Digital Light & Color

Posted: **April 10th, 2011, 7:00 pm**

Gimp scripts emulating Portra, Provia, and Velvia films are at http://silicosaur.hu/photofx/

If I read it correctly, the guts of the Provia script is simply a curve:

Code: Select all

(define (splineRed)
    (let* ((r (cons-array 8 'byte)))
        (set-pt r 0 0 0 )
        (set-pt r 1 59 54 )
        (set-pt r 2 202 210 )
        (set-pt r 3 255 255 )
        r
        )
)

(define (splineGreen)
    (let* ((g (cons-array 8 'byte)))
        (set-pt g 0 0 0 )
        (set-pt g 1 27 21 )
        (set-pt g 2 196 207 )
        (set-pt g 3 255 255 )
        g
        )
)

(define (splineBlue)
    (let* ((b (cons-array 8 'byte)))
        (set-pt b 0 0 20 )
        (set-pt b 1 35 25 )
        (set-pt b 2 205 227 )
        (set-pt b 3 255 255 )
        b
        )
)

Someone familiar with the syntax of PWP .cc curves might check my translation:

Code: Select all

ColorCurves 1.0
colorspace 2
component 2
npts1 4
style1 spline
histexpand1 0
point1 0 0 0
point1 1 59 54
point1 2 202 210
point1 3 255 255
npts2 4
style2 spline
histexpand2 0
point2 0 0 0
point2 1 27 21
point2 2 196 207
point2 3 255 255
npts3 4
style3 spline
histexpand3 0
point3 0 0 0
point3 1 35 25
point3 2 205 227
point3 3 255 255
end

One way to use a script is simply to apply it, perhaps adjusting the amount. The problem is, apply it to what? Different digital cameras process their raw files distinctly, and then the question is how much work do you do before applying the script?

Another way to use the script is to preview it and compare the change for suggestions. This description of the action helps:

It involves a very mild S curve in the red channel, a slightly stronger and slightly pulled-up one in green, and a moderate one with fairly contrasty midtones in the blue channel. The overall effect could be described as "clarifying" -- a slightly cooler, visibly contrastier look with emphasized blues.

http://www.prime-junta.net/pont/How_to/ ... _films.pdf

In short, contrast G favoring highlights, and contrast B narrowly around the midtones.

Posted: **April 11th, 2011, 9:33 pm**

The problem is, apply it to what? Different digital cameras process their raw files distinctly,...

I did some work along these lines, but it got bogged down. The idea was to photograph Makbeth chart, process the RAW using some standard settings then use Match Reference to figure out standard settings for Curves and Colour Correct for this Camera/RAW processor combination that will bring the image to the common standard. Applying film-specific curves or tone mapping (or any other colour/tonal tweak) and perhaps some simulated grain to this standard base image should in theory make such adjustment more or less universal.

I had some successes, but I could never get the procedure to the point that would allow it to be used casually.

Posted: **April 13th, 2011, 8:27 am**

Thanks Charles2... I was able to use your translation for the Provia script with no difficulties in the ColorCurves transform after saving the text as a *.cc file and using the top OPTions-Load drop down.

Further investigation indicates that the four film type toning curves [crossprocess, portra, provia, and velvia] on which the scripts are based are also available as PhotoShop *.acv files...

Petteri Sulonen 02-04-2007 "Simulating Films with Photoshop Curves" [http://www.prime-junta.net/pont/How_to/ ... films.html]:

"filmcurves.zip" at http://www.prime-junta.net/pont/How_to/ ... curves.zip

...which can be read directly by the Load Color Curves dialog [select Load from the Color Curves top OPTions] then change the bottom right "Color Curves Files" to "Photoshop Curve Files".

Still leaves the questions of "when" and "how much to apply"... perhaps best determined by one's preferences, artistic sense, and image content.

Posted: **April 13th, 2011, 12:45 pm**

den wrote:

Still leaves the questions of "when" and "how much to apply"... perhaps best determined by one's preferences, artistic sense, and image content.

Is there a download for artistic sense?

That's why I like the conceptual recipe, too. For Provia, "contrast B narrowly around the midtones." That suggests doing the opposite sometimes, and indeed I found one way to warm foliage is to reduce the contrast of the blue curve/histogram around the midtones.

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