Issue with g-input: as I click on an image from north to south, the y values are positive values that progressively get larger and larger

Hello,

I have a pressing issue here about g-input. it on an image that I have loaded, the more south of the picture I click, the more larger and positive values of y I get. I'd appreciate it if someone would let me know why this happens. Thank you.

Answer 1

[Geoff Hayes]

Arian - the pixel in the top left corner of the image corresponds to the first row and first column of your image (matrix). So if you navigate down (south) in the image, the row number (along the y-axis) will increase.

[Arian Kolahi Sohrabi]

Got it. Thank you. Is there a modification to the ginput command that would triangulate the coordinates based on a default (or custom) cartesian coordinate system?

[Geoff Hayes]

You may have to do that transformation yourself...

[Arian Kolahi Sohrabi]

Got it. Thank you. For now, I set my final y value to be -(the y from the input).

Answer 2

[Walter Roberson]

The axes YDir property is set to 'reverse' by imshow() and image() and imagesc() unless hold on is in effect.

This is done so that the first row and column of the array of data corresponds to the upper left of the displayed image, as people tend to find that easier for images than the normal convention that lower y coordinates should occur near the bottom.

[Arian Kolahi Sohrabi]

Got it. Is there a modification to the ginput command that would triangulate the coordinates based on a default (or custom) cartesian coordinate system?

[Walter Roberson]

No. What you can do is

set(gca, 'YDir', 'normal')

[Arian Kolahi Sohrabi]

Got it. Thank you.