Position of scatter points problem

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Yu Li
Yu Li 2019-1-5
回答： David López 2022-3-8
I have a set of scatter points (A), with unit of 'meter'. Then I plot them using:
f = figure(1)
scatter(A(:,1), A(:,2), 3, 'k', 'filled')
but what I noticed is that, the default Units of a figure is 'Pixel', so my question is: is there anyway to know what are the positions of these scatter points in figure?
Thanks!
Yu
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Image Analyst
Image Analyst 2019-1-5
Why do you care? As you resize the figure, the pixel location will change, however the coordinates read off the x and y axes will not change, and that's what counts. If you have a pint at (1,4) then it might be at pixel (400, 24) but if you resize the figure, it might be at pixel (135, 395) but that does not matter. The scatter plot will still show the point at (1,4) if you read the values off the x and y axes. So tell me why it matters to you what the pixel coordinates are.
Yu Li
Yu Li 2019-1-5
just want know why and how, this is a interesting thing in Matlab I think.
a simple example is that:
x=0;
y=0;
r=1;
th = 0:pi/50:2*pi;
xunit = r * cos(th) + x;
yunit = r * sin(th) + y;

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采纳的回答

Walter Roberson
Walter Roberson 2019-1-5
https://www.mathworks.com/help/images/ref/axes2pix.html can give you pixels relative to the axes position. You can
ax = gca;
oldunits = get(ax, 'Units');
set(ax, 'Units', 'pixels');
axpos = get(ax, 'Position');
set(ax, 'Units', oldunits);
and now axpos will be in pixels relative to the figure, to which you could add the results of axes2pix.
I seem to remember there being a more direct internal routine, but I do not remember the name of it.
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David López
David López 2022-3-8
Hi,
I am trying to select data in a scatter plot with the function impoly, then what I try to do is extract the original matrix position of the data. I don´t know if you know how to do this or if it is possible.
I try with this code but i cant obtain the matrix position in the original matrix.
ax = gca;
oldunits = get(ax, 'Units');
set(ax, 'Units', 'pixels');
axpos = get(ax, 'Position');
set(ax, 'Units', oldunits);
XL = xlim(ax);
YL = ylim(ax);
xpos_pixel = (xunit - XL(1)) ./ (XL(2) - XL(1)) * axpos(3) + axpos(1);
ypos_pixel = (yunit - YL(1)) ./ (YL(2) - YL(1)) * axpos(4) + axpos(2);
Thank you.
David

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