[Fwd: Re: [vtkusers] VTK SCALARS HELP!]

[Karl Garsha]

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Message-ID: 	<4036AC47.7000902 at itg.uiuc.edu>
Date: 	Fri, 20 Feb 2004 18:54:31 -0600
From: 	garsha <garsha at itg.uiuc.edu>
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To: 	Sentil Balaji <profbalse at yahoo.co.in>
Subject: 	Re: [vtkusers] VTK SCALARS HELP!
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Hello BeeJay,
Scalars refer to the amplitude component(s) associated with a pixel or 
voxel or vertex...I'm not formally trained in computer graphics either 
so my explanation may be a little rough, but I'll try.

Briefly, if you consider a greyscale image with 8 bits depth you must 
understand that each pixel that makes up the image may have an intensity 
value between 0 and 255 (0=black, 255=white).  In the world of VTK such 
an image may be of the datatype 8-bit unsigned char, and this type of 
data has 1 scalar value associated with each datapoint(pixel).  The 
scalar value represents the pixel brighteness in this simple example.  
The pixel brightness in turn may represent photons counted, velocity-all 
sorts of things depended on the instrumentation that collected the data. 
 
Expanding on this, a 24bit RGB image would have 3 scalar components: 8 
bits for red intenstiy, 8 bits for blue intensity, and 8bits for green 
intensity.  A 32 bit RGB image would have 8 bits for each of red, blue 
and green and also 8 bits for alpha (transparency).  Therefore, a 32 bit 
RGBA image has 4 scalar components.  The actual variables that each 
scalar component represents to the viewer depends on the application. 

Datasets can have many scalars per datapoint depending on the nature of 
a study.  In the context of a multidimensional laser scanning 
microscopy, each voxel in a dataset may have a location in 3D space, an 
intensity for each color channel, an emmision spectra reading for each 
voxel (which may itself have between 16 and 200 scalar values 
representing intensity at different wavelengths), and a time point 
value.  Such a dataset is rich in scalars and in order to visualize 
different aspects of the data, each datapoint would probably be a 
sophisticated data structure or miniature database object.  To view the 
data, selected scalars would be mapped to scalar values of a datatype 
that computer languages know how to handle, and graphics subsystems can 
display for viewing.  This is part of what VTK does--depending on how 
many variables one needs to visualize at once, there are several basic 
data types to choose from (see VTK documentation) to represent different 
aspects of a multidimensional dataset. 
-Karl

Sentil Balaji wrote:

>Hey,
>
>    I am not a graphics student which might make this
>question  silly enough...What are Scalars?Why are they
>used?How to define Scalars for the various
>datasets?Where can I find explanations for them?
>
>
>Thanks
>
>Cheers
>
>BeeJay
>
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-- 
Karl Garsha
Light Microscopy Specialist
Imaging Technology Group
Beckman Institute for Advanced Science and Technology
University of Illinois at Urbana-Champaign
405 North Mathews Avenue
Urbana, IL 61801
Office: B650J
Phone: 217.244.6292
Fax: 217.244.6219
Mobile: 217.390.1874
www.itg.uiuc.edu