Re: 24 bit RGB to 16 bit gray conversion

Posted by Wayne Rasband on Nov 15, 2005; 1:54am
URL: http://imagej.273.s1.nabble.com/24-bit-RGB-to-16-bit-gray-conversion-tp3704473p3704476.html

The following macro converts an RGB image with 16.7 million possible  
colors into a 32-bit image with 16.7 million possible gray values.  
Convert the 32-bit image to 16-bits and you get an image with 65536  
possible gray values.

   if (bitDepth!=24)
       exit("RGB image required");
   setBatchMode(true);
   w = getWidth;
   h = getHeight;
   rgb = getImageID;
   newImage("32-bit", "32-bit", w, h, 1);
   float = getImageID;
   for (y=0; y<h; y++) {
       showProgress(y, h);
       for (x=0; x<w; x++) {
           selectImage(rgb);
           v = getPixel(x,y)&0xffffff;
           selectImage(float);
           setPixel(x, y, v);
       }
   }
   resetMinAndMax;
   setBatchMode(false);
   //run("16-bit");

-wayne

On Nov 14, 2005, at 5:10 PM, ij-Newsgroup wrote:

> 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
>
> --
> ============================================
> ==    CSO & System Administrators         ==
> ==    ALS, Sydney, Australia              ==
> ==    [hidden email]  ==
> ============================================
>
> ======================================================================
> ==
> The information contained in this e-mail message, including any
> attachments, is confidential, subject to copyright, and maybe the
> subject of legal and other privilege. Any dissemination, distribution,
> or copying of this message, in its entirety or part by any entity  
> apart
> from the intended recipient is strictly prohibited. Please accept our
> apologies if you are not the intended recipient.  If you think that  
> you
> have received this e-mail message in error, please e-mail the  
> sender and
> destroy the original message without making a copy. Unfortunately, we
> cannot warrant that the email has not been altered or corrupted during
> transmission. Thank you.
> ======================================================================
> ==
>