OpenGL Render Pipelines

Geometry

nrm

steered by Renderer

tex

used for OptiX raycast renders, OptiX populates a texture that OpenGL presents

Gensteps

p2l

point to line geometry shader used to render gensteps

Photons

pos

point representation of photon positions (not a geometry shader)

Records : corresponding to each recorded step of the photon

rec

flying point presentation

altrec

long line strip presentation

devrec

vector (short line) presentation, vector length is controllable interactively via Composition/param.y

Unpartitioned Record structure

Below ascii art shows expected pattern of slots and times for MAXREC 5

• remember that from point of view of shader the input time is CONSTANT think of the drawing as a chart plotter tracing over all the steps of all the photons, this shader determines when to put the pen down onto the paper
  • it needs to lift pen between photons and avoid invalids
  • slot indices are presented modulo 5
  • negative times indicates unset
  • dt < 0. indicates p1 invalid

//
//
//     |
//     |
//     t
//     |          3
//     |                                          4
//     |      2                                3
//     |    1                               2
//     |  0                   2          1              1
//     |                    1         0               0          0
//     +-----------------0--------> slot ------------------------------------->
//     |
//     |              4         3 4                        2 3 4    1 2 3 4
//     |
//
//

• geom shader gets to see all consequtive pairs (including invalid pairs that cross between different photons)
• shader uses one input time cut Param.w to provide history scrubbing
• a pair of contiguous recs corresponding to a potential line

Choices over what to do with the pair:

• do nothing with this pair, eg for invalids
• interpolate the positions to find an intermediate position as a function of input time
• throw away one position, retaining the other
• https://www.opengl.org/wiki/Geometry_Shader
• http://www.informit.com/articles/article.aspx?p=2120983&seqNum=2

Cannot form a line with only one valid point ? unless conjure a constant direction. The only hope is that a prior “thread” got the valid point as the second of a pair.

Perhaps that means must draw with GL_LINE_STRIP rather than GL_LINES in order that the geometry shader sees each vertex twice (?) YES : SEEMS SO

Hmm how to select single photons/steps ?

• Storing photon identifies occupies ~22 bits at least (1 << 22)/1e6 ~ 4.19

• Step identifiers

• https://www.opengl.org/wiki/Built-in_Variable_(GLSL)

• https://www.opengl.org/sdk/docs/man/html/gl_VertexID.xhtml

  non-indexed: it is the effective index of the current vertex (number of vertices processed + first value) indexed: index used to fetch this vertex from the buffer

  • control the the glDrawArrays first/count to pick the desired range
  • adopt glDrawElements and control the indices

Geometry Shader Background

• https://www.opengl.org/wiki/Geometry_Shader
• http://www.informit.com/articles/article.aspx?p=2120983&seqNum=2