This is a short tech post to discuss the process you can use to texture the scans sold on Triplegangers. Each scan comes with the set of 22 *.CR2 (Canon RAW format) photographs that can be used to texture the scan providing enough 360 degree coverage. Using RAW .CR2's gives users the ability to correctly post-process the data to fit a Linear Pipeline or any other colour pipeline.

 

What is a Linear Pipeline?

We consulted on this with the expertise of Thomas Mansencal from Colour Science.

"Scene-referred imaging is the basis of physically-based rendering allowing to reproduce realistic light interaction using plausible light quantities. It also enables realistic camera effects such as motion-blur, defocus, glare, etc...
 
Measured scene linear-light quantities are usually normalised to a known reference. Commonly middle grey is set at luminance = 0.18 which is the reflectance of:
 
- A reference Kodak 18% Grey Card
- The background colour of a DSC Labs CamAlign ChromaDuMonde chart
 
The reflectance value of a X-Rite ColorChecker neutral 5 (.70 D) sample, the third neutral gray patch from the right, is ≈ 19%. It is generally used as the middle gray point in the VFX industry. Its exact value is [121, 121, 122] in 8-Bit integer non-linear sRGB (with the sRGB OETF applied on the linear sRGB values). This is the value you should aim for when you set the exposure and white balance of an image in Adobe Camera Raw (ACR).
 
The second critical aspect to consider is that Physically-based rendering implies inputting linear values into the rendering system: computations are only correct if the input data is linear. You need to ensure that colour textures which are typically representing reflectance factors of a given surface are proper linear reflectance factors. For that purpose, if you rely on ACR to process your acquired data, you must remove the ACR tone curve from the processing output.
 
The ACR tone curve is a creative tone curve whose purpose is to increase contrast and saturation on image. It is perfectly fine (and needed) for Photographers and in applications aiming direct representation on a display (Display-Referred imaging) as it provides a faithfull and pleasing output. However it should ABSOLUTELY be avoided when you create colour textures as it will inject non-linearity in the rendering system and thus bias the light transport equation."
 
We strongly recommend following a Linear Colour Processing Pipeline and will show you our process. Adobe Photoshop Camera Raw is used in this demonstration, but the same technique is applicable to Lightroom or even ACDSee Pro.

 

Linear Processing Pipeline

Firstly we recommend you download the DNG Profile Editor from here https://www.adobe.com/support/downloads/product.jsp?product=195&platform=Windows



You will notice if you open "Camera RAW/Camera Calibration/Camera Profile/ Name:"  that you have no Canon 100D profile!

 

Let's Create a Canon EOS 100D profile that is set for a linear pipeline!

1) Open DNG_Profile_Editor_win_1_0_4.exe

2) Go to Colour Tables/Base Profile/ -> Choose external profile..

Go to drive location C:\ProgramData\Adobe\CameraRaw\CameraProfiles\Adobe Standard <- Here you will see a list of example camera profiles already supplied by Adobe. We are mainly interested in:

Canon EOS 100D Adobe Standard.dcp

3) With your .dcp profile selected go to "Tone Curve panel, Base Tone Curve: Base Profile" and tick "Show Base Tone Curve". This is the standard Canon EOS 100D tone curve that we would normally use in Lightroom or Camera RAW. What we are interested in is the "Linear" curve from this Profile.

Base Profile Curve (make sure to tick "Show Base Tone Curve"

Linear Curve <- We want this!

4) With your Linear Curve selected go to "File/Export Canon EOS 100D profile.." we then need to find the location that Adobe (products) will reference:

Export to "C:\Users\"  "  "\AppData\Roaming\Adobe\CameraRaw\CameraProfiles" -> "Canon EOS 100D - Linear.dcp" ("  "  " is your user profile in Windows)

Find location



Success!



If you have Adobe Photoshop open, copying to the correct location will update the profile live in Camera Raw

5) Back in Adobe Photoshop "Camera Raw/Camera Calibration/Camera Profile/ Name:" choose the profile that we exported "Canon EOS 100D - Linear.dcp"



Notice the change in look to the image. This is what we want.

6) Next we can set the correct exposure using the "Colour Sampler Tool (S)". We want to pick on the grey checker under the yellow one as we are looking for that 18% refelectance value.

7) We then adjust the exposure in this case +1.48. We are looking for R/G values around 123/123

8) Next we can colour balance using the "White Balance Tool (I)" by clicking again on the grey checker under the yellow one.



You now have your CR2 photograph set to linear pipeline, with correct exposure values and colour balanced. You could then save out these settings for future use.

Save Image


You will then be able to save out the image using "Save Image..." to which ever format you prefer. You could save to TIFF or to DNG to load into Photoscan. We have a custom script that will allow you to switch format types in Photoscan. We will talk more on this soon.

Here are the example Canon EOS 100D profiles we generated for this tutorial Canon profiles 

For Adobe products to register the profiles they can be placed here - > "C:\Users\" " "\AppData\Roaming\Adobe\CameraRaw\CameraProfiles" (profile destination) . We included the profile for base curve and linear curve.

Next up we will post another tech blog on using these settings to batch process all your 20 images and any future images, ready to texture yours scans in Photoscan.