You can manually change the status of line segments: Left-Click on a line to select it (turn the color to green or blue) and Right-click to deselect it (turn the color to red or yellow). This outlier elimination involves a statistical process using random sampling which means that each time you press the “get structure” button the color pattern of the lines will look slightly different. Lines marked in red or yellow are regarded as outliers and are not taken into account during the automatic fitting step. Line segments are displayed as overlays on the image canvas, with the type of line identified by color as follows: green Vertical converging lines red Vertical lines that do not converge blue Horizontal converging lines yellow Horizontal lines that do not converge gray Other lines that are not of interest to this module Only lines that form a set of vertical or horizontal converging lines are used for subsequent processing steps. Both variations can be used alone or in combination if the default analysis is not able to detect a sufficient number of lines. Ctrl+click to apply an edge enhancement step before performing further analysis. Shift+click to apply a contrast enhancement step before performing further analysis. Icon to analyze the image for structural elements – darktable will automatically detect and evaluate line elements. This method is similar to how “keystone” correction works in the crop and rotate module. Once you are happy with your changes, re-select a “fit” icon to complete the process. You can re-enter this mode to edit your drawn rectangle at any time. Once you are happy with your rectangle, click one of the “fit” icons to complete the process. This will draw a rectangle on the screen and you can grab and move the corners of the rectangle so that the left and right sides fall on lines you wish to make vertical, and the top and bottom fall on lines you wish to make horizontal. Icon to enable perspective rectangle drawing mode. 🔗manually define a perspective rectangle Edit a line by clicking and dragging on the line or the end nodes, and right-click a line to delete it. You can re-enter this mode to edit your drawn lines at any time. Draw as many lines as you wish (the more lines, the better the fit mechanism will work) and then click on one of the “fit” icons to complete the process. The module will automatically detect whether the lines are horizontal or vertical and color them green or blue, respectively. Icon to enable line drawing mode and then click-and-drag on the image to draw lines that you wish to become horizontal or vertical. Three alternative methods are provided to do this: 🔗manually draw structure lines The first step is to obtain details about the horizontal and/or vertical structures in the image.

🔗perspective correction workflow 🔗structure This will cause the rotation parameter to be automatically adjusted to make the drawn line horizontal/vertical with respect to the image frame. While the module is active (and none of the structure buttons are selected) you can right-click and drag anywhere on the image to define a horizontal or vertical line. Click the “perspective” header to expand the controls.

Based on these (automatic or manually-drawn) line segments a fitting procedure is initiated, which determines the best values for the module’s parameters.Īs the most common use case for this module is for rotation, the perspective correction controls are hidden by default. You can also set the line structures manually by drawing a “perspective rectangle” or drawing multiple horizontal and vertical lines onto the image. In order to perform automatic correction the module is able to analyze the image for suitable structural features consisting of line segments. Corrections can be applied in a vertical and horizontal direction, either separately or in combination. This module is able to correct converging lines by warping the image in such a way that the lines in question become parallel to the image frame. Converging lines are a special case of perspective distortions frequently seen in architectural photographs – parallel lines, when photographed at an angle, are transformed into converging lines that meet at some vantage point within or outside of the image frame. Perspective distortions are a natural effect when projecting a three dimensional scene onto a two dimensional plane and cause objects close to the viewer to appear larger than objects further away. This module also allows for the rotation of the image to be adjusted. The underlying mechanism is inspired by Markus Hebel’s ShiftN program. Automatically correct for converging lines, a form of perspective distortion.

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