Each MATCH cube pixel with a valid LAT/LON is checked for a corresponding LAT/LON target body intercept in the FROM cube. If the corresponding FROM cube pixel either does not intercept the target body or is occluded, it is set to NULL in the TO cube. If the corresponding FROM cube pixel does intercept the target body, interpolation (as determined by INTERP) is applied to the FROM cube pixel to calculate the output TO cube pixel DN. This process employs the rigorous application of the FROM and MATCH cube camera distortion models and ray tracing from instrument to target body.

As an example, given overlapping Mars Global Surveyor MOC and Mars Odyssey Themis cubes, cam2cam can convert the MOC image to have the same geometric camera characteristics as the Themis cube (or vice versa). Another application is to align independent bands in a camera cube. For example, a raw Themis cube (after thm2isis) will not have aligned pixels. cam2cam can be used to align the pixels without going to a map projection. To prevent pixels from falling off the edge of the cube, run pad on the cube prior to cam2cam.

The OFFBODY parameter allows for the projection of off-body data using RA/DEC (right ascension/declination) coordinates in the MATCH image. OFFBODY is FALSE by default to preserve existing cam2cam behavior. If OFFBODY is TRUE and projection with LAT/LON coordinates fails, then it is reattempted with RA/DEC coordinates. If the RA/DEC in the MATCH image exists in the FROM image, interpolation is used to calculate the output TO cube pixel DN. It may occur that the FROM image RA/DEC coordinates will also intercept the target body in the FROM cube. This would result in the assignment of an invalid LAT/LON in the TO cube. OFFBODYTRIM=TRUE prevents this by instead assigning NULL to the output pixel.

The projection of off-body pixels has proven valuable for example in the analysis of particles in orbit around the asteroid 101955 Bennu. See for example...

Hergenrother, C.W. et al. (2020). Photometry of Particles Ejected from Active Asteroid (101955) Bennu, JGR Planets, 125, e2020JE006381 doi:10.1029/2020JE006381

The images were processed with cam2cam three times with the following OFFBODY and OFFBODYTRIM settings ...

• OFFBODY=FALSE (DEFAULT options)
• OFFBODY=TRUE; OFFBODYTRIM=TRUE
• OFFBODY=TRUE; OFFBODYTRIM=FALSE

• OFFBODY=FALSE (DEFAULT options)
• OFFBODY=TRUE; OFFBODYTRIM=FALSE
• OFFBODY=TRUE; OFFBODYTRIM=TRUE

Input OSIRIS-REx MapCam Image 20190509T180552S020

Output from Projecting DTM Image to Ellipsoid Image

In the images above, yellow represents NULL DNs and black is sky/background. In NULL regions, LAT/LON coordinates on the ellipsoid are not visible (or are occluded) on the DSK shape model from this observer-to-target configuration.

Output from Projecting Ellipsoid Image to DTM Image

Again, NULL DNs are yellow and sky/background is black. There are no NULLs in the target body interior of these images as all Lat/Lons come from the DTM image geometry and map directly to the ellipsoid. The NULLs in the rightmost image where OFFBODYTRIM=TRUE are portions of the ellipsoid that do not map to Bennu's surface. These off-limb regions between the ellipsoid boundary and DTM surface are actually sky/background data. More specifically, the MATCH image pixel coordinates here a) did not intersect Bennu's surface in MATCH/TO, b) the RA/DEC coordinates in MATCH/TO did map into the FROM image FOV, and c) the look direction vector from the FROM pixel to the RA/DEC also intersects Bennu's surface in the ellipsoid model. And so, NULL is assigned to the TO image at those pixels.

Without the use of OFFBODYTRIM, portions of FROM image surfaces could be propagated to the output TO image erroneously. Often, these are indistinguishable from valid surface and may lead to incorrect interpretation.

See noproj for a more practical use for this example and the importance of the OFFBODYTRIM parameter.

To Reproduce This Example

Download the shell script and ray tracer preference files linked here. The preference file sets the ray trace library to Bullet rather than NAIF DSK (default for shape models with a ".bds" file extension) as Bullet will detect occlusions. Before running the script, place the preference file in $ISISROOT. Download and install the OSIRIS-REx kernel into $ISISDATA. See the ISIS documentation for instructions on downloading $ISISDATA for a specific mission.