Capturing a Calibration Image
Follow this page when capturing a calibration image.
Last updated
Follow this page when capturing a calibration image.
Last updated
Marso requires a Calibration Image to produce correct results for the specific capture conditions. This includes the light source, camera, lens and exposure settings. The calibration image will be used to Create an Exposure Calibration.
You will also need to measure the distance between the light source and the grey card.
A Calibration Image is an image of a standardised grey card under your capture lighting conditions. This means lit only by your flash or light source, and with manual exposure and focus.
If any capture parameters change, you will need to capture a new calibration image, otherwise the resulting textures may be less accurate
If you do not already have a calibration grey card, M-XR recommends the Datacolor SpyderCheckr24.
If you do have a grey card, check that it is listed on the Grey Cards | Known Values page, as you will need to know the grey value of at least one tile, ideally a mid grey.
Place the grey card at a similar distance from the camera as you are you are likely to capture. Try to ensure the grey card positioned as close to perpendicular to the camera as possible.
If you are not using the on board flash, attach the light source to your camera as it will be mounted for capture.
Fix the position of the camera also. You will need to measure the distance from the light to the grey card, so this will be easier if the camera is stationary.
Measure the distance from the Light to the Grey-Card. This can be done by holding a piece of string from the light to the subject, and marking the ends, then measuring it with a ruler.
If you have multiple lights, you only need to measure the distance to one light, as they should all be a similar distance from the Grey Card if they are positioned rotationally symmetrically around the camera.
Marso also requires the position of each light relative to the sensor on the camera. The sensor is located in the centre of the lens, aligned with a mark ϕ on the body of the camera.
The Light position needs to me measured in each dimension (X, Y, Z), rather than straight line distance.
If you are using a Ring Flash, measure to the centre of the ring.
Make a note of all of these measurements. You will need them to calibrate Marso
For best results, it is recommended to shoot -2.5 stops underexposed when spot metering on a middle grey card (50%). An under-exposed image will preserve the specular highlights, whilst maintaining a good amount of the shadows.
The exposure settings used for calibration must also be used for any future scanning sessions, otherwise a new calibration will be required
Once an exposure value has been identified, take a RAW photograph. Save it with a descriptive name. This will be used in the Marso calibration process.
