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POMinimum size of rendered object using GL_LINES in iOS Open GL ES
 

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  1. POMinimum size of rendered object using GL_LINES in iOS Open GL ES
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    <p>I have just completed the first version of my iOS app, <a href="http://itunes.apple.com/us/app/corebox/id503591554" rel="nofollow noreferrer">Corebox</a>, and am now working on some new features.</p> <p>One of the new features is a "small" tweak to the OpenGL rendering to force some objects to never be drawn smaller than a minimum size. All of the objects needing this treatment are simple 2 point lines drawn with GL_LINES.</p> <p>This annotated screenshot explains what I'm after. Ignore the grey lines, the only objects I'm interested in altering are the yellow wider lines.</p> <p><img src="https://i.stack.imgur.com/cjkKM.png" alt="Corebox OpenGL scene"></p> <p>I have googled this extensively and it seems what I need to do is alter the geometry of the lines using a vertex shader. I'm quite new to GLSL and most shader examples I can find deal with applying lighting and other effects, eg: <a href="http://glsl.heroku.com" rel="nofollow noreferrer">GLSL Heroku Editor</a> and <a href="http://www.kickjs.org/example/shader_editor/shader_editor.html" rel="nofollow noreferrer">KicksJS shader editor</a>.</p> <p>My current vertex shader is extremely basic:</p> <pre><code>// GL_LINES vertex shader uniform mat4 Projection; uniform mat4 Modelview; attribute vec4 Position; attribute vec4 SourceColor; varying vec4 DestinationColor; void main(void) { DestinationColor = SourceColor; gl_Position = Projection * Modelview * Position; } </code></pre> <p>As is my fragment shader:</p> <pre><code>// GL_LINES fragment shader varying lowp vec4 DestinationColor; void main(void) { gl_FragColor = DestinationColor; } </code></pre> <p>My guess as to what is required:</p> <ul> <li>Determine the distance between the viewer (camera position) and the object</li> <li>Determine how big the object is on the screen, based on its size and distance from camera</li> <li>If the object will be too small then adjust its vertices such that it becomes large enough to easily see on the screen.</li> </ul> <p>Caveats and other notes:</p> <ol> <li>But if you zoom out won't this cause the model to be just a blob of orange on the screen? Yes, this is exactly the effect I'm after.</li> </ol> <p><strong>Edit: Here is the final working version implementing suggestions by mifortin</strong></p> <pre><code>uniform mat4 Projection; uniform mat4 Modelview; uniform float MinimumHeight; attribute vec4 Position; attribute vec4 ObjectCenter; attribute vec4 SourceColor; varying vec4 DestinationColor; void main(void) { // screen-space position of this vertex vec4 screenPosition = Projection * Modelview * Position; // screen-space mid-point of the object this vertex belongs to vec4 screenObjectCenter = Projection * Modelview * ObjectCenter; // Z should be 0 by this time and the projective transform in w. // scale so w = 1 (these two should be in screen-space) vec2 newScreenPosition = screenPosition.xy / screenPosition.w; vec2 newObjectCenter = screenObjectCenter.xy / screenObjectCenter.w; float d = distance(newScreenPosition, newObjectCenter); if (d &lt; MinimumHeight &amp;&amp; d &gt; 0.0) { // Direction of this object, this really only makes sense in the context // of a line (eg: GL_LINES) vec2 towards = normalize(newScreenPosition - newObjectCenter); // Shift the center point then adjust the vertex position accordingly // Basically this converts: *--x--* into *--------x--------* newObjectCenter = newObjectCenter + towards * MinimumHeight; screenPosition.xy = newObjectCenter.xy * screenPosition.w; } gl_Position = screenPosition; DestinationColor = SourceColor; } </code></pre>
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