Photoscan and Light

Tips and Tricks

Keeping with the theme of recent posts about light, we have some tools available in Photoscan available to us. Like any tool, its useful to know when to apply them and when the outcome might not be quite as we hoped for. Here’s a screen grab of a model as processed with no tools applied:

The Vobster Ford Escort, complete with interpretive art on the bonnet. A big thanks to Barry and Marcus for use of the model.

The Ford Escort is one of several vehicles dumped in Vobster Quay inland diving. The car may well be a teaching aid in the forthcoming Photogrammetry Workshop run by Simon Brown and Tim Clements.

We have chosen this model as it shows off how a couple of tools in Photoscan work.

Remove Lighting

This tool works by flattening out all the contrast in a model and even up the lighting. This is best done in-camera and at time of capture but may not always be possible. A statue under bright sunlight will always have a shadow side and it might not be possible to light it evenly. Underwater its very easy to end up with images of extreme contrast, with the lit areas being exposed but surrounded by dark green or blue lighting in the background. Remove Lighting tries to strip the source of light back and get rid of any contrast. Here’s the same view of the same model after Remove Lighting has been applied:

The Escort with Remove Lighting applied

This tool has its uses, but for me the lack of shadows and contrast makes the scene look less natural. The as-processed view gives the viewer a closer experience to actually being there.

Calibrate Colours

Calibrate colours looks at the images used to generate the model and applies a correction to the brightness and colour of the image. Colour correction can be tricky in the underwater world, typically there is a dominance of blue or green casts depending if we are in tropical (blue) or temperate (green) water. Either way, we lose the red band of light as depth increases, progressively losing more colour from the spectrum the deeper we go. This is why underwater photographers carry flash guns or lights to restore the colour, or shoot in natural light and have model that dominated by blue or green texture. Colour correction is applied too…to make the image look ‘right’ to the eye.

Underwater images and their colours behave differently to topside images. As depth increases, so colour is robbed. This example shows how Calibrate Colours has attempted to do as the name suggests.

Any tool intended to automatically ‘fix’ photos will often struggle with the challenges of underwater conditions. Its a niche application so no one really considers underwater light and how it behaves when thinking about the issues of general photography. Besides, when you understand how to make the camera do what you want, all of these problems go away.

The Photoscan tool does not look like its written around the demands of underwater photography. The blue cast has been picked from a few frames that caught some diver kit in the extremities. Unless these were masked out they will be taken into account. The rust on the roof of the car has been enhanced, but otherwise the scene looks very similar to the as-processed example.

Using the Orcalight

A few weeks ago I nipped along to Vobster Quay Inland Diving to make sure everything was comfortable before taking a look at a Deeper Dorset target in 45m. This gave me a chance to scan the Escort using Jimmy the double headed Orcalight and compare the results.

Shot with the Orcalight, the Escort has rendered in a different style.

And there is a link to the 3D model here:

Overall, its going to be more effective and efficient to sort the images during capture rather than rely on automatic tools to process later.