Note: Many attributes of Jitter objects use only the arguments 1 and 0 to mean "on" and "off", so it's reasonable to assume that the loop attribute of jit.qt.movie is the same. While it's true that loop 0 turns looping off and loop 1 turns it on, loop 2 causes the video to play forward and then play backward when it reaches the loopend point, rather than leaping to the loopstart point.

Technical Detail: jit.rota does a lot of internal calculation using trigonometry to determine how to rotate the image. If you're not a trigonometry buff, you might not be used to thinking of angles in terms of radians. In everyday conversation we more commonly use degrees, with a full rotation being 360°. In trigonometry, it's more common to use radians, where a full rotation equals 2π radians. That's because a circle with a radius of 1 has a circumference of exactly 2π, so you can refer to an angle by referencing the point where it would intersect the unit circle. (For example if you started at a point on the unit circle and traveled a distance of exactly π/2 around the circumference, you would end up at a 90° angle—i.e. an angle of π/2 radians—from where you started, in reference to the circle's center.)

Also, in trigonometry we consider a positive change in angle to be a counter-clockwise rotation around the unit circle, whereas in everyday life you might more commonly think of a clockwise motion as being intuitively "positive" or "increasing" (like the passage of time).

So, to convert a clockwise rotation in degrees into the same rotation in radians, you would need to multiply the degree angle by 2π, then divide by –360.

Technical Detail: Do you really want to know how we did that calculation? If so, read on.

If we think of the center point of the jit.pwindow as the origin point 0,0, and we think of the current mouse location relative to that as being a point along a circle around the origin, then we can describe a right triangle based on those two points. By taking the arctangent of the mouse's coordinates y/x, we get the angle of the mouse relative to the center of jit.pwindow.

So we take the incoming x and y coordinates, and the first thing we do is convert them so that they're relative to the center of the jit.pwindow. We do that by subtracting 160 from the x dimension coordinate (so the x values will now go from -160 to 160) and multiplying the y coordinate by –1 (so values will increase as we go up, instead of down) then adding 119.5 to it. (If we added exactly 120, then every time we had a y coordinate of 120 from jit.pwindow we'd be trying to divide by 0 in expr, which is an undefined mathematical operation.) Once we have converted the x and y coordinates, we take the arctangent of y/x to get the angle in radians, then multiply that angle value by -1 to make clockwise rotation of the mouse cause clockwise rotation of the image.

This method only works within a 180° span, because the arctangent function can't tell the difference between a mouse location and its opposite point on the circle. (The calculation of y/x will be the same for both points.) So, every time the y coordinate of the mouse goes into the bottom half of the jit.pwindow, we add an offset of -π to the theta angle to distinguish those locations from their counterparts on the opposite side. (That's the last part of the expression.)

Note that this expression only works relative to the point 160,120 in the jit.pwindow. If we wanted to make an expression that works for the central point of any size jit.pwindow, we'd need to get the jit.pwindow's dimensions with a getsize message, and use the size values as variables in our expression. As the math books say, "We'll leave that as an exercise for the reader."