AN APPROACH TO PHOTOGRAMMETRY, AGISOFT

PHOTOSCAN AND PREPARING THE MESH FOR A

CG WORKFLOW

A PART OF NCCA INNOVATIONS PROJECT BY THOMAS RICE

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AN APPROACH TO

PHOTOGRAMMETRY,

AGISOFT PHOTOSCAN

AND PREPARING THE

MESH FOR A CG

WORKFLOW

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CONTENTS:

1.0 – Introduction

2.0 – Capturing images of your subject

3.0 – Basic rules of taking photos for photogrammetry

4.0 - A technique for shooting

5.0 - How to capture the images

6.0 - Taking the images into Agisoft Photoscan

7.0 - Importing the mesh into Zbrush for cleanup

8.0 - Bibliography

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1.0 Introduction

This is a guide based on the Innovations project that I undertook at the NCCA Bournemouth University.

It will detail an approach to capturing images for photogrammetry using a single camera in the hope to

raise awareness of the technology to the average CG artist.

I have chosen to use photoscanning software Agisoft Photoscan as it is the best I can obtain at this

point in time, but there are many others available out there some of which are free (Autodesk 123D

catch).

While my methods may not be the best it is one way of tackling the problem and I hope that the reader

of this can develop the techniques to continue the creation of innovation.

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2.0 Capturing images of your subject

Firstly you should choose an appropriate subject, in general most objects can be photoscanned but

here is a small list of object that I have found to not work.

Fig.1 Reflective object - (S-media-cache-ak0.pinimg.com)

Fig. 2 Glass object - rackcdn.com

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Fig.3 Shiny and thin geometry

Objects to avoid

Reflective

Shiny

Transparent

Hard to detect thin geometry

The above objects will generate a mesh that will that is either extremely badly detailed or nothing at all

in the case of glass. This is due to the fact that the processing needs to detect points as reference and

if you have reflections on your geometry it will confuse the calculations.

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3.0 Basic rules of taking photos for photogrammetry

This section is very important to understand in order to achieve the optimal mesh.

Lighting conditions

Shooting outside:

You should avoid environments with strong casting shadows as it interferes with the

feature matching processes.

Ideally shoot on an overcast day or in the shade to try to guarantee even lighting.

Shoot in the morning or afternoon when the sun is not directly above the subject.

Shooting inside

Soft diffuse lighting should be used

Do not use the camera flash

Even lighting from all angles should be used

Do not interfere with the light rays when moving to take photographs

Equipment

Any camera can be used but the higher the quality the sharper the photos.

A fixed lens is recommended, any focal length can work but 50mm is the most favored.

A tripod should be used to avoid camera shake and to account for slow shutter speeds.

Shooting

The object should not move

Photoscan software doesn’t handle repeated backgrounds such as bricks well

Have as much of the object as possible in frame

Take a photo every 5 degrees in the pattern as suggested in fig.3

Use as low as an ISO as possible to ensure the sharpest image

A low aperture will help to focus the software on your target and not the background

Exposure should be set so the object is well exposed and the background under.

Objects that are reflective, shiny, transparent will not work with this process.

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4.0 A technique for shooting

This technique was developed during experimentation for my project. I’ve found to be most effective for

human scan with a single camera as it takes advantage of how the processing software works.

The plan view describes how you should shoot when capturing an object that is not a room or

landscape. Taking images in a 360 degree curve, you should focus on your subject and try to keep as

much in frame as possible.

The side view describes how you should shoot when capturing a landscape or interior, follow the

outside walls of the location taking images every time there is a 60% overlap.

Fig 3. The recommended shooting angles for indoor and outdoor situations. (ReCap)

Expanding on this technique the below method has been developed to generate the best results from a

scan.

5.0 How to capture the images

1. Make sure that your camera setting are correct so that you can capture the sharpest image

while moving the camera at a constant speed.

2. Using a continuous shoot mode on your camera and aiming at the top of your subject.

3. Continuously take images while moving the camera down the subject until you reach the

bottom.

4. Repeat this method in the pattern suggest in fig.3 until you have a full 360 degree coverage.

5. If you require more detail in specific areas it is then suggested to take a full 360 degree close up

scan of that area.

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Fig 4. Shooting from top to bottom of the model with continuous shot.

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Fig 5. Shooting close up images for more detail.

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Fig 6. Illustration of the new technique for shooting.

The technique developed is relatively simple but very effective for its purpose. Firstly the camera

settings need to be correct to achieve the sharpest picture possible while moving the camera

(continuous shooting). When these settings are correct the photographer should then start at either the

bottom or the top of the model and progressively bring the camera to the opposite position in the y axis

(top/bottom) while capturing images of the subject. This is then to be repeated for a full 360 degrees of

the model making sure there is at least a 60% overlap between photos. Then for areas that need to be

more detailed e.g. the head, take 360 degree images of that area making sure that most of the frame is

taken up by the target.

Using the old method for capturing a full human, you would be sacrificing pixels to make sure that the

whole body was in shot. Anything that is out of focus is useless, so when capturing the whole body in

one shot you risk the chance of part of the body not being sharp enough. With this new technique you

capture sections of the body in high detail which then allows for more accurate processing.

While this technique is very effective you will still get some blurred images so it is recommended to

remove these images from the processing stage.

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6.0 Taking the images into Agisoft Photoscan

Agisoft photoscan is my software of choice for processing the images, as mentioned before you can

use whatever you choose.

At this point I feel it will be worth explaining a way of masking that will could save you much time if your

images generate good results.

Agisoft Photoscan allows the user to generate a “mask” for each image that informs the software of the

areas that you want to calculate. This is traditionally done by taking each image into a photo editing

software and generating an alpha.

Fig 16. Left is the original image. Right image mask.

This process is lengthy and can be very awkward when the subject is not easily differentiated from the

background. When you have 30+ images this process becomes extremely time consuming there for a

new technique was needed.

A method that sometimes works is the “import mas by model” mask option. My suggested way of using

this is to put your alignment, dense point cloud and mesh setting to their lowest and generate a model.

Then use this model as your mask, Import -> Import Masks -> Import from model.

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Fig 17. Import options masks generated from simple model (black and white images).

NOTE – please note that you should use the lassoo tool to select any points which you can see

are not your subject and delete them from the calculations to save processing power for

unwanted object and to sharpen your mask.

The above should be done at the start of this process but if you are learning how to use this software

please read the below to familiarize yourself with the processes first.

Generating the mesh

Step 1

Firstly load the images into the software – Workflow -> Add Photos

You will see that your images are loaded at the bottom of the screen.

Step 2

The next step is to align the photos, this is where the software compares all of the images and

calculated the camera location and point location of your subject. To do this go to Workflow -> Align

Photos

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Step 3

The time this takes will depend on the amount of photos and complexity of your scene. You should now

have a point cloud displayed, use the selection tools underneath the Workflow menu to select any

points that you think are errors or you do not want to be in your scene. By doing this you will save

computation time on wasted elements and it allows a cleaner mesh before its generated. Once you

have selected your points (red points are the selected ones) press the X symbol next to the right of the

selection tools to delete the points.

Step 4

In order to build a mesh you have to build a dense cloud. Go to Workflow -> Build Dense Cloud and

choose the quality you desire.

Step 5

Once your cloud is built then go to Workflow -> Build Mesh to build your mesh from the cloud.

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Step 6

Finally go to Workflow -> Build Texture to build your texture.

This will give you a mesh that is textured from your photographs, it will undoubtedly need clean up in an

external program. If you want to export the model and texture go to File -> Export model and make

sure that the texture is also selected to export that with the object.

7.0 Importing the mesh into Zbrush for cleanup

Step 1 – Import into Zbrush

Import model into Zbrush

Create a duplicate

Hide duplicate model

We create a duplacite so that we can alter the topology

and poly count of our model for easier sculpting then

reporject our changes at a later time.

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Step 2 – Dynamesh

Under Geomentry -> Dynamesh. This will retopologise your mesh and delete any points

faces that can cause problems, it is also perfect for filling gaps that could be missing in your

geometry. The dynamesh resolution is up to you but should be high enough to retain most of

the detail.

Step 5 – Cleaning noise and adding back lost detail

Use a combination of a low intensity Smooth and Flatten brush to gently remove the noise on

the model. Other brushes can be used to add detail back to areas that are lacking.

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Step 6 – Retopologising

At this stage it is wise to retopologise your model in any software package that you see fit.

Export the model from Zbrush.

I prefer to use topogun as you can use your mesh as a guide and draw new lines over to top

of it, this way you do not alter the geometry.

Step 7 – Reprojecting

Import the retopologised model back into Zbrush. Once

you are happy with that model save a morph target of it.

Moph target tab – StoreMT.

Divide your new mesh until it comes close to the amount of

polygons that the original mesh had.

Turn on the visibility of the original mesh then highlight your

new mesh.

Under Subtool – project, click the Project all button to

reproject all of the detail from the original mesh onto the new

mesh.

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Step 8 – Using the morph target

You will notice that you have lost all of the work you did on the new model and that it has been replaced

by the old.

The trick now is to select the Morph brush and paint in the areas that you corrected earlier.

By storing the morph target of your new mesh it allows you to use the morph brush to paint the new

mesh back, therefore allowing you to keep the detail from the original mesh and correct any errors

present.

Step 9 – Taking the mesh back into Agisoft and creating a texture.

Once you are happy with your new retopologized and detailed mesh it is then time to take it back into

Agisoft Photoscan to reproject your texture.

Export your model from Zbrush.

Open the scene that you generated your model from and click on Tools – Import – Import

Mesh.

The mesh should be in the same position and scale as the original mesh

Finally go to Workflow – Build Texture

This should reproject the texture onto your newly topologized model. The texture will probably need

some clean up too, I recommend that you take it into Photoshop or another dedicated 3D texturing

program to make this as simple as possible.

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Bibliography

Fig.1 –

S-media-cache-ak0.pinimg.com,. N.p., 2015. Web. 3 Mar. 2015.

Fig.2 -

http://65e226e929a5af0b2a5cb2b05f995fc8916f7d7814d5d83ccb0d.r53.cf3.rackcdn.com/produ

cts/1000/wine-white-wine-glass-set-of-4.jpg

Fig. 3 –

News, Autodesk. 'What Makes Photos Good For Photogrammetry?'. ReCap Product News.

N.p., 2014. Web. 21 Feb. 2015.