Transparency

This simple tutorial shows how to use GPU accelerated ray-traced transparency in REDsdk. Two topics are covered here:

Transparency produced by custom shaders.

Enabling the ray-tracer

Turning on the GPU ray-tracer is just a matter of changing an option value, as shown below:

RED::IOptions* ioptions = viewpoint->As< RED::IOptions >();
RC_TEST( ioptions->SetOptionValue( RED::OPTIONS_RAY_TRANSPARENCY, 4, iresmgr->GetState() ) );

The ray-tracer depth will rule how deep rays will be propagated forward.

Using built-in transparent materials

The creation of transparent materials is straightforward, BUT, two constraints must be enforced, otherwise the material will be refractive and not transparent:

  1. The IOR must be 1.0.
  2. Fresnel must be disabled.

If any of these two conditions is not met, the material will NOT be transparent. The setup with a generic material is shown below for example:

RC_TEST( imatr->SetupGenericMaterial(	false, false,                                                                  // Single sided, NO FRESNEL!
                                      RED::Color::BLACK, NULL, RED::Matrix::IDENTITY, RED::MCL_TEX0,                 // Emissive
                                      RED::Color::BLACK, NULL, RED::Matrix::IDENTITY, RED::MCL_TEX0,                 // Ambient
                                      RED::Color( 0.3f ), NULL, RED::Matrix::IDENTITY, RED::MCL_TEX0,                // Diffuse
                                      RED::Color::BLACK, NULL, RED::Matrix::IDENTITY, RED::MCL_TEX0, 10.f,           // Specular
                                      RED::Color::BLACK, NULL, RED::Matrix::IDENTITY, RED::MCL_TEX0, 0.0f,           // Reflection
                                      false, false, NULL, RED::Matrix::IDENTITY,                                     // Environment
                                      1.0f, RED::Color( 0.4f ), NULL, RED::Matrix::IDENTITY, RED::MCL_TEX0, 0.0f,    // Opacity, NO IOR!
                                      NULL, RED::Matrix::IDENTITY, RED::MCL_TEX0, RED::MCL_TEX0,                     // Bump
                                      &RED::LayerSet::ALL_LAYERS, NULL,                                              // Layersets
                                      resmgr, iresmgr->GetState() ) );

Transparency using custom GLSL shaders

This part of the tutorial implements a simple transparency equation, based on custom shaders. It's way similar to the internal setup of built-in materials.

The transparency equation is OUTPUT_COLOR = 0 + ( 1 - transpar ) * DST_COLOR + transpar * AMBIENT + transpar * DIFFUSE + SPECULAR.

It's produced in turn by 3 GLSL shaders, one for each pass, as illustrated in the tutorial source code. Note that we must manually call RED::StateShader::SetSortedTransparency to tell REDsdk that this shader needs to be ray-traced.

Matching numerical accuracies

Custom transparency shaders are rendered mixed with REDsdk GPU ray-tracing shaders. Consequently, both shaders must use the same numerical pipeline, otherwise z-fighting artifacts might appear.

The default REDsdk built-in GPU ray-tracer pipeline uses double precision matrices that are calculated by REDsdk (see RED::RenderCode high definition matrices) and rendered by ARB shaders. The numerical accuracy of these matrices may differ from the numerical accuracy of those used by GLSL shaders.

Consequently, REDsdk adds an option called RED::OPTIONS_RAY_TRANSPARENCY_FFP_GPU to force the internal REDsdk ray-tracer to use OpenGL's legacy Fixed Function Pipeline. This makes it compatible with GLSL's ftransform() calls, and can be used to avoid z-fighting artifacts that would occur otherwise.