Posted by
ij-Newsgroup on
Nov 14, 2005; 10:10pm
URL: http://imagej.273.s1.nabble.com/24-bit-RGB-to-16-bit-gray-conversion-tp3704473p3704475.html
Thanks for your suggestions!
However, I still feel like having lost more information than I wanted to.
If I look at the histogram of either red, blue or green channel, then it
looks like a comb when converting from 8bit to 16bit (obviously) and
after adding the 3 channels, I again end up with a comb-like histogram
that uses only 256 (or less) of the 65535 possible colours. So in short,
I have gone from 16 million colours to 256 shades of gray and not to
65535 as intended.
What I want to do is to convert the 16 million colours into 65535 levels
of gray WITHOUT chopping it down to 256 colours in any of the
intermediate steps.
This is for a densitometry application for which the response of the
eye is not relevant, but it is important to destinguish more than 256
different gray values to quantify the staining accurately.
I understand that 24bit means 3 times 8 bit, hence 8 bit per colour
(red, blue, green) and that 8 bit allow 256 histogram channels and hence
256*256*256=16,777,216 result in 16.7 million colours. But that still
somehow does not fully convince me that I can only obtain 256 different
gray values. If I use different weights for each colour channel (as
suggested by one of you) then the resulting sum of the 3 split images
would not give me the honeycomb structure in the histogram, but
weighting colours by the wavelength-dependent sensitivity of the human
eye may not be adequate for my application.
Any more comments?
Thanks,
Art
Robert Rohland wrote:
> Art -
>
> Not too difficult. Gray scale is just the average of the red, green, and
> blue channels. If you have 16 bits (65,536 levels) to work with, you can
> simply add the red, green, and blue channels and scale them up however
> you like.
>
> Here's how to do one manually:
>
> I just opened the baboon.jpg, a 24-bit RGB. Select the baboon window.
> Then select the menu command Image / Color / RGB Split. The colorful
> baboon will go away and you will see three new windows entitled
> baboon.jpg (red), baboon.jpg (green), and baboon.jpg (blue). Each is
> 8-bit gray scale.
>
> First, you have to convert each of these 8-bit (256 levels) images to
> 16-bits (65536 levels). Select "baboon.jpg (red)." Select menu command
> Image / Type / 16-bit. Do the same with the green and blue 8-bit
> channels. Converting to 16-bits before doing math will prevent overflow.
>
> Now select menu command Process / Image Calculator. In the dialog box,
> select "baboon.jpg (red)" for Image1, "Add" for Operation, and
> "baboon.jpg (green)" for Image2. Check the "Create New Window" checkbox
> and hit OK.
> You will get a new window entitled "Result of baboon.jpg," which will be
> a 16-bit result.
>
> Now run the Image calculator again to add in the "baboon.jpg (blue)"
> layer. You will a new image called "Result of Result." This is your
> 16-bit grayscale image. Since you just added three 256-level images
> together, the maximum will be 765. You can scale this up by selecting
> menu command Process / Math / Multiply. A dialog box will ask you for
> the multiplier. The biggest multiplier you can use is 85 before you
> overflow the 16-bit maximum of 65535.
>
> If you need to adjust the brightness and contrast, use Image / Adjust /
> Brightness/Contrast.
>
> If you have a whole bunch of pictures to do, make a macro using this
> sequence of commands.
>
> - Robert M. Rohland
>
[hidden email]
>
[hidden email]
> 146 Garner Circle
> Montrose MN 55363
> 763-675-8905
> 612-226-9735 (cell)
>
> ----- Original Message ----- From: "ij-Newsgroup"
> <
[hidden email]>
> To: <
[hidden email]>
> Sent: Sunday, November 13, 2005 3:54 PM
> Subject: 24 bit RGB to 16 bit gray conversion?
>
>
>> Does anyone know how to convert a 24 bit RGB image (16.7 million
>> colours) into a 16 bit grayscale image?
>>
>> Thanks
>> Art
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