[Bug] Unprecise UIColor conversion

[tipa]

Description

The precision of the color conversion for UIColor (and perhaps also for NSColor & other platforms) can be improved.

Steps to Reproduce

1. Show UIColorPickerViewController and enter Display P3 Hex Color : #663399
2. Get UIColor object from UIColorPickerViewController & call ToSystemColor on it
3. Print Color as Hex string

Expected Behavior

#663399

Actual Behavior

#653299

This is because the CGColor has these components:
Red: 101.999986320734
Green: 50.9999969601631
Blue: 153.000021278858

With the current conversion, the values are rounded down:

Essentials/Xamarin.Essentials/Types/PlatformExtensions/ColorExtensions.ios.tvos.watchos.cs

Line 15 in 0dba844

return Color.FromArgb((int)(alpha * 255), (int)(red * 255), (int)(green * 255), (int)(blue * 255));

I think this would be a better conversion method:

return Color.FromArgb((int)Math.Round(alpha * 255), (int)Math.Round(red * 255), (int)Math.Round(green * 255), (int)Math.Round(blue * 255)); 

[jamesmontemagno]

Love it :)

[tipa]

Am I too picky? 😀
I just think when someone really wants to set their very favorite color for something, they would enter it as HEX and when returning to the color picker they expect the same HEX color to still be there

[jamesmontemagno]

I think you are correct, it should output the same numbers, and we should use Math.Round... the question is do you round up or down?

[tipa]

Not exactly sure what you mean. Math.Round rounds to nearest, so up or down depending on the number. So that 101.999986320734 would be rounded to 102 and not to 101.
Or are you asking what should be done in a the case like 101.5?

[tipa]

@jamesmontemagno created a PR in case this makes the change more likely to be merged :)

[jamesmontemagno]

I guess my question is what Midpoint rounding should we use? https://docs.microsoft.com/en-us/dotnet/api/system.math.round?view=net-5.0#System_Math_Round_System_Double_System_Int32_System_MidpointRounding_

[tipa]

In my PR I used the default Math.Round without specifying a Midpoint rounding. I think the default is using AwayFromZero which is what I would have used if I had to provide that parameter