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POHow to draw on an HTML5 Canvas, pixel-by-pixel
 

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  1. POHow to draw on an HTML5 Canvas, pixel-by-pixel
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    <p>Suppose that I have a 900x900 HTML5 Canvas element.</p> <p>I have a function called <code>computeRow</code> that accepts, as a parameter, the number of a row on the grid and returns an array of 900 numbers. Each number represents a number between 0 and 200. There is an array called <code>colors</code> that contains an array of strings like <code>rgb(0,20,20)</code>, for example.</p> <p>Basically, what I'm saying is that I have a function that tells pixel-by-pixel, what color each pixel in a given row on the canvas is supposed to be. Running this function many times, I can compute a color for every pixel on the canvas.</p> <p>The process of running <code>computeRow</code> 900 times takes about 0.5 seconds.</p> <p>However, the drawing of the image takes much longer than that.</p> <p>What I've done is I've written a function called <code>drawRow</code> that takes an array of 900 numbers as the input and draws them on the canvas. <code>drawRow</code> takes lots longer to run than <code>computeRow</code>! How can I fix this?</p> <p><code>drawRow</code> is dead simple. It looks like this:</p> <pre><code>function drawRow(rowNumber, result /* array */) { var plot, context, columnNumber, color; plot = document.getElementById('plot'); context = plot.getContext('2d'); // Iterate over the results for each column in the row, coloring a single pixel on // the canvas the correct color for each one. for(columnNumber = 0; columnNumber &lt; width; columnNumber++) { color = colors[result[columnNumber]]; context.fillStyle = color; context.fillRect(columnNumber, rowNumber, 1, 1); } } </code></pre>
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