Problem with ROI dimension - app designer

I am using the attached app to select a ROI. One of the possible bugs is when the ROI width (app.col2) is bigger than the image x dimension (app.Ix1) so I added an if statement to fix it but then I get this error:

Index in position 2 exceeds array bounds. Index must not exceed 512.

Error in appROI/otsuthresholdSliderValueChanged (line 107)
app.Icrop = app.I(app.row1 : app.row2, app.col1 : app.col2, :);

Error while evaluating Slider PrivateValueChangedFcn.

The same issue will occur with the ROI height

Any idea why? and how to soilve it?

Answer 1

[Walter Roberson]

Ir = size(app.I,1); Ic = size(app.C,1);
app.Icrop = app.I(max(1,app.row1) : min(Ir,app.row2), max(1,app.col1) : min(Ic,app.col2), :);

[Jane Bat]

Thanks. What is app.C? and were do you add these 2 lines? what shell I do with the if statments?

[Walter Roberson]

Ir = size(app.I,1); Ic = size(app.I,1);
app.Icrop = app.I(max(1,app.row1) : min(Ir,app.row2), max(1,app.col1) : min(Ic,app.col2), :);

These replace your line

app.Icrop = app.I(app.row1 : app.row2, app.col1 : app.col2, :);

I did not look are your code, so I do not know what your if are doing. This code that I posted will prevent indexing out of range. The only purpose of it is to crop the requested range against what is actually available. This code fragment makes no attempt to make sure that the "right" ranges are used, or that it makes sense to select those entries.

For example sometimes when an ROI is hanging off an image, what you "should" do is move the ROI to the closest edge of the image, leaving it the same size (unless it is larger than the image.)

... But sometimes you want to go the more difficult route of matching features between the images. Finding the small friendly dog no matter where it is in the image and cropping around where-ever it happens to be.

[Jane Bat]

Thanks for the general answer. Any chance to look on the loop?

[Walter Roberson]

If you use the min() / max() then you do not need the if statements

When I test your code and draw the ROI over to the right border, then the sum of pos(1) and pos(3) is 0.5 greater than the width of your image.

I do not see relevant material in the documentation at this time, but in the past I have seen that for at least some kinds of ROI (not necessarily this kind) that when you give a point to an ROI, that it considers the point you give to be the center of the area, and that the coordinate it returns is intended to be the edge of the selected area. The edge of a selected area is generally 1/2 a pixel further over, so if the ROI was given coordinate 512 it would consider the edge of the area to be 512.5 . And after you add pos(1) and pos(3) you ceil() -- so the 512.5 gets rounded to 513, which is outside the image.

Look for example at the details of imcrop(): it is common to find that imcrop() created an area that is one pixel larger than you expect because of similar kinds of logic.

The min() / max() code I suggest will take care of keeping the coordinates within the image.