How can I have the color of a point?

[nhanvu]

I have a problem with my project. I have to check whether the color of a point is Red or not. How can I get the color of a point? Is there a function to know the color of a point(x,y)? How to know the name of a color if I know (r, g, b)?

 

[JohnH]

Win32 API function GetPixel give the information, and with GetDC function you can get from any Device Context for example DC(0) which is all desktop, they are declared like this:

[SIZE=2][COLOR=#0000ff]Declare [/COLOR][/SIZE][SIZE=2][COLOR=#0000ff]Function[/COLOR][/SIZE][SIZE=2] GetPixel [/SIZE][SIZE=2][COLOR=#0000ff]Lib[/COLOR][/SIZE][SIZE=2][COLOR=#800000]"gdi32.dll"[/COLOR][/SIZE][SIZE=2] ([/SIZE][SIZE=2][COLOR=#0000ff]ByVal[/COLOR][/SIZE][SIZE=2] hdc [/SIZE][SIZE=2][COLOR=#0000ff]As[/COLOR][/SIZE][SIZE=2] Int32, [/SIZE][SIZE=2][COLOR=#0000ff]ByVal[/COLOR][/SIZE][SIZE=2] x [/SIZE][SIZE=2][COLOR=#0000ff]As[/COLOR][/SIZE][SIZE=2] Int32, [/SIZE][SIZE=2][COLOR=#0000ff]ByVal[/COLOR][/SIZE][SIZE=2] y [/SIZE][SIZE=2][COLOR=#0000ff]As[/COLOR][/SIZE][SIZE=2] Int32) [/SIZE][SIZE=2][COLOR=#0000ff]As[/COLOR][/SIZE][SIZE=2] Int32[/SIZE]
[SIZE=2][SIZE=2][COLOR=#0000ff]Declare [/COLOR][/SIZE][SIZE=2][COLOR=#0000ff]Function[/COLOR][/SIZE][SIZE=2] GetDC [/SIZE][SIZE=2][COLOR=#0000ff]Lib[/COLOR][/SIZE][SIZE=2][COLOR=#800000]"user32.dll"[/COLOR][/SIZE][SIZE=2] ([/SIZE][SIZE=2][COLOR=#0000ff]ByVal[/COLOR][/SIZE][SIZE=2] hwnd [/SIZE][SIZE=2][COLOR=#0000ff]As[/COLOR][/SIZE][SIZE=2] Int32) [/SIZE][SIZE=2][COLOR=#0000ff]As[/COLOR][/SIZE][SIZE=2] Int32[/SIZE][/SIZE]
[SIZE=2][SIZE=2][COLOR=#0000ff]Declare [/COLOR][/SIZE][SIZE=2][COLOR=#0000ff]Function[/COLOR][/SIZE][SIZE=2] ReleaseDC [/SIZE][SIZE=2][COLOR=#0000ff]Lib[/COLOR][/SIZE][SIZE=2][COLOR=#800000]"user32.dll"[/COLOR][/SIZE][SIZE=2] ([/SIZE][SIZE=2][COLOR=#0000ff]ByVal[/COLOR][/SIZE][SIZE=2] hwnd [/SIZE][SIZE=2][COLOR=#0000ff]As[/COLOR][/SIZE][SIZE=2] Int32, [/SIZE][SIZE=2][COLOR=#0000ff]ByVal[/COLOR][/SIZE][SIZE=2] hdc [/SIZE][SIZE=2][COLOR=#0000ff]As[/COLOR][/SIZE][SIZE=2] Int32) [/SIZE][SIZE=2][COLOR=#0000ff]As[/COLOR][/SIZE][SIZE=2] Int32[/SIZE]
[/SIZE]

And used like this with a little help from .Net ColorTranslator class:

[SIZE=2][SIZE=2][COLOR=#0000ff]Dim[/COLOR][/SIZE][SIZE=2] dc [/SIZE][SIZE=2][COLOR=#0000ff]As [/COLOR][/SIZE][SIZE=2][COLOR=#0000ff]Integer[/COLOR][/SIZE][SIZE=2] = GetDC(0)[/SIZE]
[SIZE=2][COLOR=#0000ff]If[/COLOR][/SIZE][SIZE=2] dc <> 0 [/SIZE][SIZE=2][COLOR=#0000ff]Then[/COLOR][/SIZE]
[SIZE=2][COLOR=#0000ff] Dim[/COLOR][/SIZE][SIZE=2] col [/SIZE][SIZE=2][COLOR=#0000ff]As[/COLOR][/SIZE][SIZE=2] Color = ColorTranslator.FromWin32(GetPixel(dc, 100, 100))[/SIZE]
[SIZE=2] ReleaseDC(0, dc)[/SIZE]
[SIZE=2][COLOR=#0000ff]End [/COLOR][/SIZE][SIZE=2][COLOR=#0000ff]If[/COLOR][/SIZE]
[/SIZE]

To get Color from rgb values you use one of the shared Color.FromArgb method overloads:

[SIZE=2][COLOR=#0000ff]Dim[/COLOR][/SIZE][SIZE=2] col [/SIZE][SIZE=2][COLOR=#0000ff]As[/COLOR][/SIZE][SIZE=2] Color = Color.FromArgb(123, 123, 123)[/SIZE]

 

[nhanvu]

Thank you, JohnH! I'll try it!