GIMP Version: 2.8.14
Operating System: Mac OS
GIMP Experience: New User

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Hello All,

I have no experience using design programs and I'm trying to design a waterslide decal for a guitar effects pedal that I'm building. I found a helpful youtube video from 2012 using an older version of GIMP. I'm creating guide lines using the horizontal and vertical rules to position the center of the pedal knobs on my image. The problem is that, after converting the units from pixels to inches, when I drag the rulers I cannot precisely position the guide line. For example, if I want to position the line at .625 (5/8") I can hit .620 or .630; or if I want to position it at 1.25 (1 1/4") I can hit 1.21 or 1.33. This happens whether I use my finger and track pad on my MacBook or a mouse. Is there a way to input the exact coordinates?

Any help is much appreciated.

I never really used guides before, but I did some experimenting. When creating a new image, set it to inches and the DPI to 600. Doing that, I was able to hit .625. When setting DPI to 300, it would jump from .623 ~ .627.

That was the conclusion of my test anyway. Good luck.

It has been my experience that the more you scroll into the image, the more accurate the guides. Using the control button and your mouse wheel, scroll to a greater percentage view until you are able to move the guides pixel by pixel.

Gimp works in pixels, and guide positions are in pixels. How pixels are translated to physical dimensions is set by the "Print definition" (see "Image>Print size"). Not that this is just an indication to the printer driver...

If the physical positions are .62/.63 or 1.21/1.33, this means that your pixel size is around is 0.01 and that your current print definition is around 100PPI. This is likely too small (print definition is usually 300DPI). But if you increase this, you will reduce the print size of your image, unless you make the image bigger (in pixels).

There are ways using stock Gimp tools, for example it is possible to place guides at the end of the measure tool. Might be awkward with a Mac / trackpad.

However there are a couple of utilities to place guides using other than pixels. This one was proposed by one of the Gimp team but never implemented. A script guide-new-unit.scm Put that in your user scripts folder, wherever that is in a Mac.



or
One of arakne's python plugins arakne-guide-lab Two files and a folder, go in the user plugins folder arakne-guide-lab.py , drawUI.py, locales. Might work in a Mac. Make sure the .py files are marked as executable.



Of course, Gimp still works in pixels, so there is always some rounding off. Use a large canvas to give more chance of hitting the best pixel
All zipped and attached.

I recall, 'use Gimp in fractions of an inch' has come up before. One way is set up the canvas so that a suitable number of pixels occupy one unit (inch) Then set up a grid with spacing to match.

example: Using 256 ppi a spacing of 0.125 (1/8") = 32 pixels.

Thank you one and all for your suggestions. I'll try them out and let you know what works.