Tutorial 1 Advanced MATLAB Written By: Elad Osherov Oct 2016

Transcription

1 Tutorial 1 Advanced MATLAB Written By: Elad Osherov Oct 2016

2 Today s talk Data Types Image Representation Image/Video I/O Matrix access Image Manipulation MEX - MATLAB Executable Data Visualization General Tips Tricks 2

3 Relevant data types Data Types double default in most cases single when you want to save memory uint8 [0 255] native for images uint16 [0 65,535] Logical [0 1] native for masks Simple casting: double(), uint8(). Useful when displaying images with a dynamic range not corresponding to its actual type. Conversion (of images): im2double(),im2uint8(),lab2double(),lab2uint8() 3

4 Common problem I = imread('world.jpg'); I2 = I-1.4; diffi = I-I2; fprintf('max difference between images: %d\n',max(diffi(:))); Max difference between images: 1 fprintf('max difference between images: %2.1f\n',max(diffI(:))); Max difference between images: 1.4 A much better practice is: 4 display([ 'Max difference between images: ' num2str( max(diffi(:)) ) ]); Max difference between images: 1.4

5 2D Matrix Image Representation Intensity: Each pixel value in the dynamic range [minp, maxp]. Can represent a grayscale image, results of a 2d function, etc. Useful commands: imagesc(), axis, colormap(). Binary: a.k.a masks. Can represent absolute ground truth, etc. Useful commands: bwlabel(),bwmorph(),bwdist(),im2bw(),bwperim(). 5

6 2D Matrix Image Representation Indexed: Each pixel value in the range [minp, maxp]. Can represent segmentation. Useful commands: regionprops(),label2rgb() 6

7 3D Matrix Image Representation True Color: Three 2D matrices stacked. Each represents a color component. (e.g. RGB) Can represent an RGB color image, LAB image, etc. Useful commands: imshow(),rgb2gray(),rgb2ind(). 7

8 Image/Video I/O Useful Commands imread() read image imwrite() write image im2frame() convert image to movie frame movie2avi() write avi file aviread() read avi file mmreader()/videoreader() read video (better) VideoWriter() create video file (2011b+) movie() show movie implay() show video interactively 8

9 Matrix access Useful Commands: sub2ind() convert subscript (e.g. (r,c,clr)) to index (n). ind2sub() convert index (n) to subscipt (e.g. (r,c,clr)). meshgrid() generate X,Y grids. 9

10 Image Manipulation Useful Commands: imcrop() Useful for interactive cropping. imrotate() Rotate image. imfilter() Use kernal to convolve/correlation. nlfilter() Sliding neighborhood operation. blkproc() Perform function on (semi-)distinct blocks. fspecial() Create common image filter kernels. imresize() Resize image using defined interpolation. kron() Kronecker tensor product padarray() Pad image. colfilt() Colum-stack filtering (faster) imfreehand()- Select region with mouse 10

11 MEX - MATLAB Executable Dynamically linked subroutines produced from C, C++ or Fortran source code. Useful when dealing with non efficient- Matlab algorithms (e.g. iterative algorithm implemented as loops). mex setup : Setup mex compiling configurations. 11

12 Data Visualization Useful Commands: scatter()/plot() Useful to plot points on image. imagesc() Useful for 2D data. print() Save figure as image on disk (careful with lossy compressions) 12

13 Avoid loops 13 General Tips Manage memory (Clear unused variables) Useful command: clearvars() Avoid memory duplication use nested functions function myfun A = magic(500); function setrowval(row, value) A(row,:) = value; end setrowval(400, 0); disp('the new value of A(399:401,1:10) is') A(399:401,1:10) end

14 General Tips Avoid memory duplication don t want to use nested functions? Simply use the same variable name: function x = demo x=rand(10000); x=func(x); function a=func(a) a=a*2; 14

15 General Tips Long-Term Usage (Windows Systems Only) On 32-bit Microsoft Windows, the workspace of MATLAB can fragment over time due to the fact that the Windows memory manager does not return blocks of certain types and sizes to the operating system. Clearing the MATLAB workspace does not fix this problem. You can minimize the problem by allocating the largest variables first. This cannot address, however, the eventual fragmentation of the workspace that occurs from continual use of MATLAB over many days and weeks, for example. The only solution to this is to save your work and restart MATLAB. The pack command, which saves all variables to disk and loads them back, does not help with this situation. 15

16 Efficient Programming Use Checkcode() function MATLAB profiler is a good tool to profile the code 16

17 Efficient Programming Parfor run MATLAB code on several cores\threads in parallel. Use cell arrays, and feval(). Refrain using common variables inside the loop body, instead use functions inside the loop. for i = 1:numPlayers S(:, i) = playblackjack(); end parfor i = 1:numPlayers S(:, i) = playblackjack(); end 17

18 Tricks Stenography (Wikipedia) The art of hiding a message within another larger message Original Result 18

19 Tricks Stenography (Wikipedia) The art of hiding a message within another larger message I= imread('stenographyoriginal.png'); I4=85*mod(I,4); figure; subplot(1,2,1) imshow(i); title('original'); subplot(1,2,2) imshow(i4);title('result'); 19

20 Tricks Almost Connected (Steve Eddins Blog)

21 Tricks Almost Connected (Steve Eddins Blog) url = ' bw = imread(url); lbl = bwlabel(bw); figure; imagesc(lbl); axis image;

22 Tricks Almost Connected (Steve Eddins Blog) bw2 = bwdist(bw) <= 12.5; lbl2 = bwlabel(bw2); figure; imshow(bw2); figure; imagesc(lbl2); axis image;

23 Tricks Almost Connected (Steve Eddins Blog) lbl3 = lbl2.*bw; figure; imagesc(lbl3); axis image;

24 Tricks Feature AND (Steve Eddins Blog) bw = imread('text.png'); dots = rand(size(bw))>0.99;

25 Tricks Feature AND (Steve Eddins Blog) touching_pixels = bw & dots; Overlapping

26 Tricks Feature AND (Steve Eddins Blog) out = imreconstruct(touching_pixels, bw); Reconstructed Of course this will work just as well: out = imreconstruct(dots, bw);