I use a minor relevant question to praise all the people developing and maintaining RT 
I use it for about 4 years and I am happy every time I work with it. So thanks again for this great program!!
My (unimportant) question: Coming from geosciences I always wonder about the fact that rotating a picture to the right has a negative degree value, I am used to a positive one. E.g. a right agle to the right is +90 degrees. What is the reason, am I missing something?
In most mathematics anti-clockwise motion is positive and clockwise motion is negative. It may have something to do with the gradient of a line increasing as the line goes from horizontal to vertical.
intuitive explanation
In 2D geometry itâs a universal and simple convention.
In 3D geometry, the rotation is defined by an (oriented) rotation axis. angles (rotations) are positive if they go conterclockwise for an oberver oriented as axis.
exemple: the earth axis is South-North and longitudes increase to the east
I suppose in some practical activities, the convention can be different.
Itâs sort of like that. I think it actually comes from trigonometry, where if you draw a line from the origin out to a unit circle, then the sine of the anticlockwise angle that line makes from the X-axis is equal to the y-coordinate on the unit circle, and the cosine is equal to the x coordinate. For positive angles 0 < θ < 90°, the sine is positive, and so is the y coordinate if you move anti-clockwise. If you moved clockwise, then the y-coordinate would be below the x-axis, and thus would be negative. In the complex number plane, multiplication by e^(jθ) is also an anticlockwise rotation (for positive θ), which derives from the trigonometry I just mentioned.
When I implemented the rotate tool in rawproc, I paid attention to the slider direction and the corresponding rotation. Slide right, rotate right. Slide left, well, you get the idea. That ârightâ was a positive increment of the angle and âleftâ a corresponding decrement was just coincidence⌠
Sounds like you got it backwards lol
In the end, it is all just a convention â if we draw our x and y axes so that positive x moves to the right, and positive y moves downwards (like what a lot of home computers did for their graphics back in the '80s), then weâd probably have a convention that positive angles are clockwise rotations.
You are perhaps right. Trigonometry is a very old branch of math. Perhaps greek and Classical Indian mathematicians already had this convention to represent angles?
I donât know the origin of this convention.
It should have been other way without any issue.
see also Axisâangle representation - Wikipedia
if rotate right is clockwise, @ggbutcher sliders are simply inverted, negative values on the right and positive on the left.
Thanks for these possible and plausible explanations. They satisfies my curiosityâŚ
Hmm, my ârotate rightâ is inflicted with positive increments. The slider is centered at 0, moving it right produces positive angles and a corresponding rotate to the right.
I flunked geometry in high school, so that might explain it⌠
Surely I donât understand ârotate rightâ 
If pushing slider on the right, you define a clockwise rotation, with accepted mathematic convention, the angle is negative. So on the right of slider you have negative angles and on the left of the zero position positive angles, considering usual conventions.
That said, you are free to use the opposite convention that can be more intuitive for photographers 
Sorry, should have used the clock convention: clockwiseâŚ