent overlay:
- create a temporary offscreen canvas
- getImageData to get the pixel data from the offscreen canvas
- modify the pixels as you desire
- putImageData the pixels back on the offscreen canvas
- use drawImage to draw the offscreen canvas to the onscreen canvas
Here's example code and a Demo: http://jsfiddle.net/m1erickson/CM7uY/
<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<style>
body background-color: ivory;
canvasborder:1px solid red;
</style>
<script>
$(function()
var canvas=document.getElementById("canvas");
var context=canvas.getContext("2d");
// draw an image on the canvas
var img=new Image();
img.onload=start;
img.src="https://dl.dropboxusercontent.com/u/139992952/stack1/landscape1.jpg";
function start()
canvas.width=img.width;
canvas.height=img.height;
context.drawImage(img,0,0);
// overlay a red gradient
drawSemiTransparentOverlay(canvas.width/2,canvas.height)
function drawSemiTransparentOverlay(w,h)
// create a temporary canvas to hold the gradient overlay
var canvas2=document.createElement("canvas");
canvas2.width=w;
canvas2.height=h
var ctx2=canvas2.getContext("2d");
// make gradient using ImageData
var imgData = ctx2.getImageData(0,0,w,h);
var data=imgData.data;
for(var y=0; y<h; y++)
for(var x=0; x<w; x++)
var n=((w*y)+x)*4;
data[n]=255;
data[n+1]=0;
data[n+2]=0;
data[n+3]=255;
if(x>w/2)
data[n+3]=255*(1-((x-w/2)/(w/2)));
// put the modified pixels on the temporary canvas
ctx2.putImageData(imgData,0,0);
// draw the temporary gradient canvas on the visible canvas
context.drawImage(canvas2,0,0);
); // end $(function());
</script>
</head>
<body>
<canvas id="canvas" width=200 height=200></canvas>
</body>
</html>
Alternatively, you might check out using a linear gradient to do your effect more directly.
http://jsfiddle.net/m1erickson/j6wLR/
answered Mar 5 '14 at 16:20
markE
81.3k9108126
Thanks, drawing a canvas onto another using drawImage() is exactly what I need.
– Mohsin
Mar 6 '14 at 17:18
add a comment |
You can use getImageData to create a semi-transparent overlay:
- create a temporary offscreen canvas
- getImageData to get the pixel data from the offscreen canvas
- modify the pixels as you desire
- putImageData the pixels back on the offscreen canvas
- use drawImage to draw the offscreen canvas to the onscreen canvas
Here's example code and a Demo: http://jsfiddle.net/m1erickson/CM7uY/
<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<style>
body background-color: ivory;
canvasborder:1px solid red;
</style>
<script>
$(function()
var canvas=document.getElementById("canvas");
var context=canvas.getContext("2d");
// draw an image on the canvas
var img=new Image();
img.onload=start;
img.src="https://dl.dropboxusercontent.com/u/139992952/stack1/landscape1.jpg";
function start()
canvas.width=img.width;
canvas.height=img.height;
context.drawImage(img,0,0);
// overlay a red gradient
drawSemiTransparentOverlay(canvas.width/2,canvas.height)
function drawSemiTransparentOverlay(w,h)
// create a temporary canvas to hold the gradient overlay
var canvas2=document.createElement("canvas");
canvas2.width=w;
canvas2.height=h
var ctx2=canvas2.getContext("2d");
// make gradient using ImageData
var imgData = ctx2.getImageData(0,0,w,h);
var data=imgData.data;
for(var y=0; y<h; y++)
for(var x=0; x<w; x++)
var n=((w*y)+x)*4;
data[n]=255;
data[n+1]=0;
data[n+2]=0;
data[n+3]=255;
if(x>w/2)
data[n+3]=255*(1-((x-w/2)/(w/2)));
// put the modified pixels on the temporary canvas
ctx2.putImageData(imgData,0,0);
// draw the temporary gradient canvas on the visible canvas
context.drawImage(canvas2,0,0);
); // end $(function());
</script>
</head>
<body>
<canvas id="canvas" width=200 height=200></canvas>
</body>
</html>
Alternatively, you might check out using a linear gradient to do your effect more directly.
http://jsfiddle.net/m1erickson/j6wLR/
answered Mar 5 '14 at 16:20
markE
81.3k9108126
You can use getImageData to create a semi-transparent overlay:
- create a temporary offscreen canvas
- getImageData to get the pixel data from the offscreen canvas
- modify the pixels as you desire
- putImageData the pixels back on the offscreen canvas
- use drawImage to draw the offscreen canvas to the onscreen canvas
Here's example code and a Demo: http://jsfiddle.net/m1erickson/CM7uY/
<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<style>
body background-color: ivory;
canvasborder:1px solid red;
</style>
<script>
$(function()
var canvas=document.getElementById("canvas");
var context=canvas.getContext("2d");
// draw an image on the canvas
var img=new Image();
img.onload=start;
img.src="https://dl.dropboxusercontent.com/u/139992952/stack1/landscape1.jpg";
function start()
canvas.width=img.width;
canvas.height=img.height;
context.drawImage(img,0,0);
// overlay a red gradient
drawSemiTransparentOverlay(canvas.width/2,canvas.height)
function drawSemiTransparentOverlay(w,h)
// create a temporary canvas to hold the gradient overlay
var canvas2=document.createElement("canvas");
canvas2.width=w;
canvas2.height=h
var ctx2=canvas2.getContext("2d");
// make gradient using ImageData
var imgData = ctx2.getImageData(0,0,w,h);
var data=imgData.data;
for(var y=0; y<h; y++)
for(var x=0; x<w; x++)
var n=((w*y)+x)*4;
data[n]=255;
data[n+1]=0;
data[n+2]=0;
data[n+3]=255;
if(x>w/2)
data[n+3]=255*(1-((x-w/2)/(w/2)));
// put the modified pixels on the temporary canvas
ctx2.putImageData(imgData,0,0);
// draw the temporary gradient canvas on the visible canvas
context.drawImage(canvas2,0,0);
); // end $(function());
</script>
</head>
<body>
<canvas id="canvas" width=200 height=200></canvas>
</body>
</html>
Alternatively, you might check out using a linear gradient to do your effect more directly.
http://jsfiddle.net/m1erickson/j6wLR/
answered Mar 5 '14 at 16:20
markE
81.3k9108126
edited Mar 5 '14 at 19:22
answered Mar 5 '14 at 16:20
markE
81.3k9108126
answered Mar 5 '14 at 16:20
markE
81.3k9108126
answered Mar 5 '14 at 16:20
markE
81.3k9108126
81.3k9108126
Thanks, drawing a canvas onto another using drawImage() is exactly what I need.
– Mohsin
Mar 6 '14 at 17:18
add a comment |
Thanks, drawing a canvas onto another using drawImage() is exactly what I need.
– Mohsin
Mar 6 '14 at 17:18
Thanks, drawing a canvas onto another using drawImage() is exactly what I need.
– Mohsin
Mar 6 '14 at 17:18
Thanks, drawing a canvas onto another using drawImage() is exactly what I need.
– Mohsin
Mar 6 '14 at 17:18
add a comment |
Problem
As you know, your statement
if((i/4)%30 > 15)imgData.data[i+3] = 0;
will make pixels on the right half of the image be transparent, so that any other object on the page behind the canvas can be seen through the canvas at that pixel position. However, you are still overwriting the pixel of the canvas itself with context.putImageData, which replaces all of its previous pixels. The transparency that you add will not cause the previous pixels of to show through, because the result of putImageData is not a second set of pixels on top of the previous pixels in the canvas, but rather the replacement of existing pixels.
Solution
I suggest that you begin your code not with createImageData which will begin with a blank set of data, but rather with getImageData which will give you a copy of the existing data to work with. You can then use your conditional statement to avoid overwriting the portion of the image that you wish to preserve. This will also make your function more efficient.
var imgData = context.getImageData(30,30);
for(var i=0; i<imgData.data.length; i+=4)
if((i/4)%30 > 15) continue;
imgData.data[i]=255;
imgData.data[i+1]=0;
imgData.data[i+2]=0;
imgData.data[i+3]=255;
context.putImageData(imgData,0,0);
answered Mar 5 '14 at 16:44
Joseph Myers
5,2291934
Thanks. You are right, using getImageData() you can do whatever you want with old pixels, but you need to have a formula to blend new pixels with old ones. I've tried one and it was perfect, but soon when I began to use this in a for loop (for drawing), I got a strange result when I draw something that is transparent.
– Mohsin
Mar 6 '14 at 17:31
add a comment |
Problem
As you know, your statement
if((i/4)%30 > 15)imgData.data[i+3] = 0;
will make pixels on the right half of the image be transparent, so that any other object on the page behind the canvas can be seen through the canvas at that pixel position. However, you are still overwriting the pixel of the canvas itself with context.putImageData, which replaces all of its previous pixels. The transparency that you add will not cause the previous pixels of to show through, because the result of putImageData is not a second set of pixels on top of the previous pixels in the canvas, but rather the replacement of existing pixels.
Solution
I suggest that you begin your code not with createImageData which will begin with a blank set of data, but rather with getImageData which will give you a copy of the existing data to work with. You can then use your conditional statement to avoid overwriting the portion of the image that you wish to preserve. This will also make your function more efficient.
var imgData = context.getImageData(30,30);
for(var i=0; i<imgData.data.length; i+=4)
if((i/4)%30 > 15) continue;
imgData.data[i]=255;
imgData.data[i+1]=0;
imgData.data[i+2]=0;
imgData.data[i+3]=255;
context.putImageData(imgData,0,0);
answered Mar 5 '14 at 16:44
Joseph Myers
5,2291934
Thanks. You are right, using getImageData() you can do whatever you want with old pixels, but you need to have a formula to blend new pixels with old ones. I've tried one and it was perfect, but soon when I began to use this in a for loop (for drawing), I got a strange result when I draw something that is transparent.
– Mohsin
Mar 6 '14 at 17:31
add a comment |
Problem
As you know, your statement
if((i/4)%30 > 15)imgData.data[i+3] = 0;
will make pixels on the right half of the image be transparent, so that any other object on the page behind the canvas can be seen through the canvas at that pixel position. However, you are still overwriting the pixel of the canvas itself with context.putImageData, which replaces all of its previous pixels. The transparency that you add will not cause the previous pixels of to show through, because the result of putImageData is not a second set of pixels on top of the previous pixels in the canvas, but rather the replacement of existing pixels.
Solution
I suggest that you begin your code not with createImageData which will begin with a blank set of data, but rather with getImageData which will give you a copy of the existing data to work with. You can then use your conditional statement to avoid overwriting the portion of the image that you wish to preserve. This will also make your function more efficient.
var imgData = context.getImageData(30,30);
for(var i=0; i<imgData.data.length; i+=4)
if((i/4)%30 > 15) continue;
imgData.data[i]=255;
imgData.data[i+1]=0;
imgData.data[i+2]=0;
imgData.data[i+3]=255;
context.putImageData(imgData,0,0);
answered Mar 5 '14 at 16:44
Joseph Myers
5,2291934
Problem
As you know, your statement
if((i/4)%30 > 15)imgData.data[i+3] = 0;
will make pixels on the right half of the image be transparent, so that any other object on the page behind the canvas can be seen through the canvas at that pixel position. However, you are still overwriting the pixel of the canvas itself with context.putImageData, which replaces all of its previous pixels. The transparency that you add will not cause the previous pixels of to show through, because the result of putImageData is not a second set of pixels on top of the previous pixels in the canvas, but rather the replacement of existing pixels.
Solution
I suggest that you begin your code not with createImageData which will begin with a blank set of data, but rather with getImageData which will give you a copy of the existing data to work with. You can then use your conditional statement to avoid overwriting the portion of the image that you wish to preserve. This will also make your function more efficient.
var imgData = context.getImageData(30,30);
for(var i=0; i<imgData.data.length; i+=4)
if((i/4)%30 > 15) continue;
imgData.data[i]=255;
imgData.data[i+1]=0;
imgData.data[i+2]=0;
imgData.data[i+3]=255;
context.putImageData(imgData,0,0);
answered Mar 5 '14 at 16:44
Joseph Myers
5,2291934
edited Mar 5 '14 at 16:51
answered Mar 5 '14 at 16:44
Joseph Myers
5,2291934
answered Mar 5 '14 at 16:44
Joseph Myers
5,2291934
answered Mar 5 '14 at 16:44
Joseph Myers
5,2291934
5,2291934
Thanks. You are right, using getImageData() you can do whatever you want with old pixels, but you need to have a formula to blend new pixels with old ones. I've tried one and it was perfect, but soon when I began to use this in a for loop (for drawing), I got a strange result when I draw something that is transparent.
– Mohsin
Mar 6 '14 at 17:31
add a comment |
Thanks. You are right, using getImageData() you can do whatever you want with old pixels, but you need to have a formula to blend new pixels with old ones. I've tried one and it was perfect, but soon when I began to use this in a for loop (for drawing), I got a strange result when I draw something that is transparent.
– Mohsin
Mar 6 '14 at 17:31
Thanks. You are right, using getImageData() you can do whatever you want with old pixels, but you need to have a formula to blend new pixels with old ones. I've tried one and it was perfect, but soon when I began to use this in a for loop (for drawing), I got a strange result when I draw something that is transparent.
– Mohsin
Mar 6 '14 at 17:31
Thanks. You are right, using getImageData() you can do whatever you want with old pixels, but you need to have a formula to blend new pixels with old ones. I've tried one and it was perfect, but soon when I began to use this in a for loop (for drawing), I got a strange result when I draw something that is transparent.
– Mohsin
Mar 6 '14 at 17:31
add a comment |
I wanted to copy a CRISP, un modified version of the canvas on top of itself. I eventually came up with this solution, which applies.
https://jsfiddle.net/4Le454ak/1/
The copy portion is in this code:
var imageData = canvas.toDataURL(0, 0, w, h);
var tmp = document.createElement('img');
tmp.style.display = 'none'
tmp.src = imageData;
document.body.appendChild(tmp);
ctx.drawImage(tmp, 30, 30);
What's happening:
- copy image data from canvas
- set image data to an unseen ( has to be in dom though)
- draw that image back onto the canvas
- you can delete or reuse the at this point
answered Apr 4 '17 at 14:24
Jacksonkr
17.7k34137234
add a comment |
I wanted to copy a CRISP, un modified version of the canvas on top of itself. I eventually came up with this solution, which applies.
https://jsfiddle.net/4Le454ak/1/
The copy portion is in this code:
var imageData = canvas.toDataURL(0, 0, w, h);
var tmp = document.createElement('img');
tmp.style.display = 'none'
tmp.src = imageData;
document.body.appendChild(tmp);
ctx.drawImage(tmp, 30, 30);
What's happening:
- copy image data from canvas
- set image data to an unseen ( has to be in dom though)
- draw that image back onto the canvas
- you can delete or reuse the at this point
answered Apr 4 '17 at 14:24
Jacksonkr
17.7k34137234
add a comment |
I wanted to copy a CRISP, un modified version of the canvas on top of itself. I eventually came up with this solution, which applies.
https://jsfiddle.net/4Le454ak/1/
The copy portion is in this code:
var imageData = canvas.toDataURL(0, 0, w, h);
var tmp = document.createElement('img');
tmp.style.display = 'none'
tmp.src = imageData;
document.body.appendChild(tmp);
ctx.drawImage(tmp, 30, 30);
What's happening:
- copy image data from canvas
- set image data to an unseen ( has to be in dom though)
- draw that image back onto the canvas
- you can delete or reuse the at this point
answered Apr 4 '17 at 14:24
Jacksonkr
17.7k34137234
I wanted to copy a CRISP, un modified version of the canvas on top of itself. I eventually came up with this solution, which applies.
https://jsfiddle.net/4Le454ak/1/
The copy portion is in this code:
var imageData = canvas.toDataURL(0, 0, w, h);
var tmp = document.createElement('img');
tmp.style.display = 'none'
tmp.src = imageData;
document.body.appendChild(tmp);
ctx.drawImage(tmp, 30, 30);
What's happening:
- copy image data from canvas
- set image data to an unseen ( has to be in dom though)
- draw that image back onto the canvas
- you can delete or reuse the at this point
answered Apr 4 '17 at 14:24
Jacksonkr
17.7k34137234
answered Apr 4 '17 at 14:24
Jacksonkr
17.7k34137234
answered Apr 4 '17 at 14:24
Jacksonkr
17.7k34137234
answered Apr 4 '17 at 14:24
Jacksonkr
17.7k34137234
17.7k34137234
add a comment |
add a comment |
Something that tripped me up that may be of use... I had problems with this because I assumed that putImageData() and drawImage() would work in the same way but it seems they don't. putImageData() will overwrite existing pixels with its own transparent data while drawImage() will leave them untouched.
When looking into this I just glanced at the docs for CanvasRenderingContext2D.globalCompositeOperation (should have read more closely), saw that source-over is the default and didn't realise this would not apply to putImageData()
Drawing into a temporary canvas then and using drawImage() to add the temp canvas to the main context was the solution I needed so cheers for that.
answered Nov 10 at 13:03
popClingwrap
1,30921431
add a comment |
Something that tripped me up that may be of use... I had problems with this because I assumed that putImageData() and drawImage() would work in the same way but it seems they don't. putImageData() will overwrite existing pixels with its own transparent data while drawImage() will leave them untouched.
When looking into this I just glanced at the docs for CanvasRenderingContext2D.globalCompositeOperation (should have read more closely), saw that source-over is the default and didn't realise this would not apply to putImageData()
Drawing into a temporary canvas then and using drawImage() to add the temp canvas to the main context was the solution I needed so cheers for that.
answered Nov 10 at 13:03
popClingwrap
1,30921431
add a comment |
Something that tripped me up that may be of use... I had problems with this because I assumed that putImageData() and drawImage() would work in the same way but it seems they don't. putImageData() will overwrite existing pixels with its own transparent data while drawImage() will leave them untouched.
When looking into this I just glanced at the docs for CanvasRenderingContext2D.globalCompositeOperation (should have read more closely), saw that source-over is the default and didn't realise this would not apply to putImageData()
Drawing into a temporary canvas then and using drawImage() to add the temp canvas to the main context was the solution I needed so cheers for that.
answered Nov 10 at 13:03
popClingwrap
1,30921431
Something that tripped me up that may be of use... I had problems with this because I assumed that putImageData() and drawImage() would work in the same way but it seems they don't. putImageData() will overwrite existing pixels with its own transparent data while drawImage() will leave them untouched.
When looking into this I just glanced at the docs for CanvasRenderingContext2D.globalCompositeOperation (should have read more closely), saw that source-over is the default and didn't realise this would not apply to putImageData()
Drawing into a temporary canvas then and using drawImage() to add the temp canvas to the main context was the solution I needed so cheers for that.
answered Nov 10 at 13:03
popClingwrap
1,30921431
answered Nov 10 at 13:03
popClingwrap
1,30921431
answered Nov 10 at 13:03
popClingwrap
1,30921431
answered Nov 10 at 13:03
popClingwrap
1,30921431
1,30921431
add a comment |
add a comment |
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