This is a graphical depiction of aspects of the developed image. Moving the mouse over the panel displays buttons to adjust the display. The leftmost button allows for cycling the mode between a "regular" and a waveform-style histogram. The next button over adjusts how the data for the current mode is displayed. The three rightmost colored squares are toggles which enable or disable the display of specific color channels.
When the mouse is over the histogram panel, scrolling while holding down the Ctrl key will change the height of the panel. You can hide/show the histogram entirely with a keyboard shortcut. The default shortcut is Shift+Ctrl+H.
It is possible to assign keyboard shortcuts to cycle through or switch histogram mode/type. Separate shortcuts can be set for the darkroom and tethering views. By default, none of these shortcuts are associated with keys.
For the purposes of speed, the scopes are calculated from the image preview. This is the image which is displayed in the navigation panel (see Sezione 3.3.2, «Navigazione»), rather than the higher quality developed image which is displayed in the center view. The preview is calculated at a lower resolution and may use shortcuts to bypass time-consuming image processing steps. Hence scopes may not represent fine detail in the image, and may deviate in other ways from the final developed image.
This shows the distribution of pixels by lightness for each color channel channel. In its default state, data for all three RGB color channels are displayed. The x-axis runs from 0% to 100% lightness for each channel. The y-axis height gives the count of pixels which have the given lightness, up to the maximum count for the developed image.
Clicking the second-to-leftmost button on the panel toggles between a logarithmic and a linear rendering of the y-axis data.
The leftmost button of the panel is a toggle which flips to a waveform scope. This scope includes spatial data about the image. The y-axis represents the distribution of pixels by lightness for each channel. The scope's x-axis represents the distribution of this data across the x-axis of the image. The lightness of each point of the waveform represents the quantity of pixels represented at that scope position.
The thick dotted horizontal line at the top of the waveform histogram represents 100% lightness. Hence, any channel which displays above that line may be clipped in the resulting image. The thinner dotted horizontal line represents 50% lightness. The bottom of the scope represents 0% lightness.
As with the "regular" histogram, it is possible to selectively display red, green, and blue channels, or (the default) overlay all three of them.
See Of Histograms and Waveforms for more on darktable's waveform scope.
This shows the same data as the waveform, but with the red, green, and blue channels side-by-side. When in waveform mode, clicking the second-to-leftmost button on the panel toggles between waveform (overlaid) and RGB parade rendering of the data.
The RGB parade can be useful for matching the intensities of the red, green, and blue channels. It can also help with understanding the differences between and individual qualities of each channel.
The histogram is directly linked to the exposure module described in Sezione 220.127.116.11, «Esposizione», and you can operate some of the exposure module's controls from the histogram.
For the regular histogram, you can left-click towards the right hand side of the histogram and then drag right to increase or drag left to decrease the exposure. In a similar manner you can control the black level by clicking and dragging in the left hand side.
For a waveform-style scope, dragging the upper portion of the histogram up/down will increase/decrease exposure. Dragging the lower portion up/down will increase/decrease the black level.
Scrolling in the appropriate area -- rather than dragging -- will also adjust exposure and black point. Double-clicking in the histogram resets the exposure module's parameters to their defaults.
For the regular histogram, the image data is converted to the histogram profile before the histogram is calculated. You can choose this profile by right-clicking on the softproofing or gamut checking icons in the bottom panel (see Sezione 3.3.11, «Pannello inferiore») then selecting the profile of interest.
Besides choosing a specific colorspace, you can also choose the work profile, export profile, softproof profile, or display profile.
In the case of the waveform and RGB parade scopes, the colorspace depends on the
configuration variable. In its default (true) setting, these scopes are calculated in the «Adobe RGB (compatible)» colorspace. Otherwise, they are calculated in the display colorspace.