[osg-users] Major changes to osgText merged into master, provide high quality text including SDF support

[Robert Osfield]

Hi All,

Over the last couple of months I've been working on changes to osgText that
change the way text is rendered with the aim of improving visual quality,
in particular when using outline and shadows or zooming and zooming out.

The new implementation uses shaders to implement outline and shadows, the
old multi-pass approach has been removed completely.  If your hardware
doesn't support shaders then I'm afraid you'll no longer see
outlines/shadows by as it's been well over a decade that GLSL has become
mainstream I don't think this should be an issue.  Removing the old codes
paths help simplify the internals of osgText.

The use of shaders also opens the door to implement new techniques the most
significant one is that osgText now support generation of and use of Signed
Distance Field (SDF) based glyph textures. The SDF texture data is
automatically created from the original grey scale image data that freetype
and other font reading plugins provide.  SDF strength is when you zoom in
close to the text you get smoother endges without the pixelation that is
present with traditional textured glyphs.  Previous to get readable results
with large characters on screen we'd need to push up the font resolution
really high, using SDF's this is no longer required.

The new shaders also allows us to do multi-sampling in more creative ways
that can help address rendering artefacts associated with outline and
shadows when zoomed far out from the text - previously text would saturate
to the backdrop colour (typically black) and do so with aliasing
artefacts.  The new text implementation will provide more smooth
transitions from near to far and fade out into the distance in a natural
way.

The different techniques from rendering the text are now controlled using
the following methods:

1) Setting the text directly:

    text->setShaderTechniqure(osgText::GREYSALE); // the default

    text->setShaderTechniqure(osgText::SIGNED_DISTANCE_FIELD);

    text->setShaderTechniqure(osgText::ALL_FEATURES); // create
GlyphTexture with both GREYSCALE and SIGNED_DISTANCE_FIELD, shader
defaulting to  SDF

    text->setShaderTechniqure(osgText::NO_TEXT_SHADER); // disable use of
shaders

2) Setting the global default value of Text::ShaderTechnique using
osg::DisplaySettings::setTextShaderTechnique(..)  :


    osg::DisplaySettings::instance()->setTextShaderTechnque("GREYSCALE");
    osg::DisplaySettings::instance()->
setTextShaderTechnque("SIGNED_DISTANCE_FUNCTION");
    osg::DisplaySettings::instance()->setTextShaderTechnque("SDF"); //
short for above
    osg::DisplaySettings::instance()->setTextShaderTechnque("ALL_FEATURES");
    osg::DisplaySettings::instance()->
setTextShaderTechnque("NO_TEXT_SHADER");
    osg::DisplaySettings::instance()->setTextShaderTechnque("NONE); //
short for above

3) Setting the above DisplaySettings::setTextShaderTechnique(..) string
using env var:

   export OSG_TEXT_SHADER_TECHNIQUE="SDF"


--

As part of this work I added the ability to assign osg::DisplaySetting
assigned user values to shaders using DisplaySettings::setValue(name,
value).  You can use this to pass in globals your shaders, which you can
leverage for setting things like #version and precision hint, which in turn
means you can pass in these settings for different platfrom targets, making
it much easier to share shaders between GL2, GL3 and GLES.

To help with this DisplaySetting automatically sets up the follow values:

OSG_GLSL_VERSION
OSG_PRECISION_FLOAT
OSG_VARYING_IN
OSG_VARYING_OUT

When you build the OSG specifically against a GL2, GL3/GLCORE, GLES2 or
GLES3 target the appropriate defaults will set, then in your sharer you can
use the following syntax:

$OSG_GLSL_VERSION

$OSG_PRECISION_FLOAT

$OSG_VARYING_OUT vec2 texCoord;

$OSG_VARYING_IN vec2 texCoord;

You can have a look at the new OpenSceneGraph-Data/shaders/text.vert and
text.frag to see these in action.  Just update to master of
OpenSceneGraph-Data to see these shaders.

--

Now while the internals of osgText have changed significantly the API is
largely identical so hopefully you'll just need to pull down OSG master and
build the OSG and your application and text will just work (using the
GREYSCALE default code path.)  Visually it should be pretty similar when
using this default - you should start to see improvements when doing things
like outlines/shadows or when enabling SDF.

If there are any issues let me know,

Robert.
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