Plotting an image, how to show origin tick marks?

I'm trying to show the origin in this plot, but it is not working, any suggestions?

x1 = linspace(-1,1,256);
x2 = x1.';
A = 1;
f1 = (4);
f2 = (3);
ph = pi;

u = A*cos(2*pi*(f1*x1+x2*f2)/2+ph);
figure(1)
imshow(u)
title('Cosine Function, f=(4,3)c/ph')
xlabel('x1[ph]')
ylabel('x2[ph]')
axis on
xticks([0 50 100 150 200 250])
xticklabels({'0','50','100','150','200','250'})
yticks([0 50 100 150 200 250])
yticklabels({'0','50','100','150','200','250'})

In addition to this, I need to make another plot of this 3-d cosine, is there a command I can use with imshow to alter this image to give a perspective shot? Maybe via axis?

Answer 1

[Image Analyst]

You can use imshow and pass in XData and YData. You can use line() to draw across the image at the origin/main axes if you want:

fontSize = 20;

x1 = linspace(-1,1,256);
x2 = x1.';
A = 1;
f1 = 4;
f2 = 3;
ph = pi;

u = A*cos(2*pi*(f1*x1+x2*f2)/2+ph);
imshow(u, 'XData', x1, 'YData', x2);
title('Cosine Function, f=(4,3)c/ph', 'FontSize', fontSize)
xlabel('x1[ph]', 'FontSize', fontSize)
ylabel('x2[ph]', 'FontSize', fontSize)
% Put tick marks at specified locations:
axis on
xticks([-1:0.2:1])
yticks([-1:0.2:1])
% Put lines across the entire image, in the overlay, where the main axes should be.
line(xlim, [0,0], 'LineWidth', 2, 'Color', 'r');
line([0,0], ylim, 'LineWidth', 2, 'Color', 'r');

Answer 2

[Adam]

[0, 0] is not visible on an image axes normally. The origin is at [0.5 0.5] because the first pixel is centred at [1 1], extending out in all directions by 0.5 to meet up with the next pixel.

You could force [0 0] to be on the axes by e.g.

xlim( hAxes, [0 250] )
ylim( hAxes, [0 250] )

but you may well get a small white border round your image in that case as there is no data between 0 and 0.5.

Alternatively you can use the XTickLabel property of axes instead and map and XTick at 0.5 onto a label of 0 and the same with YTickLabel.

It can all get a bit messy though, especially if you allow zooming and panning.

Answer 3

[Jan]

imshow draws the image from the coordinates 1 to 256. Then the data do not contain the 0 and therefore no ticks appear at the origin.

If you use image instead, you can specify the X and Y coordinates.

What do you want to see in the tilted view? A tilted grey image or do you want a 3D graph with the Z position defined by the value of u?

[Jan]

Whoops? @Adam: I could not see you answer while I've written mine. It seems, like I can see the other answers after a long delay of over half an hour. This happened to me several times today now.

[Adam]

Well, your answer does give additional information. I always use imagesc for my image plotting so using XData and YData is the solution I would always use in my own work if I wanted to do this.

[User1234]

@Jay Simon Thanks for the info!

As for your followup question I would like to see a 3d graph with the Z position defined by the value of u.

Right now I have something like this but it does not use imshow:

x1 = linspace(0,1,256);
x2 = x1.';
A = 1;
f1 = (4);
f2 = (3);
ph = pi;

u = A*cos(2*pi*(f1*x1+x2*f2)+ph);

b=figure(2);
u2=mesh(x1,x2,u);
colormap hsv;
title('Cosine Function, f=(4,3)c/ph');
xlabel('x1[ph]');
ylabel('x2[ph]');
saveas(b,'A1_3b.png');

[Image Analyst]

Sounds like you want surf() then, not image(), imagesc(), or imshow().