developing monochrome images

Photography has a long history of producing monochrome images, and many still enjoy this aspect of photography. While there are some specialized/modified cameras with a truly monochrome sensor, most still use a regular camera to capture a color image and transform it into a monochrome image during post-processing.

There are two main approaches to this conversion:

  • A physical approach, where we attempt to simulate how a silver-based photographic film emulsion would react to the light captured at the scene.

  • A perceptual approach, where we develop a color image and reduce the color saturation in a perceptual color space such as CIE Lab.

These approaches, and other monochrome-related features in darktable, are discussed in the following sections.

🔗importing and flagging images as monochrome

When importing an image, there are a number of properties that can be used to indicate that the image requires a monochrome treatment:

  • If the image was taken using an achromatic camera, the image will be automatically flagged as monochrome.

  • When you capture a scene from which you would like to produce a monochrome image, it can be helpful to put your camera into a “black & white” creative mode. This allows you to visualise what the scene would look like in monochrome through your camera’s liveview screen or electronic viewfinder. The camera will still capture the full color data in the raw file, but the embedded JPEG preview image will be monochrome. When you import such an image, darktable can automatically flag the image as monochrome based on the preview image.

    Checking whether the preview is monochrome slows down the import process, so this is disabled by default. You can enable this check in preferences > processing > detect monochrome previews

  • When processing a raw file, one of the first steps is to demosaic the image. If you set the demosaicing method to “passthrough (monochrome)”, this discards color information during the demosaicing process, and darktable will flag the image as monochrome.

    Note: You should only use this for images taken on a camera where the color filter array has been removed.

  • After you have imported the image, you can manually flag an image as monochrome (or not) using the metadata tab on the lighttable’s actions on selection module,

If any of the above methods result in an image being flagged as monochrome, darktable modules can use this information to present the user with some monochrome-specific module controls and/or apply special processing to the image.

The darktable|mode|monochrome tag will be automatically applied to any images flagged as monochrome and, if you have enabled any permanent overlay information on your lighttable thumbnails, such images will be marked with a visual indicator B&W next to the file type information. By automatically applying this tag and visual indicator, darktable makes it easy to set up filters to single out images for monochrome treatment, and to see at a glance which images in the current collection bear the monochrome tag.

If darktable detects a true monochrome image or one from a monochrome-converted camera (using the “passthrough monochrome” demosaicer) some modules (e.g. demosaic, white balance) are automatically disabled.

🔗monochrome conversion

🔗physical approach

This approach tends to work with linear scene-referred data from the sensor, and attempts to mimic the response of a photographic film with a silver emulsion. It consists of three steps:

  1. Map the color channels from the sensor into a single monochrome channel. Different types of monochrome photographic film have different levels of sensitivity to various wavelengths of light, and this can be simulated by giving the three color channels different weightings when combining them together into a single monochrome channel. The color calibration module allows the three channels to be mixed into a gray channel by varying amounts, and it includes a number of presets that are designed to emulate the characteristics of some well-known types of photographic film.

  2. Apply a luminosity saturation curve. As a piece of photographic film is exposed to more intense light, its response will fall off as the silver emulsion becomes saturated. This saturation curve can be simulated within the filmic rgb module.

  3. Developing a monochrome film in the darkroom traditionally involves “dodging and burning” to control the level of exposure across different parts of the image. This can be emulated in darktable by using either the exposure module with manually created masks, or by using the tone equalizer module, which generates a mask using a guided filter.

🔗perceptual approach

The other option for producing a monochrome image is to reduce the color saturation in the image, which can be done in a linear colorspace, or in a color space oriented towards modelling human perception.

  • The color balance module operates in linear RGB, and allows you to reduce the color saturation in the image using either the input or output color saturation slider – which you choose depends on whether you want to make any other adjustments to either the color or monochrome image in the color balance module. The color balance module tends to give a predictable and perceptually-uniform result.

  • The monochrome module works in Lab color space, and it allows the user to graphically define a weighted combination of colors to determine the density of the blacks in the monochrome image. The interface can be somewhat sensitive to the settings, with small changes producing large effects, and you may experience problems with the global contrast and/or black pixel artifacts.

  • Other modules such as color zones can also be used to remove color saturation from the image, but these don’t offer any real advantage over the simplicity of the color balance module’s saturation sliders.