Advanced MATLAB046746

Topics

• Data Types• Image Representation• Image/Video I/O• Matrix access• Image Manipulation• MEX - MATLAB Executable• Data Visualization• General Tips• Tricks

Data Types

• Relevant data types• double – default in most cases (usually 64 bit)• single – when you want to save memory (usually 32 bit)• 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()

• rgb2ind(),ind2rgb(),makecform()

I = imread('world.jpg');I2 = I-1.4;diffI = I-I2;fprintf('Max difference between images: %d\n',max(diffI(:)));

Common problem

fprintf('Max difference between images: %2.1f\n',max(diffI(:)));

Max difference between images: 1.4

Max difference between images: 1

display([ 'Max difference between images: ' num2str( max(diffI(:)) ) ]);

A much better practice is:

Max difference between images: 1.4

Image Representation

2D Matrix

• 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().

Image Representation

2D Matrix

• Indexed: Each pixel value in the range [minP, maxP].• Can represent segmentation.• Useful commands: regionprops(),label2rgb()

Image Representation

3D Matrix

• 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().

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

Matrix access

Useful Commands:• Ind = sub2ind(matrixSize, rowSub, colSub) convert subscript to

index.• [I,J] = ind2sub(siz,IND) – convert index to subscipt.• meshgrid() – generate X,Y grids.• F = scatteredInterpolant(x,y,v) - creates an interpolant that fits

a surface of the form v = F(x,y). (replaced TriScatteredInterp)

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

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.

Data VisualizationUseful 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)

General Tips• Avoid loops• Manage memory (Clear unused variables)• Useful command: clearvars()

• Avoid memory duplication – use nested functions

function myfunA = 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

General Tips• Avoid memory duplication – don’t want to use nested

functions? Simply use the same variable name:

function x = demox=rand(10000);x=func(x); function a=func(a)a=a*2;

General Tips• Preallocating Arrays

• Preallocate correctly

ticx = 0;for k = 2:1000000 x(k) = x(k-1) + 5;endtocElapsed time is 0.301528 seconds.

ticx = zeros(1, 1000000);for k = 2:1000000 x(k) = x(k-1) + 5;endtocElapsed time is 0.011938 seconds.

A = int8(zeros(100));A = zeros(100, 'int8');

General Tips• You will be amazed of the variety of built in functions that

Matlab offers.• In fact, assume that the function you need is built in Matlab.

It’s probably true!• Use Matlab “help” to understand how to use functions• Use Google to search for new functions!

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.

Tricks

Original Result

Stenography (Wikipedia) The art of hiding a message within another larger message

?

Tricks

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');

Stenography (Wikipedia) The art of hiding a message within another larger message

TricksAlmost Connected (Steve Eddins’ Blog)

Tricks

url = 'http://blogs.mathworks.com/images/steve/2010/blobs_in_clumps.png';bw = imread(url);lbl = bwlabel(bw);figure; imagesc(lbl); axis image;

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Almost Connected (Steve Eddins’ Blog)

Tricks

bw2 = bwdist(bw) <= 12.5;lbl2 = bwlabel(bw2);figure; imshow(bw2);figure; imagesc(lbl2); axis image;

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Almost Connected (Steve Eddins’ Blog)

Tricks

lbl3 = lbl2.*bw;figure;imagesc(lbl3); axis image;

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Almost Connected (Steve Eddins’ Blog)

Tricks

bw = imread('text.png');dots = rand(size(bw))>0.99;

Feature AND (Steve Eddins’ Blog)

Tricks

touching_pixels = bw & dots;Overlapping

Feature AND (Steve Eddins’ Blog)

Tricksout = imreconstruct(touching_pixels, bw);

Reconstructed

Feature AND (Steve Eddins’ Blog)

out = imreconstruct(dots, bw);

Of course this will work just as well: