[Gimp-gui] Inconsistencies in angle of rotation measurements

[Elle Stone <ellestone ninedegreesbelow com>]

Stopping to add/subtract the appropriate angle to make various tools and 
filters match the angle measured by the Measure Tool is a bit of an 
interruption.

Is there an option somewhere (that I haven't found - I looked in the 
toolbox and in Preferences) to tell the Measure Tool to show angles from 
0 to 360 counterclockwise to match the angles shown by the Paint Tools?

And a second option to tell the Measure Tool to show angles from 0 to 
+/- 180 degrees to match the layer arbitrary rotation operation?

And a third option to tell the Measure Tool to show angles from 0 to 360 
degrees *clockwise*, to match Filters that measure angles clockwise 
instead of counterclockwise from the positive x-axis? And etc if there 
are other starting points/directions for other Filters/operations that 
take angles as user input?

If there aren't already such options available, maybe it would be a good 
idea to have all angle measurements for all tools and filters be 
counterclockwise from zero at the positive x-axis? Though some angle 
measurements are more comfortably done starting from 0 degrees at the 
positive y axis.

Here are various ways current GIMP tools and operations measure angles:

* The "click/shift to position and paint" angle measurements measure 
counterclockwise from zero to 360 degrees, with zero being at the 
positive x-axis.

* Filters/Blur/Circular Motion Blur and Linear Motion Blur angle of blur 
both run from 0 to 360, counterclockwise (matches the Paint tools).

* The "View/Rotate View/Other Rotation Angle" angle of rotation 
measurements from 0 to 360, but *clockwise* (opposite direction from the 
Paint tools) from the positive x-axis.

* For arbitrary rotation of a layer, the rotation measurements from 
always from 0 at the positive y-axis, running from -180 
(counterclockwise) degrees to +180 (clockwise) degrees.

* The "Measure Tool" measures the angle from the closest of the positive 
and negative x axes. So the maximum angle the Measure Tool will show is 
90 degrees, which means there are four angles (one per quadrant) that 
measure 1 degree, four angles that measure 2 degrees, four that measure 
3 degrees, and so on, all the way from "just over 0" to "just under 90 
degrees", with 0 and 90 degrees only being measured at two angles each, 
being the positive and negative x and y axes, respectively.

Rotating a layer or selection to match an angle measured by the Measure 
Tool requires some fairly complicated adding/subtracting. Let's say you 
measured an angle of 27 degrees in the third quadrant. So starting from 
0 rotation, add 90 degrees to get to the negative x-axis, and then add 
27 more degrees, so rotate the layer or selection by 90+27=117 degrees.

Painting a straight line at a given angle that was measured using the 
Measure Tool requires adding/subtracting 0 or 90 or 180 or 270 degrees, 
depending on the direction of the painted line vs the angle measured by 
the Measure Tool.

And etc.

Here are some possibly relevant bug reports:

add easy way to set rotation angle to straighten an image: 
https://bugzilla.gnome.org/show_bug.cgi?id=526719


Measure tool: saveable preset parameters: 
https://bugzilla.gnome.org/show_bug.cgi?id=679562


The Angle range in brush dynamics should be -180°/+180° and not 0°-360°: 
https://bugzilla.gnome.org/show_bug.cgi?id=787651

implement free angled guides: 
https://bugzilla.gnome.org/show_bug.cgi?id=344109

Best,
Elle