Logical Approach tothe PHOTO QUALITY
Typical CCD image < VS > Photo Quality Image
A lot of people have suceeded before to attain the 'photo-Quality CCD images', but, in most cases, the process has seemed to be a 'Trial and Error' aproach. In order to generalize the process, we have to know;
What is the 'photo-quality'? What is the difference between the above two images
1. CCD vs Film
Good linearity of CCD sensor is important for accurate measurement of magnitude.
For most amateurs, however, their goal is "beautiful pictures", not "measuring magnitudes".
If this is so, the next question is
Is the CCD linearity also important
for getting those beautiful pictures?
The answer is usually NO, it is not very important.
Two non-linear processes in film photography play an important role in making photographs look natural to the eye.
1) A non-linear sensitivity
("S-shape gamma curve")
2) An edge emphasis effect
1) Gamma curve non-linearity
CCD data has an extremely large dynamic range. The range is too large to be reproduced on print or in a video display.
In the highlight region of an image, photographic film saturates gradually while a CCD image saturates suddenly, at a maximum point. The low level, the contrast of film is low at nature.This comes from the S-shape Gamma curve.
The S-shape gamma curve is shown in fig-1 with the curve of CCD sensor. This difference in saturation behavior of the films expands the effective dynamic range of film, compared to CCD images.
Keeping this behavior in mind, we consider the "gamma value" which is the gradient of the linear region on the gamma curve.A larger gamma results in a higher contrast image.
When we require the same gamma value for the major part of image, the effective dynamic range of the processed (and printed) CCD image will be "much narrower" than film.
The down side of this non-linear expansion of the effective dynamic range is low contrast in highlight region, which results in a "flat" appearance.
In the chemical process of films, an edge emphasis is usually used to compensate for this defect.
EXAMPLE: CCD image of M77
The core of M77 is very bright.
a) Convensional process
b) S-shaped Gamma curve
c) 'b' plus edge emphasis (discussed below)
2) Edge emphasis effect
Although the edge emphasis is very important to achieve the Photo-Quality, most film users may be not aware of this fact, because it is always done automatically in the chemical process. You can clearly see this effect in pictures of the moon, at the boundary (fig-2), which was taken by Fuji Color.
fig-2
Summarizing the above:
It is a non-linear gamma curve plus edge emphasis effects in the highlight region that result in images with a wide dynamic range. This is automatically done in the chemical process of film.
How to achieve this in the digital process of CCD images?
2. Digital development of CCD images
1) Hyperbolic conversion
X: the original image data
Y: the converted data
i x j : Array of CCD pixcels
X=[Xij]=
X11,X12,X13,------X1jX21,------
X31,------
------
------Xij
Y=[Yij]
The one step process to the photo-quality, what is called, "digital development process DDP" in Japan, is written as:
Yij = k [ Xij / ({Xij} + a ) ] + b ....(equation-1)
k : A constant to control the maximum value of Yij, which is not important. (k=1)
'a' & 'b' : Artificial pedestals, which are the important parameters.
{Xij} : the unsharpened (blured) data of the original image [Xij].
(a) Case without edge emphasis
(b)
(c) When {Xij} = [Xij], i.e. no low pass filter was used, the equation results just a hyperbolic conversion of gamma curve. The relation between Xij and Yij is shown in Fig-3. The curve at X>b becomes a hyperbolic curve. You can see that the curve is very similar to the S-shape gamma curve of conventional film.
The meanings of the key parameters 'a' and 'b' are clear in the figure.
1) When b < Xij < a, the gamma is linear.
2) When a<Xij, the gamma is gradually reduced.
Xij =a is the turning point.
Fig-33) The minimum value of Yij is b.
This is similar to the base level of chemical film, which is never zero. The base pedestal results in the low contrast in the low level region, which will improve the visual S/N ratio in background level. This fact plays a very important role to attain the 'photo quality' by films. But, this has been overlooked in CCD processing. (Do not set at b=0!)
4) The gamma value is unity (=1.0) at b < Xij < a in this example.
If you replace Xij by Xij to the G power (xij^G), the gamma value will became G, while we assume G=1.0 througthout this report for simple discussion.
The curve of the Log-conversion, which is available in most image processing softwares, is also shown. Note that there is no linear region in this Log curve.
I believe that the linear region of gamma curve should be important to achieve the 'photo quality', because any photographic films have this linear region. Therefore the 'photo quality' will not be attained by the simple log conversion.
The Hyperbolic conversion will be the simplest and easiest pass to 'photo quality'.
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. where s=R+G+B.
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? No.
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.