Thursday, July 4, 2013

A simple ray tracer in Python - with CGKit and RTree

First of all I am over-reaching by calling this a ray-tracer, it is more appropriately called a "Splatter". I am simply projecting all the top-triangles in an OBJ model onto a raster for True Ortho generation purposes. It was a fun project for playing with Kd-trees and line-triangle intersections. The OBJ models are geo-referenced so it does not make sense to take them into a purely CG/Game oriented ray-tracer like Blender and lose the geographic context, as well as fiddle around with setting up orthographic cameras and render resolution to match ground-sampling distance. Doing it through a proper ray-tracer typically also requires setting up appropriate lighting, while I just want to sample the pre-exisiting texture in the model.
Direct vertical projection render from script
Properly lit render using Terragen

As usual a bit of googling located the python batteries that will perform the intersection as well as parse the obj mesh I have at hand. CGKit fits the bill perfectly both in terms of parsing as well as ray-triangle intersection.

However testing every triangle for intersection for every grid-cell quickly becomes horridly slow and begs for a hierarchial index to speed up the look-up and only process the triangles in the path of the ray. Here I picked up RTree to build a 3D index , a kD-Tree of Bounding volume hierarchy to speed up intersection testing.

After building the tree I went back to only testing relevant triangles in CGKit and dug through the parsed OBJ data structure to extract the texture co-ordinates of the intersected face. The barycentric description of the intersection point is sufficient to sample the texture for the right pixel colour and transfer it to the raster grid. For those interested in the implementation, the code follows, improvement suggestions are welcome. Multiprocessing is thrown in for tile-by-tile processing.

The point of the ray-tracer was to generate true-orthos for GIS use, as a side effect of implementing the ray tracer we can now generate true-orthos from both the top and the bottom of the model. The main article about true-orthos can be found here.

Wednesday, July 3, 2013

Making 64-bit installers with CMake and NSIS

I was getting tired of my 64-bit programs getting installed by NSIS into the x86 folder under windows, just because the NSIS installer itself is 32-bit.

I started piecing the solution together from this bug report for CMake. NSIS has been ported to 64bit but the inclusion in CMake is only available in the latest release. So I had to get NSIS 64,  and latest version of CMake 2.8.11 and then make some modifications to CPack to enable the new 64-bit installer.

Change installer type to NSIS64: SET(CPACK_GENERATOR NSIS64) and re-package your application. Voila you get correct program file target location and here is a screenshot of the running application just for fun.
Some products such as PCL do a platform detection while building to suggest an install folder, using the snippet below. This method will also work if you don't feel like installing NSIS64.

    set(win_system_name win64)
    set(win_system_name win32)