Bloom: what it is and how it works

Bloom

Film color simulation in digital image processing is usually discussed in the context of film profiles or other color transformations. Also, sometimes we’re talking about grain and how it can be superimposed.

In fact, color on itself is necessary but not enough to obtain a so-called ‘warm film look’ because film image is also distinguished by other properties of physical nature.

Some of them are particularly important. The most significant are grain, bloom, halation and highlights compression.

Grain is already discussed in detail in this article, but today we will talk about the Bloom effect and the corresponding tool in Dehancer Film plugin.

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How does film grain work in Dehancer Film

There are many ways to simulate film grain in digital post-processing. Either way, there are two different methods and types of grain simulation:

  1. Natural grain scanned from real footages shot on film.
  2. Procedural (computer generated) grain.

Both approaches have rights to exist and both have their pros and cons. At first glance, scanned grain is the only ‘honest’ way to add grain to digital picture. But no matter how real it is in its origin, this type of grain isn’t related to the image, doesn’t represent it and therefore will always look unnatural and superimposed. Also there is significant limitation in controlling the characteristics of sampled grain – size, texture, density etc.

After many years of movie and photographic film study and many experiments on darkroom printing and film scanning, we managed to create a reliable mathematical 3D model that allows us to generate flexibly controlled analogue-like procedural grain.

This model is based on physical properties of real emulsion. Here are some of them:

  • Grain isn’t just overlaid on the image. Image consists of grain. One of the key points (but not the only one) that follows from this is that image detail depends on grain size, and usually visible details do not exceed grain size.
  • Film emulsion has a certain thickness, i.e. it is not flat. When passing through multiple layers of emulsion, light is reflected, refracted and diffused in many ways.
  • Silver halides (as well as dye granules) have volume. Accordingly, they are turned differently relative to film plane and therefore inevitably cast shadows.
  • There are different shapes of granules – classical rounded and flat grain, also called ’T-grain’.
  • Silver halides are distributed unevenly in the depth of emulsion layer. Tiny grains do not exist alone, forming complex conglomerations.
  • – There is always considerable amount of grain in highlights because these areas correspond to maximum negative density, i.e. maximum accumulation of silver halides (Dmax). These always affect positive image printed from film to film or from film to photo paper. At the same time, due to the nature of human perception, grain is less noticeable in the highlights (but cannot be null).
  • Grain is always found in deep shadows too, as these areas correspond to minimum exposure, i.e. minimal amount of silver halides (Dmin, most transparent areas of the negative). Accordingly, when printing from film to film (or from film to paper), print media is getting maximum amount of light in the shadows of the final image. In this way, the grain of print media itself becomes visible, along with slight grain of the source negative film.

Our algorithm generates grain based on local colour and brightness characteristics of the image, and thus is always related with it.

The granules are in fact 3D particles in the volume. Each granule is rotated relative to the image plane by a certain angle. Granules are randomly shifted and sometimes grouping in clusters. The resulting structure of ‘halides’ is superimposed over the original image (not vice versa like with traditional approach), taking into account the reflection, refraction and diffusion effects in the layers of ‘emulsion’.

In the process of printing simulation, the initial negative grain becomes positive and is built into the image as an integral part of it, that is, it does not look like overlaid on top.

On the one hand, procedural grain eliminates the need to use quite large scan files, which should be included in the installer bundle. In this case, tens or even hundreds of high-resolution scans would be required to avoid repeating patterns. And also results in additional tricks – rotation and mirroring for each frame, etc.

On the other hand, grain generation requires computational resources and inevitably slows down the framerate in the viewport and during the rendering process. However, we managed to accommodate these losses to a mere 5% of the plugin’s total performance.

How it works?

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What is Push/Pull and how it works?

All films behave differently depending on how much light they received during exposure.

Therefore, each film profile in Dehancer Film in fact consists of 3 separate profiles. Each profile is defined by its own sampling conditions:

  • Normal exposure
  • Underexposed –2 Ev
  • Overexposed +2 Ev

Exposure correction during shooting is a creative tool that allows you to vary color-contrast look of each film.

In Dehancer Film exposure control is implemented with the Push/Pull (Ev) tool. This parameter allows one to set any exposure continuously between film states within -2 to +2 stops of exposure index (ie, within 5 stops total).

Changes in the Push/Pull setting result in darker or lighter image, changing color and contrast – just like with real film. Film type defines the character of changes.

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How we build film profiles

How we build film profiles

The way we’re building film profiles for Dehancer differs significantly from typical approaches, which are basically used in the film and photo industries. In this article we’ll discuss our profile building technique and how it differs from other applications and plugins.

This article is big, so to get straight to the business, we offer a brief summary:

Dehancer film profiles key features:

  • over 45 next generation quality film profiles (the list is constantly updated)
  • full range of current Kodak Vision 3 negative movie films
  • all profiles are built around sampling the real analogue media, combined with non-linear image processing
  • film targets are printed optically from negatives to print media to ensure that they are correctly interpreted in terms of color and contrast
  • there are 3 target samples made for each film: underexposed –2 Ev, normal exposure and overexposed +2 Ev. So in fact each multi-LUT consists of 3 separate LUT-profiles, smoothly adjustable between the extreme values
  • not only films are sampled but many other analogue photographic processes as well – like Ambrotype, Prokudin-Gorsky color separation technique etc.
  • basic set of conversion profiles for movie cameras, including Arri Alexa
  • Exposure, Contrast and Analogue Range Limiter tools are based upon the real optically printed media samples, so these tools do behave like real analogue media does
  • brand new Color Density tool emulates analogue media saturation
  • unique Expand tool meant to control black and white points independently
  • False Color mode added for visual grading quality control
  • all film profiles are updated in one click within the plugin

Below is a detailed description of the basic principles of film profile sampling technique used by Dehancer team.

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Modern motion picture color negative films

Dehancer film profiles are created using unique technology of accurate sampling of color-contrast characteristics of the film. You can simply scroll and try film profiles in the list until getting the most interesting or desired results. Or you can arm yourself with the knowledge of how film works physically and thereby make profile choice even more conscious and effective. First of all, it is necessary to understand what lighting conditions a particular film is suitable for.

This article reviews main film stocks that are produced and used nowadays. We are interested only in color negative films now, as they are commonly used in modern movie production — both in fully analogue process (hello, Tarantino) and for hybrid technology with scanning and digital post-production as well.

Continue reading “Modern motion picture color negative films”