## Archive for the ‘Uncategorized’ Category

### Paper-worthy rendering in MATLAB

Thursday, July 20th, 2017

MATLAB is not a great tool for creating 3D renderings. However, the learning curves for most commercial rendering tools are quite steep. Other tools like Mitsuba can create beautiful pictures, but can feel quite cumbersome for rendering pure geometry rather than the physical scenes their designed for.

Over the years, I’ve developed a way of creating plots of 3D shapes in MATLAB using a few extra functions in gptoolbox. This started as a way to just make images from research prototypes more palatable, but eventually became the usual way that I render images for papers. If the code for my research is already written in MATLAB then one huge advantage is that every image in my paper can have a *.m script that deterministically generates the result and the corresponding image with user intervention. This helps with reproducibility, editing and sharing between collaborators.

Here’s a “VFX Breakdown” of rendering a 3D shape in MATLAB.

t = tsurf(F,V);
set(gcf,'COlor',0.94*[1 1 1]);
teal = [144 216 196]/255;
pink = [254 194 194]/255;
bg_color = pink;
fg_color = teal;
for pass = 1:10
switch pass
case 1
% blank run
axis([-209.4       119.38      -181.24       262.67      -247.28 247.38]);
case 2
axis equal;
axis([-209.4       119.38      -181.24       262.67      -247.28 247.38]);
axis vis3d;
case 3
t.EdgeColor = 'none';
case 4
set(t,fphong,'FaceVertexCData',repmat(fg_color,size(V,1),1));
case 5
set(t,fsoft);
case 6
l = light('Position',[0.2 -0.2 1]);
case 7
set(gca,'Visible','off');
case 8
set(gcf,'Color',bg_color);
case 9
case 10
end

vidObj = VideoWriter(sprintf('nefertiti-%02d.mp4',pass),'MPEG-4');
vidObj.Quality = 100;
vidObj.open;
thetas = linspace(30,-30,450);
for theta = thetas(1:end-1)
view(theta,30);
drawnow;
vidObj.writeVideo(getframe(gcf));
end
vidObj.close;

end


### Read animated gif and convert to rgb frames

Wednesday, June 14th, 2017

Matlab’s built in imread (as of 2017) doesn’t load animated gifs correctly. You can fix this by changing the line:

map = info.ColorTable;


in /Applications/MATLAB_R2017a.app/toolbox/matlab/imagesci/private/readgif.m with

map = reshape([info(:).ColorTable],[],3,n);


For a single frame indexed image you can use ind2rgb to convert to an rgb image. To do this on entire animated gif. You can use an arrayfun for-loop hack:

[X,M] = imread('input.gif');
Y = cell2mat(permute(arrayfun(@(C) ind2rgb(X(:,:,:,C),M(:,:,C)),1:size(X,4),'UniformOutput',false),[1 4 3 2]));


Update:

map = zeros(0,3,n);
for j = 1:n
map(1:size(info(j).ColorTable,1),:,j) = info(j).ColorTable;
end


### Delete all git branches except current

Saturday, March 18th, 2017
git branch | grep -v "^\\*" | xargs git branch -D


### Matlab imresize with bilinear methods computes different result than bilinear interpolation

Thursday, May 12th, 2016

Strangely, it seems that matlab’s builtin function imresize does not reproduce the usual bilinear interpolation.

Let’s consider a toy 4×4 image:

F = matrixnormalize(magic(4));


BF = imresize(F,[256 256],'bilinear');


Compare that to explicitly computing the bilinear interpolation:

[X,Y] = meshgrid(1:size(F,2),1:size(F,1));
[BX,BY] = meshgrid(linspace(1,size(F,2),size(BF,2)),linspace(1,size(F,1),size(BF,1)));
BF2 = interp2(X,Y,F,BX,BY,'bilinear');


The difference is obvious in the corners and along the edges. The imresize result has some flat (piecewise constant) patches). Why? It actually looks as if the image was padded by repeated values before upsampling.

### Purge big files (and their version history) from a git repository

Wednesday, December 23rd, 2015
1. fork the repo (e.g. using github web interface) and never touch the original again until confident that everything below worked
2. clone fork
3. cd fork
4. git rm all big files
5. git commit -m “removed big files”
6. bfg –strip-blobs-bigger-than 20K .
7. git reflog expire –expire=now –all && git gc –prune=now –aggressive
8. git push origin –force master

Verify (e.g. on github website) that both repositories look as expected.