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POHow to speed up band selection tool in a Dart WebGL application
 

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  1. POHow to speed up band selection tool in a Dart WebGL application
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    <p>The task at hand is to add a band selection tool to a Dart WebGL application. The tool will be used to draw a rectangle over multiple objects by dragging the mouse. Thus multiple objects can be selected/picked in a single user action. I'm currently using gl.readPixels() to read colors from an off-screen renderbuffer. Problem is, when a large area is band-selected, gl.readPixels() issues millions of pixels. Scanning such a big amount of colors wastes precious seconds just to locate few objects.</p> <p>Please anyone point possibly faster methods for band-selecting multiple objects with Dart+WebGL.</p> <p>For reference, I show below the current main portion of the band selection tool.</p> <pre class="lang-dart prettyprint-override"><code>Uint8List _color = new Uint8List(4); void bandSelection(int x, y, width, height, PickerShader picker, RenderingContext gl, bool shift) { if (picker == null) { err("bandSelection: picker not available"); return; } int size = 4 * width * height; if (size &gt; _color.length) { _color = new Uint8List(size); } gl.bindFramebuffer(RenderingContext.FRAMEBUFFER, picker.framebuffer); gl.readPixels(x, y, width, height, RenderingContext.RGBA, RenderingContext.UNSIGNED_BYTE, _color); if (!shift) { // shift is released _selection.clear(); } for (int i = 0; i &lt; size; i += 4) { if (_selection.length &gt;= picker.numberOfInstances) { // selected all available objects, no need to keep searching break; } PickerInstance pi = picker.findInstanceByColor(_color[i], _color[i+1], _color[i+2]); if (pi == null) { continue; } _selection.add(pi); } debug("bandSelection: $_selection"); } // findInstanceByColor is method from PickerShader PickerInstance findInstanceByColor(int r, g, b) { return colorHit(_instanceList, r, g, b); } PickerInstance colorHit(Iterable&lt;Instance&gt; list, int r,g,b) { bool match(Instance i) { Float32List f = i.pickColor; return (255.0*f[0] - r.toDouble()).abs() &lt; 1.0 &amp;&amp; (255.0*f[1] - g.toDouble()).abs() &lt; 1.0 &amp;&amp; (255.0*f[2] - b.toDouble()).abs() &lt; 1.0; } Instance pi; try { pi = list.firstWhere(match); } catch (e) { return null; } return pi as PickerInstance; } </code></pre>
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    1. COcan we see how ‘findInstanceByColor‘ is implemented ?
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    2. COHow did the pixels get to where they are in the first place, are they objects you already know the shape and position of because you rendered them? If so could you use the intersection of the drawn band shape with the known shapes to get what you want or is there some other reason you need to select by colour?
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    3. CO@nesderl: method findInstanceByColor included.
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