Convert unsigned 24-bit RGB BMP to 16 gray

NickB

(Originally posted by jswang on 7/31/2006)

Convert unsigned pseudo 24-bit RGB image file to 16-bit grayscale file.

Our original data is 16-bit grayscale data acquired using Labview. As the Labview can not handle 16-bit grayscale image, the data was saved as unsigned 24-bit colour BMP image. In the saved BMP image format, it used the blue channel to store the lower 8-bit and green channel to store the higher 8-bit, i.e., the 16-bit data D is expressed as D = B + (2^8)*G, where B is 8-bit blue channel value and G is 8-bit green channel value. (The red channel was not used here and was 0 for all pixels).

For such pseudo 24-bit RGB image, is there a simple way that I can convert it to a 16-bit grayscale image in the ImagePro?

 

NickB

(Originally posted by KevinR on 7/31/2006)

That's an interesting way to get the data. I would have tried floating point TIFF, if Labview supports it, but if it works, it works. This does lead to some interesting issues in generating multiple outputs from a single command, though.

Extract the desired color channels, convert them to 16-bit, scale the green and add the blue.

The following code does this. Note that both the blue and green channels should be converted to the same type, 16-bit, before combining them in order to _not_ autoscale the data.

' Convert a mixed channel image into a 16-bit output
Sub Color_extract()
	Dim chanGreen As Integer, chanBlue As Integer, blue16 As Integer
	Dim source As Integer, out As Integer

	' Identify the source document
	ret = IpDocGet(GETACTDOC, 0, source)

	' Extract green/blue color channels.
	' Blue will be the last image created, green will be the
	' image before that numerically. 
	ret = IpCmChannelExtract(CM_RGB, CM_RGB, 14)
	ret = IpDocGet(GETACTDOC, 0, chanBlue)
	chanGreen = chanBlue - 1

	' Convert the green channel to a 16 bit image, scale it
	ret = IpAppSelectDoc(chanGreen)
	out = IpWsConvertImage(IMC_GRAY16, CONV_DIRECT , 0, 0, 0, 0)
	ret = IpOpNumberArithmetics(256.0, OPA_MULT, 0)

	' Convert the blue channel to a 16 bit image
	ret = IpAppSelectDoc(chanBlue)
	blue16 = IpWsConvertImage(IMC_GRAY16, CONV_DIRECT , 0, 0, 0, 0)

	' Add the blue
	ret = IpAppSelectDoc(out)
	ret = IpOpImageArithmetics(blue16, 0.0, OPA_ADD, 0)

	' Clean up scratch images
	ret = IpAppSelectDoc(chanGreen)
	ret = IpDocClose()
	ret = IpAppSelectDoc(chanBlue)
	ret = IpDocClose()
	ret = IpAppSelectDoc(blue16)
	ret = IpDocClose()
End Sub

Note that this simple bit of code doesn't do any error checking - I would suggest checking the image class with IpDocGet(GETDOCINFO...) before attempting the conversion.