Contrast

This algorithm adjusts the contrast of specified image in RGB color space. This algorithm works by increasing RGB values of bright pixels and decreasing RGB values of dark pixels.

   

• C#
• VB.Net

									Public Shared Sub ApplyContrast(ByRef bmp As Bitmap, contrastValue As SByte)
	If contrastValue < -100 OrElse contrastValue > 100 Then
		Return
	End If

	Dim bmpData As BitmapData = bmp.LockBits(New Rectangle(0, 0, bmp.Width, bmp.Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb)
	Dim ptr As IntPtr = bmpData.Scan0
	Dim stopAddress As Integer = CInt(ptr) + bmpData.Stride * bmpData.Height
	Dim pixel As Double = 0, contrast As Double = (100.0 + contrastValue) / 100.0

	contrast *= contrast

	While CInt(ptr) <> stopAddress
		pixel = Marshal.ReadByte(ptr) / 255.0
		pixel -= 0.5
		pixel *= contrast
		pixel += 0.5
		pixel *= 255
		If pixel < 0 Then
			pixel = 0
		ElseIf pixel > 255 Then
			pixel = 255
		End If
		Marshal.WriteByte(ptr, CByte(Math.Truncate(pixel)))

		pixel = Marshal.ReadByte(ptr + 1) / 255.0
		pixel -= 0.5
		pixel *= contrast
		pixel += 0.5
		pixel *= 255
		If pixel < 0 Then
			pixel = 0
		ElseIf pixel > 255 Then
			pixel = 255
		End If
		Marshal.WriteByte(ptr + 1, CByte(Math.Truncate(pixel)))

		pixel = Marshal.ReadByte(ptr + 2) / 255.0
		pixel -= 0.5
		pixel *= contrast
		pixel += 0.5
		pixel *= 255
		If pixel < 0 Then
			pixel = 0
		ElseIf pixel > 255 Then
			pixel = 255
		End If
		Marshal.WriteByte(ptr + 2, CByte(Math.Truncate(pixel)))

		ptr += 3
	End While

	bmp.UnlockBits(bmpData)
End Sub
								

Example

									DIm b As Bitmap = CType(Image.FromFile("rose.jpg"), Bitmap)
ApplyContrast(b, 30)