[osg-users] Some issues with osgText / Signed Distance Field rendering

[OpenSceneGraph Users]

Hello Robert,

 

I toyed around with SDF and discovered some problems in the fragment shader 
that I managed to solve.

 

There seems to be a bug in the multisampling/blending code. Affected are 
semi-transparent texts. It’s most obvious for glyphs with a big fill area, 
like e.g. for the bullet character. In the attached picture, the upper 
bullet character was rendered with the original shader and an alpha value 
of 0.2. It looks totally wrong. For comparison, the second line was 
rendered with an alpha value of 1 and is fine.


In order to find out what’s wrong I rewrote the code applying the basic 
blending “trick” – doing linear combinations of colours with premultiplied 
alpha only. (Let’s call them colours in pma format, the transform function 
being pma[ (r,g,b,a) ] -> (r*a,g*a,b*a,a).) This resolved the issue for me, 
semi-transparent characters now render visually correct.


So, what I did, is:

   1. Apply pma transformation to the source colours (vertex colour, border 
   colour) 
   2. Do the multisampling. Simply calculate the mean of the sampled 
   colours. Each sampled colour is a linear combination of the source colours, 
   so too in pma format. 
   3. Apply inverse pma transformation for to get the resulting fragment 
   colour (to be blended with default blend mode (GL_SRC_ALPHA, 
   GL_ONE_MINUS_SRC_ALPHA)). 

This algorithm can be simplified. Obviously, the resulting fragment colour 
too is just a linear combination of the source colours. So, I changed the 
multisampling loop to only sample the coefficients for the source 
colours.(It’s applying distributive law.) This in turn made me get rid of 
the pma and inverse pma transformations.

 

And it also helped resolving the second issue that I had with the shader: 
Shadow backdrops were not rendered correctly. The problem was caused but 
following lines of code:

 

    total_color.rgb /= total_color.a;

    total_color.a *= scale;

    return total_color;

 

I think the division could turn out to be 0.0 / 0.0 which may have produced 
NaNs or unspecified results.

 

A minor issue I encountered that prevented the shader to compile on my GLSL 
ES 1.0 system. 

The function texture2DLod is specified by the GLSL ES 1.0, GLSL 1.1, GLSL 
1.2 specifications only for use in vertex shader. It may be available 
though. In my case it wasn’t, but the extension texture2DLodEXT was.

So I made an extra check to prefer that extension, if available, for GLSL 
ES 1.0 only.

 

I experienced another issue that I wasn’t able to solve so far.

On my embedded ES 2.0 system, the GL_OES_standard_derivatives extension is 
available but the functions dFdx / dFdy just seem to deliver bad results.

I don’t know why. Could it be non-conforming usage? A driver or GPU bug? As 
a workaround, I’ve switched to using a simpler shader for that system only, 
which doesn’t do multisampling. The result seems still fine.

 

Please check out my changes. I’ll do a pull request. I hope this is 
helpful, SDF is really a cool feature.

The shader could surely further be simplified when adapting it to blend 
mode (GL_ONE, GL_ONE_MINUS_SRC_ALPHA), which would be a bit easier to deal 
with.

I also wondered if the extensive bounds checking for smoothstep when 
sampling the colours/coefficients is still adequate for performance.

 

Regards,

Hannes

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