quiver plot on an image

Question

Reza Zh 2024-3-1

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2089106-quiver-plot-on-an-image

评论： Voss 2024-3-1

采纳的回答： Voss

Capt2ure.PNG

Hello

I am trying to quiverplot on an image, but quiver plot shows up very small on the upper left corner (attached here)

I have to use 'imshow' instead of 'imagesc' becasue I want the image in black and white and colored quiver vectors. When I assigne colormap for 'imagesc' it messes the quiver vectors.

Is there any way to superimpose quiver plot on imshow function?

thank you

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Answer 1

Voss 2024-3-1

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2089106-quiver-plot-on-an-image#answer_1419726

编辑：Voss 2024-3-1

在 MATLAB Online 中打开

"Is there any way to superimpose quiver plot on imshow function?"

Yes.

% read an image:

img = imread('peppers.png');

% imshow the image:

imshow(img)

% make a quiver plot:

hold on

[m,n,~] = size(img);

[x,y] = meshgrid(1:n,1:m);

quiver(x,y,randn(m,n),randn(m,n))

% zoom in to see some details:

xlim([101 115])

ylim([101 110])

It looks like the x and y you use in quiver are incorrect is why your quiver plot is so small.

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Voss 2024-3-1

在 MATLAB Online 中打开

Here's an example where the image and x and y are the same sizes as yours:

% an image the size of your image:
img = imresize(imread('peppers.png'),[2304 2401]);
size(img)
ans = 1×3
        2304        2401           3
figure
im = imshow(img);

By default the XData of a plotted image is [1 size(img,2)] and the YData is [1 size(img,1)], so the pixel coordinates are 1:size(img,2) in the x-dimension and 1:size(img,1) in the y-dimension.

im.XData
ans = 1×2
           1        2401
im.YData
ans = 1×2
           1        2304
% some arbitrary x and y of the same size as yours:
x = linspace(2,78,31); % x goes from 2 to 78
y = linspace(3,14,29); % y goes from 3 to 14
[x,y] = meshgrid(x,y);
size(x)
ans = 1×2
    29    31

You either have to (1) scale your x to span from 1 to size(img,2) and scale your y to span from 1 to size(img,1), or (2) you have to set your image's XData and YData to match the limits of your quiver's x and y.

Option 1: rescale the x and y to the location of the image:

minx = min(x(:));
maxx = max(x(:));
miny = min(y(:));
maxy = max(y(:));
[m,n,~] = size(img);
X = (x-minx)./(maxx-minx).*(n-1)+1;
Y = (y-miny)./(maxy-miny).*(m-1)+1;

Check the min and max of X and Y (these match the image now):

min(X(:))

ans = 1

max(X(:))

ans = 2401

min(Y(:))

ans = 1

max(Y(:))

ans = 2304

% make a quiver plot:

hold on

quiver(X,Y,randn(size(x)),randn(size(x)))

Option 2: adjust the image XData and YData:

% new figure:

figure

% show the image in the new figure:

im = imshow(img);

% now adjust the XData and YData (makes the image tiny in this case):

im.XData = [min(x(:)) max(x(:))];

im.YData = [min(y(:)) max(y(:))];

% make a quiver plot:

hold on

quiver(x,y,randn(size(x)),randn(size(x)))

% the axes limits are still for the image's original

% XData and YData, so adjust those too:

set(gca(),'XLimMode','auto','YLimMode','auto') % setting the limit modes to auto

% xlim(im.XData+[-0.5 0.5]) % alternatively, explicitly set the limits to

% ylim(im.YData+[-0.5 0.5]) % match the new XData and YData

Reza Zh 2024-3-1

thanks

Voss 2024-3-1

You're welcome!

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quiver plot on an image

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