Question

Why this simple skin detection code always returns Message=Arithmetic operation resulted in an overflow.

xMax = bmp.Width - 1 : yMax = bmp.Height - 1
For y = 0 To yMax
    For x = 0 To xMax
       tmpColor = fixColor(bmp.GetPixel(x, y))
       If (((tmpColor.R > 95) And (tmpColor.G > 40) And (tmpColor.B > 20) And (tmpColor.R - tmpColor.G > 15) And (tmpColor.R > tmpColor.G) And (tmpColor.R > tmpColor.B)) Or _
                ((tmpColor.R > 220) And (tmpColor.G > 210) And (tmpColor.B > 170) And (tmpColor.R - tmpColor.G <= 15) And (tmpColor.R > tmpColor.B) And (tmpColor.G > tmpColor.B))) Then bmp.SetPixel(x, y, Color.Black)
    Next x
Next y

Answer 1

Assuming that tmpColor is defined as System.Color, an educated guess would be that, when this error occurs tmpColor.G is greater than tmpColor.R, which would render the result less than zero, and unable to be stored into a byte.

One possible solution would be to do this

Dim r as integer = tmpColor.R
Dim g as integer = tmpColor.G
Dim b as integer = tmpColor.B

and then use these new values within your calculation. It would make the code cleaner some (and much cleaner if you were to put casting within that if statement).

Another option would be to re-order the tests relying on that subtraction, and use the AndAlso operator:

(tmpColor.R - tmpColor.G > 15) And (tmpColor.R > tmpColor.G)
'To
(tmpColor.R > tmpColor.G) AndAlso (tmpColor.R - tmpColor.G > 15)

AndAlso is VB.Net’s short-circuiting logical-and operator, and will cause evaluation of the expression to stop at the first False. As all you use is Ands, replacing them all with AndAlso might see a minor performance increase.

A combination of these two items might make the code more readable, overall.