Photo Quality Color


1) What is 'real'?

2) Color Emphasis Processing





1) What is real?

The color by the present-day photo films is not 'true color'. The color chrominance by photo films is highly enhanced (color emphasis). This is because most people tend to prefer 'saturated color' rather than true color, especially in the highlight region.

It has been shown that the preferred colors by most people are different not only from the true colors but also from the colors in mind. Possibly the preferred color will depend on the country and the culture.



The color by the present-day photo film tends to reproduce the preferred color rather the true color. Most people tend to believe the color by photo film to be the true color. But it is a misunderstanding.

What color do you want for your astronomical images?

True color technology has been established already, for example, in the case of the latest color copier. This case is simple. The original image is printed on paper. And the copy is printed on paper too. The copy must be as simular as possible to the original print.

On the other hand, the case of astronomical images is much more difficult. Because the 'original' astronomical images are not printed on paper. They glitter on the sky. It is impossible to reproduce the true image on a paper. Even if you reproduce the true color of astronomical images on paper, it may not be the image what you prefer.





My target of astronomical images is
the color preferred by most people


In order to obtain the preferable color on the paper or display, we need some sophisticated image processing.The preferable color may depend on the culture, country and various background of the observers. In my case, the target is the Japanese amateur astronomers. However, in my feeling, the colors preferred by the Japanese amateurs are not so different from the color preferred by US people. Anyway, the color processing technology should be in common.






2) Color Emphasis Processing

Why we need the color emphasis?

1. Human's eye have some color emphasis function.
Simple RGB composit never reproduce true (preferred) color.



2. We have to remember that, in the latest color film, the color chrominance is highly enhanced, because most people prefer it.

3. When we use the DDP, the dynamic range is highly compressed. This means that the chrominance is compressed too. Therefore, the color contrast decreases by the DDP. The color emphasis will compensate it.



The color emphasis technique is an extension of the DDP. This technique results in drastic effect for some object, especially for galaxies.



Color emphasis by DDP

Remember again, the DDP basic equation:

Yij = k [ Xij / ({Xij} + a ) ] + b

When you process the Red image Xij(red),
use the mask from Blue image
{Xij(blue)+ a}

Yij(red) = k [ Xij(red) / {Xij(blue)+ a} ) ] + b

When we process the "Red image with blue mask", "Green image with red mask" and "Blue image with red mask", we call this process RGB/brr.

There are various combinations: RGB/bgr, RGB/ggg, RGB/sss, etc.


Why the chrominance is enhanced in this process?

Example: R/b, B/r case:

Red becomes more red, blue becomes more blue as well as the total dynamic range compression by the DDP effect. When you use the blured images for the masks, the edge emphasis is done at same time.

The DDP realized 'One step process to the photo quality'



Is a similar effect available by changing 'Color Saturation' in PhotoShop?



1. It is processed with 8 bits, where some color informations is destroyed. The DDP uses the raw data (12~16 bits).

2. Changing 'Color saturation' results in increasing color noise in the low level region. The DDP affects in the 'Xij>a' region only, which results in preferable images in most cases.




The left image of the Hale-Bopp comet was processed by RGB/rgb (i.e. the simple DDP without color emphasis), while the right was processed by RGB/bgr. You can see the drastic effect of the color emphasis.





Another drastic color effect is found in the image of NGC2403 galaxy:


In this image of M13, the colors of stars are clearly shown.






Although the DDP is very powerful tool to get 'Photo quality', it required a lot of effort and skill. In order to make 'easy to use' possible, a new image processing software Stella Image 2 has been developed by Astro Arts Corp. with the advice and the beta-test by myself.

It suport not only DDP but also the quad color composit (LRGB process). The suported formats are SBIG, 16 or 32 bits FITTS, 8 or 16 bits TIFF etc, with no limit in the image size.

In Japan, the 'Stella Image Effect' highly improved the quality of CCD images by the Japanese amateurs.

The Stella image 2 is now converted in the English version.


I would like to demonstrate the DDP and LRGB process by the Stella image 2 in this conference, because it will be very convenient tool for CCD users in US.



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