using darktable-chart on darktable 5.6 user manual

overview

Mon, 01 Jan 0001 00:00:00 +0000

darktable-chart is a tool for extracting luminance and color values from images of color reference cards such as IT8.7/1 charts. Its main purpose is to compare a source image (typically a largely unprocessed raw image) to a target image (typically a JPEG image created in-camera) and produce a darktable style that is able to use the luminance and color values of the source image to produce the target image. This style employs the tone curve module, the input color profile module, and the color look up table module for that purpose.

usage

Mon, 01 Jan 0001 00:00:00 +0000

The tool is organized into three tabs in the upper part and a text output frame in the lower part.

The first tab is used to define the source image, the second tab defines the reference (target) and the third tab contains the controls to generate the resulting darktable style.

source image

Mon, 01 Jan 0001 00:00:00 +0000

In the “source image” tab you set your source image, which requires two elements. The first element is an input file in Lab Portable Float Map format (extension .pfm). The source file represents the largely unmodified data as the camera sees it. Information about how to take photos of a color reference card and produce a .pfm output file are described below. The second element is a chart file that contains a formal description of the underlying color reference card’s layout (extension .cht). Chart files are usually shipped with your color reference card or can be downloaded from the internet.

reference values

Mon, 01 Jan 0001 00:00:00 +0000

The “reference values” tab determines the target values to which the source image must to be modified by the resulting style. You can either supply reference values in the form of measured data of your color reference card (mode “cie/it8 file”), or you can supply a photographic image (mode “color chart image”) much in the same way as described above. This second image must also be supplied in Lab Portable Float Map format. There is no need to supply the chart file again as darktable-chart takes the same one as defined under “source image”. You only need to again align the layout grid and the image and potentially adjust the “size” slider.

process

Mon, 01 Jan 0001 00:00:00 +0000

If all of the required settings in the “source image” and “reference values” tabs are ready you can move on to the “process” tab.

First, you need to tell darktable-chart which of the patches represents the gray ramp. In the screenshot displayed above, the gray ramp is positioned in the lower part of the color reference chart, denoted as “GS00…GS23”.

The “number of final patches” input defines how many editable color patches the resulting style will use within the color look up table module.

making input images for darktable-chart

Mon, 01 Jan 0001 00:00:00 +0000

To start with, you need a suitable photo of your color reference card in RAW+JPEG format. It goes beyond the scope of this manual to explain the details of how to take this photo, but in a nutshell you need to make the shot on a sunny day around midday with the light source (the sun) shining at an angle onto the card. You need to avoid any glare in the image. The neutral white color patch in the gray ramp (G00) should end up at the L value specified in the description of your card. Often this is L=92 and requires you to overexpose the shot by about 1/3 EV. Ideally you will make several shots with slightly different exposures and later select the right one in darktable. Make sure that the chart fills up most of the frame. Use a lens with a “normal” focal length (e.g. 50mm equivalent) and stop down a bit to avoid vignetting.