OK here I am, bigshot, giving out advice on how to rotate an image when I'm just trying to learn that myself.(adsbygoogle = window.adsbygoogle || []).push({});

I'm trying to do this in Matlab; it's rotating, but for some reason the rotated image is acquiring a black background and has various lines and dots superimposed on it. Can anyone tell me why?

Here's the code:

Images are attached. triangles.png is the original image. tri.png is just the red layer of that file; that's the array I'm rotating. newtri.png is the result.Code (Text):% rotation.m

% reads in and rotates a figure in file triangles.png

a = imread('triangles.png');

image(a); % this just displays the original image

pause;

close; % this closes it

[rows cols layers] = size(a); % this declares 3 variables named rows, cols

% & layers & assigns to them the dimensions of array a

s = min(rows,cols);

%% want to make sure the new image is square, just for convenience

rows=s;

cols=s;

%% declare a 32-bit int vector p (1 x 2) to hold the coordinates of the new point, initialized (0 0)

p=int32(zeros(1,2));

%% for simplicity, I will only deal with one color so define two 2D matrices, size rows x cols,

%% type double to allow multiplication, initialized to all 0s

b = double(zeros(rows,cols));

c = double(zeros(rows,cols));

%% loop through the array, copying just the red layer of a to the corresponding pixels of b

for row = 1:rows,

for col = 1:cols,

b(row,col)=a(row,col,1);

end

end

%% define the transformation matrix t

t = [ cos(pi/4) -sin(pi/4); sin(pi/4) cos(pi/4)];

%% now loop through the arrays computing rotated & shifted coordinates for array c

%% and copy pixels from b to their new locations in c

for row = 1:rows,

for col = 1:cols,

%% first compute the coords of the new point; note I'm rotating

%% and shifting to prevent any negative indices

%% got the 140 right-shift by trial & error

p = round([row col]*t) + [0 140];

%% now copy the current pixel value to the new matrix

c(p(1,1),p(1,2)) = b(row,col);

end

end

image(c);

hold on; % this holds the image window open so I can view one superimposed over the other

image(b);

imwrite(b,'tri.png');

imwrite(c,'newtri.png');

Edit: I just noticed that tri.png got rotated somehow in the process of uploading, so it wasn't oriented the same as the original image. Now it's OK.

Edit: added more comments to the code (everything starting with % is a comment)

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# Rotating images in matlab

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