Processing Imagery Using Pix4D

Overview of the Software

• What is the overlap needed for Pix4D to process imagery?
• A high overlap between images is required in order to get high accuracy results automatically.
• What if the user is flying over sand/snow, or uniform fields?
• Use a high overlap with at least 85% frontal overlap and at least 70% side overlap
• Set exposure settings accordingly to get as much contrast as possible in each image
• What is Rapid Check?
• Rapid check is used in the field to get a quick preview of the outputs
• Can Pix4D process multiple flights? What does the pilot need to maintain if so?
• Yes, Pix4d can process multiple flights, and to do so, the flight height should not vary much between the flights.
• Can Pix4D process oblique images? What type of data do you need if so?
• Yes, Pix4D can process oblique images, and to do so, it is strongly recommended to use GCPs or Manual Tie Points.
• Are GCPs necessary for Pix4D? When are they highly recommended?
• Adding GCPs are optional, but are highly recommended when processing images without image geolocation
• What is the quality report?
• The quality report displays how well the imagery was processed

Walking Through the Software

Professor Hupy provided two data sets for students to process in Pix4D: 'Litchfield Flight 1' and 'Litchfield Flight 2'. The imagery did not contain ground control points (GCPs). 

Processing the Imagery

Step 1: Start a new project

Figure 1: Starting a new project on the home screen

Open Pix4Dmapper and click on 'Projects' > 'New Project...' (Figure 1)

Step 2: Name project and save it in a folder

Figure 2: Naming a project and choosing a folder to save it in

It is recommended that when naming a project to include the following within the name:

• Date
• Site
• Platform/Sensor
• Altitude

Enter the name of the project into the 'Name:' bar and select the folder where it will be saved by clicking the 'Browse...' button to the right of the 'Create In:' bar and locating the correct folder (Figure 2). For this project, it should be saved in the student's personal folder.

Click 'Next'

Step 3: Adding images

Figure 3: Adding images

Add the images that will be used for processing by clicking 'Add Images...', locating the folder(s) containing the images to be used, and selecting the desired images and adding them to the image menu (Figure 3).

Images for this exercise are located in the 'Litchfield Flight 1' and 'Litchfield Flight 2' folders.

At least three images must be selected and must also be in either TIFF or JPG format.

Click 'Next'

Step 4: Image Properties

Figure 4: Image properties 

Once the images are added, Pix4D will check to see if the images are geotagged and what coordinate system is associated with them (most UAS data defaults to WGS 84 decimal degrees)(Figure 4).

Pix4D also detects the camera model and associated info, but these defaults are not to be trusted!

To edit the camera model, click 'Edit...' under the 'Selected Camera Model' section.

Step 5: Edit Camera Model 

Figure 5: Edit camera model

Look under 'Camera Model Parameters' and notice next to 'Shutter Model' it is defaulted as 'Global Shutter or Fast Readout' -- this is incorrect.

Click on the tab next to 'Shutter Model' and select 'Linear Rolling Shutter' as this will be important for processing (Figure 5).

Click 'OK' 

This will lead back to the image properties menu (figure 4). Click 'Next'

Step 6: Select Output Coordinate System

Figure 6: Select output coordinate system 

Leave in defaulted selection (Figure 6). The imagery can be reprojected later in a GIS program if need be. 

Click 'Next'

Step 7: Processing Options Template

Figure 7: Processing options template

Pix4D offers many processing templates to choose from for processing imagery (Figure 7).

For this project, select '3D Maps'

Click 'Finish'

Step 8: Processing (Part 1) 

Figure 8: Processing and the map view screen

The map view will appear on the screen. Uncheck processing steps 2 and 3 right away (Figure 8), then click on 'Processing Options' in the lower left corner

Step 9: Processing Options

Figure 9: Processing options

Click on '3. DSM, Orthomosaic, and Index' and under the 'Raster DSM' section select 'Triangulation' next to 'Method' (Figure 9)

Click 'OK'

This will return to the map view screen 

Step 10: Processing (Part 2)

Figure 10: Processing steps 1, 2, and 3

Click 'Start' and this will begin the Initial Processing. After Initial Processing is completed, a quality report will be generated. If the report is is good, continue on to processing steps 2 and 3 by unchecking step 1, checking steps 2 and 3, and clicking 'Start' once again (Figure 10). Once processing is complete, all steps will become green.

Quality Report

Figure 11: Quality report summary

Figure 12: Overlap section of quality report

The quality report provides extensive feedback on how well the imagery was processed. The report is generated after Initial Processing is complete. The 'Summary' (Figure 11) provides information such as the date and time of processing, camera model name, and the area covered. According to the 'Quality Check' section that appears just after the 'Summary', all 155 images that were selected for processing were utilized, a 100% image calibration. Further down the quality report is the 'Overlap' section which shows the number of overlapping images processed for each pixel of the orthomosaic. Green areas represent high overlap while red and yellow areas represent low overlap and can lead to poorer results in these areas. Looking at the overlap for this project (Figure 12), there appears to be good overlap overall except for some areas on the edge of the processing area, particularly on the south and north edges. This may be due to there not being as many images that can be overlapped with each other at the edges as compared to areas towards the middle of the orthomosaic. 

Fly-Through Animation

The final part to be completed in Pix4D is to create an animation that flies through the orthomosaic. This can be done through the following steps:

• Click on 'rayCloud' on the left side underneath 'Map View'
• Click on the 'New Video Animation' button (the one that looks like a video camera) underneath the 'Create' tab
• This will bring up the 'Video Animation Trajectory Creation' screen; accept the default and click 'Next'
• This will bring up the 'Record Video Animation Trajectory Waypoints' screen; from here select the desired camera angle and orientation and click 'Record Trajectory Waypoint' and keep doing this until all desired waypoints have been added
• Click 'Next' to continue to 'Rendering Options'
• Select the desired length of time of the animation, give it a name, and click 'Finish'
• The animation can be played back and the duration/speed of the animation can be adjusted accordingly; when the animation is suitable, click 'Render' under the 'Video Rendering' section and save it to the appropriate folder 
• The animation for this project can be viewed in the 'Results/Discussion' section below

Results/Discussion

Figure 13: Final maps including hillshaded DSM (left)
and orthomosaic (right)

Pix4D created a DSM and an orthomosaic which could then be used in ArcMap for further analyses and map-making. Figure 13 features the DSM with hillshade on the left side while the orthomosaic is on the right. A hillshade was ran on the DSM, then given a diverging color symbology and set over the top of the DSM with a translucent effect of 35%. Green areas represent low shade while red areas represent high shade. When comparing these areas of extreme low to high shaded relief to the orthomosaic, many of them represent large piles of sand. In the orthomosaic, the grey/golden areas are the mine and the sand itself. A road running north/south on the right side of the image is clearly visible as it turns into the mine in two different places. A aqua blue body of water just barely appears on the northwestern edge of the 

image while a group of thick trees appears in the middle of the southern edge of the image and just barely on the east side of the image as well. 

 Here is the animation created in Pix4D of the processed Litchfield Flight data which shows the 3D imagery very well by capturing the image from multiple angles at different orientations.


Conclusion

Based simply off of this one activity, which just introduces students to the software without getting into "very advanced" territory, Pix4D seems like a pretty easy program to use and it runs and processes smoothly. All of the steps in setting up the project seem pretty straight forward, although if Dr. Hupy hadn't assisted students in a couple of sections with tips (such as changing the 'Raster DSM' 'Method' to 'Triangulation' in 'Processing Options') the project may not have gone as smoothly. If a user does get tripped up, the program offers many resources for assisting those who are in need of help along the way. I was surprised how easy it was to create the animation of the orthomosaic at the end of the project. Overall Pix4D seems like a good program to use for processing UAS imagery.