Integrated Software for Imagers and Spectrometers v3. ISIS3 is a digital image processing software package to manipulate imagery collected by current and past NASA and International planetary missions.
Description
I am not a programmer but I have been trying to understand the PointPerspective mapping. I had the same question as the 2020 issue #3873 (units of DIST and reference from the surface or center). That issue was apparently closed with the finding that the documentation was correct and the distance was from the center, and the current documentation reflects that: "Distance from the center of the target in kilometers: value should be greater than the target radius."
Trying to confirm this for myself in the code, I see the following (PointPerspective.cpp::77):
// Get the distance above planet center (the point of perspective from
// the center of planet), and calculate P
m_distance = mapGroup["Distance"];
m_distance *= 1000.;
m_P = 1.0 + (m_distance / m_equatorialRadius);
So m_P is effectively (m_distance + m_equatorialRadius)/m_equatorialRadius, which suggests that the input is the distance from the surface (or, at least, the equator). Again, I am not a programmer...I am assuming that mapGroup["Distance"] is returning the DIST input.
Importantly, while inspecting the code and checking the use of m_P, I noticed line 161:
bool PointPerspective::SetGround(const double lat, const double lon) {
// Convert longitude to radians & clean up
double projectionRadius = m_equatorialRadius*sqrt((m_P+1)/(m_P+1));
I suspect that (m_P+1)/(m_P+1) is a bug; this logic always sets projectionRadius == m_equatorialRadius. Elsewhere in the file we have logic that looks more functional (line 326):
ISIS version(s) affected: 8.3.0
Description
I am not a programmer but I have been trying to understand the PointPerspective mapping. I had the same question as the 2020 issue #3873 (units of DIST and reference from the surface or center). That issue was apparently closed with the finding that the documentation was correct and the distance was from the center, and the current documentation reflects that: "Distance from the center of the target in kilometers: value should be greater than the target radius."
Trying to confirm this for myself in the code, I see the following (PointPerspective.cpp::77):
So
m_Pis effectively(m_distance + m_equatorialRadius)/m_equatorialRadius, which suggests that the input is the distance from the surface (or, at least, the equator). Again, I am not a programmer...I am assuming that mapGroup["Distance"] is returning the DIST input.Importantly, while inspecting the code and checking the use of
m_P, I noticed line 161:I suspect that
(m_P+1)/(m_P+1)is a bug; this logic always setsprojectionRadius == m_equatorialRadius. Elsewhere in the file we have logic that looks more functional (line 326):How to reproduce
Reported from documentation and code inspection; no MWE available.
Possible Solution
Doc update and correction of line 161.