June 10, 2004

Contents

False Color

False color, or the use of the LUT (Look Up Table) is an extension of the idea of brightness and contrast adjustment. Here, we look at other intensity scales and custom LUT's.

On the PC with versions earlier than Scion Image Beta 3b, I have found that if an optional LUT (other than gray scale) is applied to an image, subsequent images (.tiff files, anyway) are not read correctly. The pixel values of the image are corrupted so that the image looks 'solarized' (the contrast is much too great). To correct this, close the corrupted image (by clicking on the "X" at the right end of the title bar of the image), temporarily apply the gray scale to any images that are already open (use the Options -> Grayscale menu), and open the image again.

 Image with normal gray scale.

In ImageJ, the Map window and the LUT are not there.

  

 This is a 'negative' of the original. Note that the LUT has not changed. The pixels values have been subtracted from 255.  


The image has been edited by this menu - the LUT was not changed. The same appearance can be obtained by inverting the LUT.



 A 'negative' image like the one above, but this time the LUT is inverted (note that black is now at the top on the LUT window), and the pixel values of the image have not been changed.

The Map window and the double-arrowed LUT tool can now be used to adjust this inverted LUT.		  


 

The thermal color scale (or 'Hot Body' color scale) is the most natural looking and most easily interpreted color scale (other than the gray scale). This color scale sometimes renders dark image features more visible, without degrading the bright features. (This is not the case for this example).



 Image with thermal scale.

 ImageJ

In ImageJ, the fire1 lut, without the blue/purple tones between black and red, is not available directly from the menu.  This is the fire lut, which is the same as the Fire-2 LUT in Image, shown below.


This image resembles sepia prints of black and white images. The colors go from black through red, orange and yellow to white, without any greens, blues, purples, thus the ease in interpreting the image. The ImageJ Fire LUT has the blues and purples, making the image less "natural".

If this color table had been applied after the Edit -> Invert step above, the image would have not been 'restored' before application of the thermal scale. The result would be the same as applying the thermal scale (as we have just done above) , and then inverting the thermal scale using Options -> LUT Options... menu, and clicking on the Invert box as was done with the gray level image above, or in ImageJ, using Image / Lookup Tables / Invert LUT.

Adding any colors at all to the thermal scale degrades the interpretation of shapes of three dimensional objects, although the extra colors may be used to highlight areas of interest or visualize some phenomena that otherwise would appear as a slight change in gray level.

Here are examples of other color scales:



 

Options -> Color Tables -> Fire-2

ImageJ:  Image / Lookup Tables / Fire

Purple added between red and black. The steps in the LUT window show that the number of gray levels has been reduced to about 30.



 

Rainbow

Not available in ImageJ directly..



 

Spectrum



 

Fire-1, with LUT inverted.  

 



 

8 Grays The 'contoured', 'pixellated', or 'banded' effect is due to the reduced number of gray levels. Images sometimes appear this way when the Monitors or Screen control panels of the computer are set incorrectly.

  • ImageJ: 
    •  Process / Math / Divide / 25
    •  Image / Adjust / Brightness & Contrast / Auto



 

20 Colors.

  • To see a similar effect in ImageJ:
    • Click the Apply button in the Brightness & Contrast dialog.
    • Image / Lookup Tables / Spectrum