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I am trying to manipulate an image in equirectangular projection with ImageForwardTransformation. I want to change the projection's inclination to the ecliptic. For this, I created a transformation function

changeEcliptic = 
 Compile[{{θφ, _Real, 1}, {rpy, _Real, 1}},
  With[{θ = θφ[[2]], φ = θφ[[1]], roll = rpy[[1]], pitch = rpy[[2]], yaw = rpy[[3]]},
   {
    ArcTan[
      Cos[θ] Sin[pitch] + Cos[pitch] Cos[roll + φ] Sin[θ],
      -Cos[pitch] Cos[θ] Sin[yaw] + Sin[θ] (Cos[roll + φ] Sin[pitch] Sin[yaw] + 
        Cos[yaw] Sin[roll + φ])
    ],
    ArcTan[
      Cos[pitch] Cos[yaw] Cos[θ] + Sin[θ] (-Cos[yaw] Cos[roll + φ] 
        Sin[pitch] + Sin[yaw] Sin[roll + φ]), 
      Sqrt[(Cos[θ] Sin[pitch] + Cos[pitch] Cos[roll + φ] Sin[θ])^2 +
        (Cos[pitch] Cos[θ] Sin[yaw] - Sin[θ] (Cos[roll + φ] Sin[pitch] Sin[yaw] + 
        Cos[yaw] Sin[roll + φ]))^2]
    ]
   }
   ],
  CompilationTarget -> "C", RuntimeOptions -> "Speed"
  ]

which I derived from

CoordinateTransform["Cartesian" -> "Spherical", 
 RollPitchYawMatrix[{roll, pitch, yaw}].# &@
  CoordinateTransform["Spherical" -> "Cartesian", {r, θ, φ}]]

I then import my image

mwpic = Import["https://upload.wikimedia.org/wikipedia/commons/thumb/a/a1/ESO_-_The_Milky_Way_panorama_%28by%29.jpg/640px-ESO_-_The_Milky_Way_panorama_%28by%29.jpg"]

And apply the transformation:

ImageForwardTransformation[mwpic,
 changeEcliptic[##, {0 Degree, -40 Degree, 0 Degree}] &, 
 640, Background -> Green,
 PlotRange -> {{-π, π}, {0, π}}, DataRange -> {{-π, π}, {0, π}}
]

transformed image

The transformed image has a strange black band at the bottom which should not be there. How to achieve a proper transformation of an equirectangular image with roll, pitch and yaw? Is this a bug of ImageForwardTransformation?

UPDATE

ImageTransformation seems to work fine (the angle has to be reversed). Looks like a bug to me.

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It seems like a nuisance coming from the interpolation used by ImageForwardTransformation when you use negative values of rpy[[2]]. The image looks grainy but overall OK without interpolation:

gr = ImageForwardTransformation[mwpic, 
         changeEcliptic[##, {0 Degree, -40 Degree, 00 Degree}] &, 640, 
         PlotRange -> {{-π, π}, {0, π}}, 
         DataRange -> {{-π, π}, {0, π}}, "Interpolated" -> False]

Mathematica graphics

You may perform an "after the fact" interpolation (not with precise results, of course):

Inpaint[gr, ColorNegate@Binarize[gr, .0001], Method -> "TotalVariation"]

Mathematica graphics

But since your scheme works OK with positive values of rpy[[2]] you may (better) do:

ImageReflect@
 ImageForwardTransformation[ImageReflect@mwpic, 
  changeEcliptic[##, {0 Degree, 40 Degree, 0 Degree}] &, 640, 
  PlotRange -> {{-π, π}, {0, π}}, 
  DataRange -> {{-π, π}, {0, π}}]

Mathematica graphics

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