Using imref2d with reverse axes

Good morning,

I have a picture impiv of size 2004*1218*3 and I know the world coordinates of every corner of the pictures :

   (-2,41)                                (-26,41)
    +-----------------------------------+         
    |                                   |         
    |                                   |         
    |                                   |         
    |                                   |         
    |                                   |         
    |                                   |         
    |                                   |         
    |                                   |         
    |                                   |         
    |                                   |         
    |                                   |         
    |                                   |         
    |                                   |         
    |                                   |         
    |                                   |         
    |                                   |         
    |                                   |         
    |                                   |         
    |                                   |         
    |                                   |         
    |                                   |         
    |                                   |         
    |                                   |         
    |                                   |         
    +-----------------------------------+         
(-2,-1)                                  (-26,-1)

I also know the x and y resolution :

sc_x = sc_y = 0.02 (cm/Px)

I want to use imref2d to display my picture in the world coordinate system so I said

ymax = 41
ymin = -1;
yWorldLimits = [ymin, ymax];

xmin = -26 ;
xmax = -2 ;
xWorldLimits = [xmin, xmax];

imageSize = size(impiv1);

R = imref2d(imageSize,xWorldLimits,yWorldLimits);
imshow(impiv,R);

But then my axes are in the reverse order as I want them (see picture below) and if I try to flip

xWorldLimits

and

yWorldLimits

I get an error saying that the values must be in ascending order.

Does anybody have an idea on how to procede ? Thank you.

Answer 1

[Gitesh Nandre]

In the first diagram of the picture in the world coordicate system, you have shown point (-2,-1) to the left side of point (-26,-1). It should be in other direction because (-2) > (-26). Hence your picture in world coordinate system should be like below.

   (-26,41)                                (-2,41)
      +-----------------------------------+         
      |                                   |         
      |                                   |         
      |                                   |         
      |                                   |         
      |                                   |         
      |                                   |         
      |                                   |         
      |                                   |         
      |                                   |         
      |                                   |         
      |                                   |         
      |                                   |         
      |                                   |         
      |                                   |         
      |                                   |         
      |                                   |         
      |                                   |         
      |                                   |         
      |                                   |         
      |                                   |         
      |                                   |         
      |                                   |         
      |                                   |         
      |                                   |         
      +-----------------------------------+         
  (-26,-1)                                  (-2,-1)

Now, it is clear that X-axis for final output shown in your case is correct. However, Y-axis is reversed and it is not what you are expecting. Origin setting in upper-left corner is a default one and for plotting images it makes sense. For matrix plotting, you want it to be at lower-left corner. We can flip Y-axis in following two ways:

Flip 'impiv' upside down and draw xy axes in default cartesian axes format.

imshow(flipud(impiv),R);
axis xy;

Another way is to flip YTick labels as follows:

ylims = get(gca,'YLim');    %get y limits for current axes
yticks = get(gca,'YTick');  %get y-ticks
flipped_yticks = ylims(2)-yticks; ...
                            %flip ticks by subtracting from upper limit
set(gca,'YTickLabel',num2str(flipped_yticks'));

You will get final result as follows:

[Luc Rébillout]

Thank you, but my coordinates are correct according to my experimental setup. But I can just use the same trick you used for the y axis.