Lab hints and tips

Methodology 1) Do selective unit testing. Don't implement a lot of code and then expect it all to work. If it doesn't (it won't ...) how will you locate the error? Divide and conquer.

2) Use a pen and paper to solve problem subsets first. Mathematics programming is more than maths and programming combined. You are trying to solve both issues at once when you move directly to the computer. First solve and understand the math using pen and paper. Then solve the programming issues.

3) Be critical of instructions. Real instructions are always inaccurate and/or vague. You need to correct them and fill in the gaps where necessary. Implementing equations without trying to understand them (see step 2), will guarantee many hours of wasted time in these labs and the real world.

Lab setup Arguably one of the most difficult aspects of the labs is getting your programming environment up and running with the skeleton code. The large number of various operating systems, development environments and versions make it difficult to provide a single solution. The following details may therefore be helpful for setting the labs up on your particular configuration:

Ubuntu A quick tutorial on Ubuntu system can be found here.

Windows and Microsoft Visual Studio A useful guide can be found here by previous DH2323 student Victor Dahlin on conducting a setup in Microsoft Visual Studio 2013. A new version on VS 2013 can be found here. Note that in order to easily enable cout output to the console, you may need to set the project to be a console application in the properties menu. For the students using VS2015, you may encounter the problems as posted in the News feed. The reason is in visual studio 2015, stdin, stderr, stdout are defined differently. The lib files downloaded from SDL homepage were compiled in VS version lower than VS2015. I compiled a new lib files for VS2015. Please use the lib files instead of the downloaded one. Then it will work. The recompiled lib files can be downloaded here.

MacOS (with XCode) A useful guide by previous DH2323 student Andreas Kramer can be found here. Chengqiu Zhang posted a follow-up tutorial this year using the latest Mac OSX (Version 10.11.1) with Xcode 7.0.

MacOS (without XCode) Previous DH2323 student Mikael Hedin recommends the following:

Install fink (http://www.finkproject.org/)fink install sdlInclude in your Makefile: CXXFLAGS=$(shell sdl-config --libs) $(shell sdl-config --cflags)make

General hints and tips

Lab 3 Rasterisation (lab track 1) There are some misprints in the Lab 3 documentation (not difficult to spot, hopefully...):

1. Eqn. (3) and (4) on page 2 should refer to W/2 instead of W2 and H/2 instead of H2, respectively.

2. In section 5 on page 11, it repeatedly mentions the inverse "1=z" - it should be "1/z"

3. On page 14, the reflectance element of the Vertex struct should be represented a vec3 and not vec2 as printed.Hints:Issues with triangle borders (or gaps) being drawn may relate to missing out the drawing of pixels in one of the loops related to triangle filling operations. Check the starting and finishing conditions of those loops.On page 14, the Light power of 1.1f * vec3(1,1,1) is not enough to give clear illumination. Try the value from the previous lab of 14.1f * vec3(1,1,1), especially if you are not getting good results for the illumination questions at the end of the lab.Per vertex illumination is implemented in the vertex shader using eq. 10 and 11. The normal vectors of vertices are identical to those of triangles in this scene.

Perspective correct interpolation (last section of lab 3)

Per pixel illumination involves 3D position stored in the pixel structure. Do not interpolate the original 3D positions linearly, interpolate the transformed position (which is multiplied by inverse z value).

More detailed explanation:

Since a 2D point (x, y) in the screen space image is derived by x = fX/Z+W2; y = fY/Z+H2; (the perspective view), the interpolated points in the 2D screen space image changed linearly with 1/z. For example, consider a triangle edge AB in world-space, with A(Xa, Ya, Za), B(Xb,Yb,Zb) and the equivalent perspective projected edge in camera-space ab, with a(xa, ya), b(xb, yb). If we create linearly interpolated points between A and B in 3D world-space and then project them into the 2D screen space, the results will not be linear. This creates distortion in terms of lighting and so on.Therefore, we should do a linear interpolation on the 2D screen space between a and b first. We then use these 2D interpolated points to reconstruct 3D interpolated points in 3D space.To do this, we find a variable in 2D camera-space which changes linearly. Given x = fX/Z+W2 and y = fY/Z+H2, the variable 1/Z changes linearly. This is because x and y change linearly and (x, y) is the linear combination of 1/Z in 2D screen space. 1/Z should then be inverted to obtain Z in the 3D position.¶

More detailed descriptions of the problem and solution is available here:¶

http://www.scratchapixel.com/lessons/3d-basic-rendering/rasterization-practical-implementation/perspective-correct-interpolation-vertex-attributes¶

https://www.comp.nus.edu.sg/~lowkl/publications/lowk_persp_interp_techrep.pdf¶

You should try to understand the problem, although deriving the solution is a lot more involved.Be careful of the following:Accidentally converting floats to integers: Make sure your floats are being evaluated as such, and not being converted to integers. E.g. be careful of using "zinv = 1/vtx.z" instead of the correct "zinv = 1.0f/vtx.z"If you are having segmentation fauls, you need to implement some form of bounds checking i.e. if it is possible for a vertex of a triangle be placed outside of the view, it would access the depth buffer out of bounds resulting in a segmentation fault.