Tan of 90 lets sprites go past normal position limits

[jwzimmer-zz]

Discussed with @cwillisf, reported by user -Handle-. If you use tan(90) for sprite position, the value will be a very large number and the sprite will not appear within the visible part of the stage.

Repro: https://scratch.mit.edu/projects/212387052/#editor

@cwillisf said:

• Google's built-in calculator gives the same result for tan(90), https://www.google.com/search?q=tan+90+degrees
• Apparently it [returns that value specifically] because Math.pi in Flash (and in JavaScript, and a number of other places) is represented the same way, with a 64-bit floating point number (standard IEEE 754 format), which is imprecise enough that the common way of implementing tan gives a result that's really big but not actually infinity (inf). It could also be related to converting from degrees to radians.
• I'm not sure why it lets you go past the normal limits, though

[Kenny2github]

tan(x) where x is 90 technically should have the same result as x/0, i.e. NaN. However, values of x approaching 90 should get progressively larger (e.g. tan(89) ~= 57.3 but tan(89.99) ~=5730).

[TheLogFather]

I've come across this numerous times, and it's probably worth noting that it has nothing to do with tan, specifically – it's all to do with the large numbers.

I've always suspected this issue comes down to rounding errors when performing the calculations on such large values (i.e. maths operations on x & y in function keepOnStage() in ScratchSprite.as) – the final values still end up outside the expected range (i.e. not within screen limits).

[TheLogFather]

I think a fairly quick-n-easy fix might be to assume that the dimensions of the sprite will not exceed some suitably large value (a million pixels?), and then check if the x and/or y positions do exceed this by somewhat more (positively or negatively). If so, then bound the co-ordinate without any reference to the actual supplied x and/or y (i.e. using only the sprite's bounding box, along with the sign(s) of the relevant x/y co-ord(s) to tell which screen edge to bound it on).