→ Category: 3d

Installing Cuda on Ubuntu 11.10

Written 2011-10-27 by Timo Dickscheid, tagged as 3d, ubuntu

To install Cuda, I followed some hints on bottom of this thread, but also had to fix a few more issues.

  1. Download the current Cuda Toolkit for Ubuntu and the GPU Computing SDK, and save the .run files somewhere.

  2. Install and select gcc/g++ 4.4 

    sudo apt-get install \
    gcc-4.4 g++-4.4 build-essential
sudo update-alternatives \
    --install /usr/bin/gcc gcc /usr/bin/gcc-4.6 40 \
    --slave /usr/bin/g++ g++ /usr/bin/g++-4.6 
sudo update-alternatives \
    --install /usr/bin/gcc gcc /usr/bin/gcc-4.4 60 \
    --slave /usr/bin/g++ g++ /usr/bin/g++-4.4

    Check with

    sudo update-alternatives —config gcc
gcc —version

    that you have now version 4.4.x of the compilers.

  3. Install the nvidia drivers

    sudo apt-get install \
    nvidia-current\
    nvidia-current-dev\
    nvidia-current-updates\
    nvidia-current-updates-dev

  4. As root, run the two .run files from nvidia (see 1.).

  5. For compiling the SDK examples, you also need to install

    sudo apt-get install freeglut3-dev libxi-dev

    and create the following links

    sudo ln -s /usr/lib/libXmu.so.6 /usr/lib/libXmu.so
sudo ln -s /usr/lib/nvidia-173/libGL.so /usr/lib/libGL.so

    Then go to the NVIDIA_GPU_COMPUTING_SDK_... folder. In the file C/common/common.mk change the line

    LINKFLAGS +=

    to

    LINKFLAGS += -L/usr/lib/nvidia-current

    Then run make. This should compile everything, indicating that the CUDA stuff works.

How to convert Blender camera settings into a projection matrix

Written 2010-05-14 by Timo Dickscheid, tagged as 3d, programming

I was looking for an easy way to derive a projection matrix corresponding to the camera of images rendered with blender. Given such a 3x4 projection matrix P, I want to define a 3D point in the blender scene, multiply it with the matrix and paint it into the existing image. For example, using Matlab to draw the scene origin as a red plus sign onto the image, I want to write

imshow('blender_img.jpg');
hold on;
X = [0 0 0 1]';
x = P*X; 
x = x(1:2)/x(3);
plot(x(1),x(2),'r+');
hold off;

Deriving P turns out to be not documented at all, and besides knowledge on the computer graphics pipelines requires some trial and error. Today I managed to solve the problem, so here's the solution.

Blender transform

First of all, collect the camera position and orientation from the "Transform Properties" dialog, as shown in the image above. In Matlab you get the translation vector and rotation matrix via

X0 = [2 -10 4]'; 
o = 70*pi/180;
p = 15*pi/180;
k = -10*pi/180;
Ro=[1 0 0; 0 cos(o) -sin(o); 0 sin(o) cos(o)];
Rp=[cos(p) 0 sin(p); 0 1 0; -sin(p) 0 cos(p)];
Rk=[cos(k) -sin(k) 0; sin(k) cos(k) 0; 0 0 1];
R = Rk*Rp*Ro;

Now we need to derive the calibration matrix. Unfortunately, the focal length value in blender is neither documented nor standard. However, it is possible to display the lens angle alpha in degrees, referring to the wider image dimension, as shown in the image below. In the example we have alpha=49.13.

Blender lens

From the image resolution, which is here assumed to be 800x600, we get the focal length in pixel from fl=-400/(49.13*pi/180/2). Observe the minus!

The final projection matrix P is obtained in Matlab using 

fl = -400/tan(49.13*pi/180/2);
K = [fl 0 400; 0 fl 300; 0 0 1];
P = K*diag([1 -1 1])*R'*[eye(3) -X0];

Thanks to Daniel Berjon at Disney Research for pointing me to the tangens that was missing in the above code snippet. Note the flipping of the y axis which results in a left handed system. A student is planning to write an export script for blender to write P to file.