C41

The C41⁸ plugin takes a 16 bitC41 digital intermediate negative film as input and outputs a positive image. It became necessary because C - 41 negatives can fade or color-shift over time which was a problem early on. It is still important today because there is a large amount of documentaries, video clips, and other media out there that was shot on super 16 film. This works for RGB-float, RGB, and also YUV variations.

There are two sets of data – the scanned input values and your corrected values. Simple functionality of the plugin is to compute the data, transform to get corrected values, then apply that.

Basic usage strategy:

first time the controls come up, nothing is checked and everything is set to 0
check the box Compute negfix values to see the current media input values
check Activate processing and you see a 1 - colored screen in the Compositor due to zero values
check the Apply values box to see the input values on the left side propagate to the right side
check Apply default box if you want to make sure that the borders of the image are not used
correct the output values as desired on the applied right side

It is important to note as you play or change the frame, the plugin re-computes the data as you move along, but it is not propagated to the applied side.

Checkboxes:

Activate processing: when checked, the c41 operation is used to render the image.
Compute negfix values: computes the current negative values of the image (inside the box).
Show active area: draws horizontal and vertical grid lines displaying the boxed area.
Postprocess: when checked, applies contrast/brightness values as defined in coef ${\frac{{1}}{{2}}}$ .

Values:

Compute negfix values: (left side) and
negfix values to apply: (right side):
Min/Max R/G/B: minimum and maximum values for Red, Green, and Blue.
Light: value of light; a smaller number is lighter.
Gamma G/B: values for gamma Green and Blue.
Contrast: simple color contrast.
Brightness: white brightness.

Buttons:

Apply values: copies computed RGB/Light/Gamma/Contrast/Bright from negfix to applied values.
Apply default box: copies default computed Box column/row from negfix to applied values.

Shading box:

The boxing option allows for calculating the inversion of the digital negatives in a given area of the frame as opposed to the entire frame. The program will automatically calculate the columns and rows to shave from the frame when compute negfix values is checked. A default box area is initially calculated, called the shaving box, based on where the min/max difference in a row/column is less than the program defined tolerance. This row/column minimum and maximum difference must be greater than 0.05. The effect is to cut away the border areas with constant color. If you check the Show active area, you can see the box in the compositor window. The boundary search is constrained to a range of 0.1 to 0.9 times the frame dimensions, to create a 10 percent shaved margin to avoid over-scan and negative edge bleeding. Manual adjustment of the shaving box is controlled via the four sliders on the bottom right which move each of the left, right, top and bottom shaving margins. The slider bar new values automatically take effect as you move the box and you will see the right-hand side applied values change. When you have either the rows or the columns where the minimum slider is greater than or equal to the maximum slider, the default box will be in effect instead.

Optional postprocessing:

In order to have the values of Contrast and Brightness take effect, you must check the Postprocess checkbox.

Contrast: is the difference in brightness between objects or regions.
Brightness: refers to the overall lightness or darkness of the image.

Figure 10.28 shows the C41 controls on the left and part of the Compositor window with grid lines showing the default shading box since the Show active area box is checked. Changes have been made to the left-hand side original computed values as seen in the right-hand side such as Gamma G which contains the hairline cursor and has a partial red outline value box.

**Figure 10.28:** C41 - Control window and compositor window in action

Footnotes

... C41 ⁸: credit Florent Delannoy, original program code author, and Edouard Chalaron

The CINELERRA-GG Community, 2021
https://www.cinelerra-gg.org