The roll off of gamma curve at "Xij > a" will result in a flattened image in the highlight region, without the edge emphasis.
Again, the DDP basic equation:
Yij = k [ Xij / ({Xij} + a ) ] + b ....(equation-1)
The edge emphasis is possible by using the "low pass filtered image of Xij'" (={Xij}), in the mask data.
The basic idea to enhance the edge is very similar to the conventional unsharp-mask.
Yij = k [ Xij / ({Xij} + a ) ] + b ....(equation-1)
The edge emphasis becomes conspicuous at {Xij} >> a.
As described in Section 1, the edge emphasis + the non-linear gamma curve is essential in the 'photo quality'. Therefore, in my opinion, any types of histogram equalization and/or simple gamma conversion never reproduce the photo quality without edge emphasis.
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
Please note:
if b=0,
(Equation-1) : Yij = Xij / [{Xij} +a ] + b= Xij / [{Xij} + a]
That is, this equation is just identical to
Flat field correction of Xij by [{Xij} + a ]
The digital development process (DDP) does not require any specialized software.
If you regard ({Xij} + a ) as the flat frame for Xij,
Flat field command is available for the DDP.
With the flat field command, the DDP process are done as follows:
(The original image data = X)
( If you taken the image with heavy light pollution, I recommend to substract the background from the raw data before DDP.)
1. Unsharpen (blur) X : this is {X}
2. Make {X}+a (simply add 'a')
3. Do 'flat field correction' of X,
considering {X}+a as the flat field for X.
This is just X/[{X}+a}].
4. Add a pedestal 'b' to X/[{X}+a]
This process will be achievable with any softwares for CCD. However, if a specialized software for DDP is available, it will be more convenient. Therefore we developped the Stella Image.
