Photoprocessing - Brightness and Unsharp Mask

[Wilhelm Sanke]

On Sat, 3 Jun 2006, Sivakatirswami <katir at hindu.org> wrote:

> You people all amaze me... are there no limits? (smile)
>
> Question (duh type): are these filters limited to image data that is  
> an exact square? (implied by "3X3" "7X7")
>
> Or, would a "filter" that supports 7 X 7 also handle an image that  
> was, for example, 345 pixels wide and 500 pixels tall?  I assume the  
> answer is yes, but wanted to check.


The answer is indeed yes. The size of of the filter matrix is totally 
unrelated to the size of the image to be processed. In Chipp Walter's 
original scripts the height and the width of the image are among the 
parameters that are assembled before the matrix formula (with a DLL or 
completely scripted) is being applied. In my adapted versions I use an 
image size of 640 X 480 pixels, but they could easily be changed to work 
for other sizes.

The matrix-filter size defines the number of pixels involved in 
computing the color value of the center pixel. With a 3 X 3 matrix, the 
color values of the eight pixels surrounding the center pixel - along 
with a so-called "division" factor and optionally a "bias" factor - are 
processed by means of a specific formula to arrive at the new value for 
the center pixel. This process is repeated for each image pixel. 
Similarly, a 7 X 7 matrix would use 48 surrounding pixels to compute a 
filtered value of the center pixel. The greater the matrix, the more 
variable effects in changing an image with a filter are possible.

Border and corner pixels pose a special problem as they have less 
neighbouring pixels. With a 3X3 filter a border pixel possesses 5, and a 
corner pixel only 3 adjacent pixels, meaning the processing formula has 
to take this into account and find a workaround.
Chipp's no-external formula produces a "black" 1-pixel border in this 
context, his DLL formula produces a "white" 2-pixel border, Derek Bump's 
DLL a black 2-pixel border. Photoshop and PaintShop Pro, for that 
matter, do not produce such border effects after filter use, as they use 
a more sophisticated equation when border pixels are involved.

>
> it would be interesting if this whole thing could run without a GUI  
> on a web server as a faceless processor.... that could have major  
> implications for web apps where users upload source images, the rev  
> processor goes to work and the CGI incorporates downsized images back  
> to some web pages or image repository directory ...  I would  
> certainly put this to use almost immediately. Quality is the big  
> issue for some potential uses, not for others.
> (snip)
>
> Sivakatirswami



My guess is that once a satifactory and versatile solution for filter 
use inside Rev is in place, this could also be used with a CGI. I assume 
there are a sufficient number of web-experienced Revolution users that 
could then recommend and implement CGI versions.

Best regards,

Wilhelm Sanke
<http://www.sanke.org/MetaMedia>