Poked around the code but am not a hardcore c coder- hope this helps focus efforts:

in view3d_ruler.c, line 319:

	BKE_gpencil_free_strokes(gpf);

would delete and regenerate the strokes per frame? since there isn't any reference to a palcolor per stroke it would add a new one when the draw function runs.

Maybe define a palcolor in this function and make it default to a "RulerData3D" palette like it did with the GPencil layer, then per stroke apply?

Patch which seems to work, but it doesn't do the UI GP Color panel initializing if the first thing done is the Ruler Set.

I can reproduce the error. Thanks for the investigation, I will take a look.

@Ryan (shiprex) Your solution is good and I like the idea to check if "rulename" color exists and reuse it.

I have only a doubt about the default color.

• I don't like to set a color harcoded, so we need a parameter to control it.

• I could use the layer default color (defined in user preferences) and the stroke would have the same color as if were draw manually or I can use the default wire color (used while the ruler is active).

@Joshua Leung (aligorith) What do you think?

@Antonio Vazquez (antoniov) Thanks! Much of that code block is duplicated over from gpencil_utils.c:473 (refactor if appropriate). I like the idea of a parameterized default color, but only on construction, not reference. Once the color is initialized, set and saved in a .blend, it would be great to be whatever the user wants it to be.

Best,
Ryan

Yes, this is used in the first creation only, after the creation of the color, the user can customize or move the stroke to a new color (as any other stroke).

We can use any user preference color value, but at this moment we cannot add a new user preferences color (it's too late for 2.78 release).

I will wait for @Joshua Leung (aligorith) answer before commmit the fix.

Awesome :) While we're at it, has there been a change to the draw order for X-ray strokes? they seem to disappear in objects with just a bit of transparency.

Here's a minimum effective blend for example, there is a GP stroke in the cube, either put the viewport alpha all the way up, or all the way down, the GP stroke is visible, but mixed it disappears.

No change for X-ray strokes. I will test your file.

Woot, first patch :D (question, why the strcopy? Still learning...)

RE: X-Ray, Thank you, no rush :) FWIW, I don't imagine you signed up to troubleshoot ruler/protractor users ;) MeasureIt has a lot more features which are super cool (will it work with modifier-deformed/shapekey-ed meshes in the future? I imagine it would be tough to pull this off with the Python API...)

Wonder if separating measurement system from the GP would save headache in the long run?