Databases
StackOverflow2013
Postsdbo
POCreating an image with canvas in Javascript
Body
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POCreating an image with canvas in Javascript
    text
    copied!<p>I seem to have hit an error message that I'm having a hard time debugging. What I'm trying to do is dynamically create an image such as this one ( <a href="http://www.webreference.com/programming/javascript/mk/column3/creating/cp_mini_gradient_details.png" rel="nofollow noreferrer">http://www.webreference.com/programming/javascript/mk/column3/creating/cp_mini_gradient_details.png</a> ) with canvas.</p> <p>The error message I'm getting is: </p> <pre><code>[Exception... "An invalid or illegal string was specified" code: "12" nsresult: "0x8053000c (NS_ERROR_DOM_SYNTAX_ERR)"] </code></pre> <p>I've googled and there seems to be a number of causes for this error message, but I can't seem to find any relating to canvas specifically.</p> <p>Here is my code:</p> <pre><code>var newCanvas = document.createElement('canvas'); newCanvas.height="30"; newCanvas.width="113"; document.body.appendChild(newCanvas); var context = newCanvas.getContext('2d'); context.fillStyle = '#FFFFFF'; context.fillRect(0, 0, 113, 15); context.fillStyle = '#000000'; context.fillRect(0, 15, 113, 30); var numCanH = Number(newCanvas.height); var numCanW = Number(newCanvas.width); var imgd; if (context.createImageData) { console.log('context.createImageData'); imgd = context.createImageData(numCanW, numCanH); } else if (context.getImageData) { console.log('context.getImageData'); imgd = context.getImageData(0, 0, numCanW, numCanH); } else { console.log('else'); imgd = {'width' : numCanW, 'height' : numCanH, 'data' : new Array(numCanW*numCanH*4)}; } var pix = imgd.data; var ndv = numCanW/6; for (var i = 0; i &lt;= numCanH; i++) { var a=1-Math.abs(2*i-numCanH)/numCanH; for (var j = 0; j &lt; numCanW; j+=4) { var bitUp = Math.ceil((255/130)*j); var bitDown = 255-bitUp; if(j&lt;(ndv)){ //Red to Yellow - (rgb) 255,0,0 to 255,255,0 pix[j] = 255; // red channel pix[j+1] = bitUp; // green channel pix[j+2] = 0; // blue channel } else if(j&gt;(ndv) &amp;&amp; j&lt;(ndv)*2){ //Yellow to Green - (rgb) 255,255,0 to 0,255,0 pix[j] = 255; // red channel pix[j+1] = bitDown; // green channel pix[j+2] = 0; // blue channel } else if(j&gt;(ndv)*2 &amp;&amp; j&lt;(ndv)*3){ //Green to Cyan - (rgb) 0,255,0 to 0,255,255 pix[j] = 0; // red channel pix[j+1] = 255; // green channel pix[j+2] = bitUp; // blue channel } else if(j&gt;(ndv)*3 &amp;&amp; j&lt;(ndv)*4){ //Cyan to Blue - (rgb) 0,255,255 to 0,0,255 pix[j] = 0; // red channel pix[j+1] = bitDown; // green channel pix[j+2] = 255; // blue channel } else if(j&gt;(ndv)*4 &amp;&amp; j&lt;(ndv)*5){ //Blue to Magenta - (rgb) 0,0,255 to 255,0,255 pix[j] = bitUp; // red channel pix[j+1] = 0; // green channel pix[j+2] = 255; // blue channel } else if(j&gt;(ndv)*5 &amp;&amp; j&lt;(ndv)*6){ //Magenta to Red - (rgb) 255,0,255 to 255,0,0 pix[j] = 255; // red channel pix[j+1] = 0; // green channel pix[j+2] = bitDown; // blue channel } pix[j+3] = a; // alpha channel+ console.log('bitUp '+bitUp); console.log(typeof bitUp); console.log('bitDown '+bitDown); console.log(typeof bitDown); console.log('a '+j); console.log(typeof a); console.log('j '+j); console.log(typeof j); console.log('i '+i); console.log(typeof i); console.log(imgd); console.log('before context.putImageData'); context.putImageData(imgd, j,i); console.log('after context.putImageData'); } } </code></pre> <p>The weird thing is, the error only occurs during the second iteration of the loop.</p>
 

Querying!

 
Guidance
An individual column

Larger individual text columns get their own page to allow for proper reading.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload