I want to measure the precise pixel coordinates within an image where the mouse cursor sits.
Are there stock OS X or third party tools that can easily do this?
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You can do this simply by dragging from the desired pixel to the upper left corner of the screen. This will create a selection rectangle, and the size of that rectangle is shown in the screen. Since the rectangle can only stretch until it reaches the origin (the corner), its size will be the same as the pixel coordinates you're looking for.
I needed something very similar (coordinates of bounding boxes around certain objects in image for computer vision research). I build a simple tool that runs in the browser: http://nicodjimenez.github.io/boxLabel/annotate.html
Not that I know of. The closest stock solution I personally use is to take activate the utility to take a screenshot of a screen portion (Command ⌘+Shift ⇧+4). The cursor will turn into a crosshair with the screen's horizontal and vertical pixel coordinates. You then have to apply some math to determine the distance between the end-points you want to measure. Not ideal, but better than nothing.
You can also try using xScope, a nice 3rd party utility which makes life a little bit easier with dedicated tools, but it is zoom agnostic, so make sure you measure at 100% zoom.
Photoshop is the only tool that effectively takes pixel distances regardless of the zoom level, but you have to open the image or at least a screenshot (if you take it from a website or PDF) in Photoshop.
Via this answer to a related question:
Preview (included in OSX): Select a Text Box then from
Tools -> Show Inspectorpick the 4th tab that has a little ruler icon. This gives coordinates with an upper-left origin.
While it may be a bit cumbersome, I use the Rectangular Selection Tool, then go to a corner of the image that suits your coordinate system (top-left in some cases, bottom-left in others) and stretch the rectangle to the desired position.
The size of the rectangle shown in the tooltip will be your position.
Zooming in helps to be more precise.
I would suggest using GIMP for this. Firstly, import your image, right-click on it, and select
Image -> Crop to Content:
This ensures that the canvas is the same size as your image so that if you hover at the top left of the image, the coordinate display at the bottom left shows
I found this to be superior to the
Cmd + Shift + 4 method in Preview.app because it shows a true image coordinate, whereas with
Cmd + Shift + 4, I found that if I resized/panned around the image and hovered over the same point, it would show different coordinates.