I joke. So far it’s just been a lot of reading papers on graphics, most of which I do not understand. :(
Anyway, as a fun little side project I’ve been working on a 3D Ray Caster using my old favourites, OpenCL, OpenGL, and C++. It’s quite similar in concept to the renderer for my dissertation project last year but with a simplified rendering method and faster performance.
Currently the program has seven hardcoded Axis Aligned Bounding Boxes (AABB’s) which it renders as the camera orbits around them. I’m working on a method to organise AABB’s into a flat packed Oct-Tree which can be passed to the GPU. Once this is working it should be trivial to construct an AABB Oct-Tree of the CT Scanned skull I used before, or to construct a simple game.
Another thing I may look into is modifying the Aliens game to have textured floors and ceilings using floor-casting and possibly to have maps with multiple vertical levels.
So today I thought I’d try my hand at Voxel rendering. Rather than existing in a 2D image plane with an X & Y coordinate Voxels exist in 3D space with a position in the X, Y, & Z axes. A prime example of the use of voxels in rendering is the popular game Minecraft. In Minecraft randomly generated terrain is modelled as large fields of voxels with each voxel rendered as a textured cube. Because the terrain is not just a surface coated in triangles but rather a volume with internal data, the terrain in minecraft can be modified and changed during runtime with little effect on performance.
Voxels are useful for displaying other kinds of volume data too. CT Scans are medical images which capture 3D density plots of a patient’s body. By capturing density at each location throughout the scanned tissue the data can be replayed using a computer to literally look inside the body and see where possible irregularities and illness may reside.
2nd year CT Scan coursework
In my second year at university I was given a Computer Graphics coursework to read in and display the data for a CT Scan. It was very low resolution, only 256 * 256 * 113 voxels in size but it worked well for the purposes of the coursework which was simply to demonstrate the ability to parse the volume data into usable slices (not that hard). There were several bonus sections to the project such as histogram equalization and density bounding to eliminate tissue we didn’t care about. Another one of the bonuses was for rendering the volume data in 3D with a hint of using trilinear interpolation. At the time the work load we were under was horrendous due to the year long group engineering project, so while I eagerly completed all of the other bonuses I just didn’t have time to get 3D working. Until now.
It’s not pretty, and in fact it’s really not pretty when you make it 3D due to the low input resolution of the scan. But there it is! The same CT Scan now rendered in 3D in OpenGL. I used the Java bindings for OpenGL (JOGL) so I could use the same loader code I wrote in 2nd year to parse the binary CT file into a 3D array of shorts.
I’ve been meaning to do it for a while but I have finally gotten around to making a Template for OpenCL & OpenGL Projects in Visual Studio 2012.
The host application is written in C++ using GLew and freeGLUT to interface with OpenGL, and the Amd APP SDK to interface with OpenCL as I am running a ATI 5850 GPU. Building upon the template is fast and easy and really comes down to just a few core functions which differ for each program. Below is a simplified version of the functions each program must implement.
Using this frame as a base to build off of I was able to quickly and easily get a demo running. The program in the video above and the images below generates a 1000 * 1000 (1 million) vertex triangular mesh and displays an exponential sine function over it. At each frame OpenCL computes the 3D location of all 1,000,000 points in the mesh which OpenGL then renders. Due to the massively paralleled nature of the OpenCL update code frame rates exceeding 100 frames per second are achieved! (In the video Fraps capped the frame rate at 30fps for smooth capture)
Below is a simplified version of the SineMesh Demo, most of the work is performed in the initialization phase to create the 1000 * 1000 mesh and link the triangles together. The program could be made a lot more efficient if the triangle linking phase was done such that it creates a Triangle Strip as opposed to the current Triangle List. A strip has the benefit of chaining touching triangles together to save the number of vertices which need to be stored in the linking array.
Rejoice! For after one hell of a long year finally the dissertation is done dusted and thankfully handed in!
This was the final result, a real-time path tracer written in C using OpenCL to compute frames and OpenGl to render them. Here is a simple cornell box style scene that was left to converge for a couple of minutes.
My project fair demo. My graders seemed to like it (I bagged 92% for the Viva!) and so did the PhD students and my coursemates… Not so much praise from the school kids, the phrase “Realistic? Looks nothing like Call of Duty” was used…. First time I’ve ever wanted to smack a child jokes but what can you do. The tech industry people didn’t seem to care for it either which was rather miserable because I had to stand there for 5 hours in a boiling hot room while no one wanted to know about my work.
But I can’t really complain about not getting any job offers because… I got offered a summer research position unconditional fully funded PhD Studentship at the uni! So this summer I’ll be staying in Swansea to effectively continue this project with the goal of getting Path Tracing working much much faster and on mobile devices (tablets most likely).
Here’s an album showing the progress from start to finish throughout the year.