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
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
* 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.
Here are some possibly relevant bug reports:
add easy way to set rotation angle to straighten an image:
Measure tool: saveable preset parameters:
The Angle range in brush dynamics should be -180°/+180° and not 0°-360°:
implement free angled guides:
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