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Tamila  
#1 Posted : Wednesday, July 8, 2009 9:08:53 PM(UTC)
Tamila

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Joined: 3/9/2008(UTC)
Posts: 554

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Suppose, you have color RGB picture and you want to convert it to a red-white, yellow-white or blue-white (and so on) image analogous to standard grayscale one.

As a solution I suggest the following algorithm for each pixel:
  1. Get the values of channels (R,G,B) in source bitmap.
  2. Calculate lightness.
  3. Compose HSL (Hue, Saturation, Lightness) color. Lightness of this pixel equals to the value calculated on the previous step. H and S values are defined as input values fixing monochrome color.
  4. Convert from HSL to RGB.
  5. Write down pixel into new bitmap.
The following code represents the algorithm above. Take into account h and s parameters passed to this function forms monochrome color (in HSL format, L channel is omitted) we transform the image to.
Code:
Private Function HslToRgb(ByVal h As Double, ByVal s As Double, _
   ByVal bmp As Aurigma.GraphicsMill.Bitmap) As Aurigma.GraphicsMill.Bitmap

	Dim bitmapData As Aurigma.GraphicsMill.BitmapData = bmp.LockBits()
	Dim stride As Integer = bitmapData.Stride
	Dim pointer As IntPtr = bitmapData.Scan0
	Dim pixelSize As Integer = bitmapData.BitsPerPixel / 8

	Dim newBmp As New Aurigma.GraphicsMill.Bitmap(bmp.Width, _
               bmp.Height, Aurigma.GraphicsMill.PixelFormat.Format24bppRgb)
	Dim newBitmapData As Aurigma.GraphicsMill.BitmapData = newBmp.LockBits()
	Dim newPointer As IntPtr = newBitmapData.Scan0
	Dim newStride As Integer = newBitmapData.Stride

	Dim colors(pixelSize - 1) As Byte
	Dim lChannel As Double
	Dim hChannel As Double
	Dim sChannel As Double

	Dim j As Integer = 0

	For i As Integer = 0 To bitmapData.Height - 1
		j = 0
		Do While j < bmp.Width
			System.Runtime.InteropServices.Marshal.Copy(IntPtr.op_Explicit(pointer.ToInt32()+ _
                        i * stride + j * pixelSize), colors, 0, pixelSize)
			Dim blue As Byte = colors(0)
			Dim green As Byte = colors(1)
			Dim red As Byte = colors(2)

			Dim lightness As Byte = (30 * blue + 59 * green + 11 * red) / 100

			lChannel = lightness / 255.0
			hChannel = h
			sChannel = s

			Dim v, r, g, b As Double

			r = lChannel   ' default to gray
			g = lChannel
			b = lChannel
			v = IIf((lChannel <= 0.5), (lChannel * (1.0 + sChannel)), _
                       (lChannel + sChannel - lChannel * sChannel))

			If v > 0 Then
				Dim m, fract, vsf, mid1, mid2, sv As Double
				Dim sextant As Integer

				m = lChannel + lChannel - v
				sv = (v - m) / v
				hChannel *= 6.0
				sextant = CInt(hChannel)
				fract = hChannel - sextant
				vsf = v * sv * fract
				mid1 = m + vsf
				mid2 = v - vsf
				Select Case (sextant)
					Case 0
						r = v
						g = mid1
						b = m
					Case 1
						r = mid2
						g = v
						b = m
					Case 2
						r = m
						g = v
						b = mid1
					Case 3
						r = m
						g = mid2
						b = v
					Case 4
						r = mid1
						g = m
						b = v
					Case 5
						r = v
						g = m
						b = mid2
				End Select
			End If

			colors(2) = Convert.ToByte(r * 255.0F)
			colors(1) = Convert.ToByte(g * 255.0F)
			colors(0) = Convert.ToByte(b * 255.0F)

			System.Runtime.InteropServices.Marshal.Copy(colors, 0, _
              IntPtr.op_Explicit(newPointer.ToInt32() + i * newStride + j * pixelSize), pixelSize)
			j = j + 1
		Loop
	Next i
	bmp.UnlockBits(bitmapData)
	newBmp.UnlockBits(newBitmapData)
	Return newBmp
End Function



See also:

Edited by moderator Monday, May 28, 2012 8:31:38 PM(UTC)  | Reason: Not specified

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