orthophoto

Tutorial #A2 – Orthophoto generation

Welcome to the 3DF Zephyr tutorial series. In this recipe, you will learn how to generate an ortophoto with 3DF Zephyr.

This tutorial requires 3DF Zephyr Pro (prior 3.700) or 3DF Zephyr Aerial.

This tutorial is not about reconstruction: if you are not interested in geographical analysis and survey you can probably skip this tutorial.
  • Step 1 – introduction

3DF Zephyr is used in a wide variety of case scenario, from asset generation for videogames to precise geographic measurements. Orthophoto generation is critical for these case studies where – usually – a drone or other aircraft takes pictures of a certain area and surveys needs measurements  without having to worry about distorsions. If you are using 3DF Zephyr for this purpose, you probably know all of this very well, but for those unfamiliar, let us quote Wikipedia:

An orthophoto, orthophotograph or orthoimage is an aerial photograph geometrically corrected (“orthorectified”) such that the scale is uniform: the photo has the same lack of distortion as a map. Unlike an uncorrected aerial photograph, an orthophotograph can be used to measure true distances, because it is an accurate representation of the Earth’s surface, having been adjusted for topographic relief, lens distortion, and camera tilt.

With 3DF Zephyr you can generate an orthophoto just in few clicks: we will be using the same dataset used for control points. Please prepare your workspace and then proceed.

  • Step 2 – Orthomosaic/Orthophoto generation
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orthomosaic_tutorial
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orthophoto_tutorial
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 From the “Tools” Menu (1) select Orthophoto and orthomosaic” (2) and then “Generate Orthophoto” (3). The “Orthophoto Wizard” window (4) will appear. To create the orthophoto, you must have at least three control points defined. To select the control points needed to generate the orhophoto, simply use the Add (5) and Remove (7) buttons or use the “Add all points” (6) button depending on your scenario. Once you have chosen your control points, simply click the “Next” Button (8). The Orthophoto Wizard will now present a whole new set of options while also rendering a preview in your scene. You can zoom, translate and rotate the camera with the camera controls (distance, rotation, X,Y ) and see the changes happen in real time in the Zephyr viewport. You can save a preview screenshot with the Take Screenshot button, or actually proceed to the orthophoto generation by clicking the “Generate Orthophoto” Button.

Similarly, you can clikc “Generate Orthomosaic” if you want to generate an orthomosaic: the orthomosaic generation requires a dense point cloud in the workspace. After selecting the generation mode in the first wizard page (by control points or by axis) the orthophoto generation wizard will appear:

3DF Zephyr Aerial will show the current coordinate system (A).

The “Position” subsection (B) (in this example the axis mode has been selected) will define the initial and final position of the orthomosaic. You may set the limits via control points or by dragging a rectangle with the rubber band tool

You may control the appearance of control points and control distances using the “show control poitns and distances in preview”  checkbox.

The “Export” (C) subsection allows to export:

  • World file (3DF Zephyr Pro and 3DF Zephyr Aerial)
  • Autocad script (3DF Zephyr Aerial) – exports a scr file that will automatically load the orthomosaic within autocad, with the correct scale settings set in Zephyr
  • .KML file (3DF Zephyr Aerial)

The Output subsection (D) allows to define the width and height of the output file (thus controlling the ground sample distance)

The Advanced subsection (E) allows to control the color generation mode for each single pixel. This is system is very similar to the texture generation system. We advise to leave the color balance on with 1 camera per pixel. If you wish, you may however use more cameras for a different color weight. If this happen, you may choose between a fusion algorithm:

blending – will weight the color source depending on the view angle and the distance. Although it’s robust to lighting changes, the mean won’t keep high frequencies.

multiband – will use the mean to to compute low frequencies and the best images to compute the high frequences. This is usually a better choice than blending because while it is still robust to lighting changes (usuaully just a little worse than blending) by preserving the high frequencies the resulting image will be more in focus and sharp.

To proceed with the orhtophoto generation you must have generated a mesh first.

Every option in this wizard has the same explanation as the orthomosaic. Please note how in this screenshot the control point mode has been selected to define the orthophoto plane, and thus the “Position” subsection is slightly differnt, offering parameters such as distance, rotation and X,Y positions.

As a last note, it is possible to export the .DEM file in the Export subsection (by specifiying the elevation axis)
  • Final notes

The next tutorial will show how to merge more file projects togheter. Click here to proceed to the next tutorial.