[Issue 838] Update ASCeilPixelValue and ASRoundPixelValue

[rcancro]

Layouts can come back for 3x devices as a repeating decimal. Floats/doubles have a hard time representing these values and often the last digit or two will be garbage. This garbage can result in unexpected values from ceil or round. I try to fix this by subtracting FLT_EPSILON from the value before calling ceil or round

#838

[nguyenhuy]

LGTM!

If you have time, please update ASFloorPixelValue as well. Thanks!

[wiseoldduck]

This makes sense for ceil() but I don't see the same reasoning applying to round()

[wiseoldduck]

Yes I think ASFloorPixelValue would want +FLT_EPSILON, and ASRoundPixelValue would want to be left alone.

[Adlai-Holler]

I am not a smart man… is it worth using ASCeilPixelValue? It seems like it's asking for this kind of trouble, and subpixel clipping of content isn't a serious risk.

Anyway, I support landing this change. I can't think of how it could produce bad results in practice.

[wiseoldduck]

Yes fair warning I am quite naive as to how this is used in practice!

But am pretty sure if you apply this to ASRoundPixelValue and can come up with examples where it mattered it would be incorrect about half the time.

[wiseoldduck]

OK I see the example in the comments cites 100.666666666669, not what I was expecting to see like 100.666666666667, where we were addressing just the inability to capture a nonterminating rational in a CGFloat. There is apparently some other source of inaccuracy than that! I will take this conversation somewhere else as it is now more about my enlightenment than constructive review. 😄

[rcancro]

@wiseoldduck I think you are right -- We probably shouldn't do anything to round. I'll remove that.

@Adlai-Holler When creating an ASLayout, it uses ASCeilPixelValue on the given size. I don't know the performance implications, but in the past making sure you staying on pixel bounds was important for keeping text and images crisp.

I don't love this solution, but it with 3x devices in particular floats/doubles will try to represent a value they can't leading to garbage at some point. If we treat that garbage as true then we will see weird issues like these.

[Adlai-Holler]

Good change.