Film Negative: Difference between revisions

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Negatives are images with reversed lightness and colors, such as those produced by film cameras. RawTherapee (version 4.2 at the time of writing) does not yet have a flawless single-click solution for dealing with them, so this page serves to inform you of the possible workaround solutions:
<div class="pagetitle">Film Negative</div>
# Invert a diagonal [[Exposure#Tone_Curves|tone curve]] either in the Exposure tool, or all of the curves in the [[RGB Curves]] tool. The downside is that there are tonal shifts when doing this.
# Use a negative Hald CLUT via the [[Film Simulation]] tool. The "RawTherapee Film Simulation Collection" contains one, get it from the [[Film Simulation]] page. The downside is that some controls might operate in reverse, such as the Exposure slider, and you may experience clipping in the shadows and/or highlights as these tools are not designed to work with negatives.
# In addition to using the neutral negative Hald CLUT as described above, if you have a successful workflow of not only inverting negatives but also toning them to your liking in RawTherapee or in other software, you could [[Film_Simulation#Make_Your_Own|make your own negative Hald CLUT]] which reproduces the whole look including negative inversion. To do that, apply the same steps to the "identity Hald CLUT image" shipped with the RawTherapee Film Simulation Collection as you would to a photo negative, save it under a new name, then open a photo negative in RawTherapee and apply that new Hald CLUT image. This lets you instantly achieve not only negative inversion but also your own toning with the click of a button, leaving you only needing to expand the histogram by adjusting the Exposure slider or using curves.
# Currently the best method is to use the DCP (DNG Camera Profile) for your camera model but edited in DNG Profile Editor so that the diagonal tone curve is inverted, then manually loading this DCP in RawTherapee for all negative shots. Method outlined below.


== Creating a DCP for Negatives ==
== Introduction ==
[[File:DNG Profile Editor inverted tone curve.png|frame|right|Screenshot of DNG Profile Editor's Tone Curve tab showing an inverted diagonal tone curve for use on negatives.]]
[[File:Rt negative dcp l curve.png|frame|right|L* curve used to treat a negative raw with an applied negative DCP. The left spike is caused by the background around the scanned photo, which this curve clips to black. The right spike is the actual image on the photo.]]
# Get [http://www.adobe.com/support/downloads/detail.jsp?ftpID=5494 DNG Profile Editor]. It runs fine in Linux through [https://www.winehq.org/ wine].
# Convert one of your camera's or scanner's raw photos (it can be the photo of the negative) to DNG by following the guide "[[How to convert raw formats to DNG]]".
# Open the DNG image in DNG Profile Editor.
# In the Color Tables tab, see whether the Base Profile called "Adobe Standard (''<your camera model>'')" is available. If it is, then select it. If it is not, then select "Choose external profile" and find the file titled "''<your camera model>'' Adobe Standard.dcp". The guide [[How to get LCP and DCP profiles]] explains how to get them and where to find them.
# In the Tone Curve tab invert the diagonal so that it goes from top-left to bottom-right (move the top point to the bottom).
# Still in the Tone Curve tab, there are three "Base Tone Curves" for you to choose from. "Base Profile" and "Camera Raw Default" are usually identical and have more contrast, while "Linear" makes the image look flat. We recommend you save one DCP which uses "Base Profile" and another which uses "Linear", and see for yourself which one better suits your needs in RawTherapee. Both DCPs will require further image tweaking in RawTherapee, but the "Linear" one will require more tweaking than the "Base Curve" one, though the latter might over-saturate colors - keep an eye on that.
# To create the DCP, click "File > Export ''<your camera model>'' profile...". As recommended in the previous step, save two versions.


To use this new DCP for negatives, once you have your negative raw opened in RawTherapee go to the Color tab > Color Management section > Input Profile, and select Custom, then find this new DCP file. Enable "Use DCP's tone curve".
Negatives are images with reversed lightness and hues, such as those produced by film cameras. RawTherapee 5.7 introduced the Film Negative tool to make developing raw photos of negatives simple.


When tweaking images in RawTherapee using these DCPs, remember that using tone curves which operate on RGB channels (Tone Curve 1 and 2 in the Exposure tool) will change not only the lightness but also the color saturation the stronger your curves. You can control color saturation by using the "Weighted Standard" and "Saturation and Value Blending" modes, or you can avoid the problem by working in L*a*b* space. The advantage of working in L*a*b* space as the L* curve does is that colors are not changed while you modify the lightness, so you can fix the lightness first using as strong an L* curve as you need, and then fix the color saturation using for example the CC curve.
The tool supports raw photos of a Bayer or X-Trans sensor. Other raw types and non-raw formats are not supported.
 
In the negative image, each channel value is proportional to a power of the reciprocal of the corresponding channel in the original exposure (see [https://en.wikipedia.org/wiki/Photographic_film#Film_basics Photographic Film - Basics] for more info). Each channel value is raised to a different exponent, depending on the film type, age and possibly other factors, such as shooting conditions. These exponents can be specified in order to better adapt the correction process to the characteristics of each film. To simplify manual tweaking, these three R,G,B exponents are specified as one "reference" exponent (which gets applied to the Green channel), and two ratios of the Red and Blue exponents to the reference. Default values should produce reasonably good results out-of-the-box with some common Kodak film types like the ColorPlus 200 or Gold 200.
 
== Usage ==
 
# Open a raw photo of a negative.
# In the Raw tab, activate the Film Negative tool.
# Optionally, you can try to automatically set more accurate red and blue ratio values. To do so, click the "Pick neutral spots" button, then click on a neutral light and dark spot in the photographed scene.
#:* These spots should have had no color tint in the original scene, they should have a neutral hue. Keep in mind that these spots might not appear neutral in your photographed film negative until you use the [[White Balance]] tool.
#:* The spots should differ in brightness, and should not be clipped.
#: Picking the spots needs to be done only once per film roll, then the processing profile can be copied and pasted onto the other photographs from the same roll. This allows you to use any image in the whole roll to pick the neutral spots.
# White-balance the photo. Picking the white balance off a spot which should be neutral in hue, if the image has one, is easiest.
#: Picking the neutral spots changes the raw data's values in the [[Toolchain_Pipeline|pipeline]] before the white balance tool takes effect; therefore it is recommended to white-balance the photo after having picked the spots. If you white-balanced it before picking the spots that is fine, but you might want to re-white-balance it again afterwards.
 
That's it as far as correcting the negative goes. Resume adjusting the photo just as if it was a normal "positive" raw photo.
 
== Interface ==
 
=== Reference exponent (contrast) ===
 
Exponent applied to the Green channel, and proportional to the other exponents applied to the Red and Blue channels. Changing this value alters the general image contrast without altering its colors. The default value is good for an average-contrast negative image. In case of a very faint, or incorrectly exposed negative, this value may have to be increased. In case of a very high contrast negative, the converted, positive image could reach clipping, so this value will need to be decreased.
 
=== Red ratio ===
 
Ratio of the Red channel exponent to the reference exponent. This coefficient indicates how "bent" the Red channel transfer curve is, with respect to the Green transfer curve. Changing this value alters the color characteristics of the correction, while keeping the general image contrast.
 
=== Blue ratio ===
 
Ratio of the Blue channel exponent to the reference exponent. This coefficient indicates how "bent" the Blue channel transfer curve is, with respect to the Green transfer curve. Changing this value alters the color characteristics of the correction, while keeping the general image contrast.

Latest revision as of 21:09, 9 December 2019

Film Negative

Introduction

Negatives are images with reversed lightness and hues, such as those produced by film cameras. RawTherapee 5.7 introduced the Film Negative tool to make developing raw photos of negatives simple.

The tool supports raw photos of a Bayer or X-Trans sensor. Other raw types and non-raw formats are not supported.

In the negative image, each channel value is proportional to a power of the reciprocal of the corresponding channel in the original exposure (see Photographic Film - Basics for more info). Each channel value is raised to a different exponent, depending on the film type, age and possibly other factors, such as shooting conditions. These exponents can be specified in order to better adapt the correction process to the characteristics of each film. To simplify manual tweaking, these three R,G,B exponents are specified as one "reference" exponent (which gets applied to the Green channel), and two ratios of the Red and Blue exponents to the reference. Default values should produce reasonably good results out-of-the-box with some common Kodak film types like the ColorPlus 200 or Gold 200.

Usage

  1. Open a raw photo of a negative.
  2. In the Raw tab, activate the Film Negative tool.
  3. Optionally, you can try to automatically set more accurate red and blue ratio values. To do so, click the "Pick neutral spots" button, then click on a neutral light and dark spot in the photographed scene.
    • These spots should have had no color tint in the original scene, they should have a neutral hue. Keep in mind that these spots might not appear neutral in your photographed film negative until you use the White Balance tool.
    • The spots should differ in brightness, and should not be clipped.
    Picking the spots needs to be done only once per film roll, then the processing profile can be copied and pasted onto the other photographs from the same roll. This allows you to use any image in the whole roll to pick the neutral spots.
  4. White-balance the photo. Picking the white balance off a spot which should be neutral in hue, if the image has one, is easiest.
    Picking the neutral spots changes the raw data's values in the pipeline before the white balance tool takes effect; therefore it is recommended to white-balance the photo after having picked the spots. If you white-balanced it before picking the spots that is fine, but you might want to re-white-balance it again afterwards.

That's it as far as correcting the negative goes. Resume adjusting the photo just as if it was a normal "positive" raw photo.

Interface

Reference exponent (contrast)

Exponent applied to the Green channel, and proportional to the other exponents applied to the Red and Blue channels. Changing this value alters the general image contrast without altering its colors. The default value is good for an average-contrast negative image. In case of a very faint, or incorrectly exposed negative, this value may have to be increased. In case of a very high contrast negative, the converted, positive image could reach clipping, so this value will need to be decreased.

Red ratio

Ratio of the Red channel exponent to the reference exponent. This coefficient indicates how "bent" the Red channel transfer curve is, with respect to the Green transfer curve. Changing this value alters the color characteristics of the correction, while keeping the general image contrast.

Blue ratio

Ratio of the Blue channel exponent to the reference exponent. This coefficient indicates how "bent" the Blue channel transfer curve is, with respect to the Green transfer curve. Changing this value alters the color characteristics of the correction, while keeping the general image contrast.

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