Stellarium: Warping quality comparisons

The following comparisons are provided in an attempt to address some confusion over how the image quality of a fisheye frame is affected by the warping process applied for projection using a spherical mirror. The image on the left is the fisheye frame from Stellarium. The image on the right is an optimal warping of the identical view settings. The algorithm is not particularly special, the fisheye is rendered to a texture slightly larger (by between 30 to 50%) than the screen height. This resulting fisheye texture is then mapped onto a mesh with appropriate vertex and texture coordinates to accomplish the correct distortion. The slightly larger image rendering and the texture mapping has a negligible performance hit on modern graphics cards.

Fisheye	Warped fisheye
XGA (768x768)	XGA (1024x768)
SXGA+ (1050x1050)	SXGA+ (1400x1050)
HD (1080x1080)	HD (1920x1080)

• The thickening of the lines/text towards the top of the image should not be interpreted as a thickening of the corresponding image on the dome. All the lines appear the same thickness irrespective of where they are on the dome.

• Star fields are very hard to successfully project with a single projector, this is especially so with only an XGA projector, fisheye or spherical mirror. As might be imagined while an HD projector is of little use for full fisheye projection using a fisheye lens, it is ideal for the spherical mirror which can take advantage of the additional width.

• Better results can be achieved with a fisheye lens configured in a truncated arrangement which is a closer match to the spherical mirror in terms of dome coverage.

• If the images are zoomed in there is an obvious difference between the sharp pixel edges in the fisheye and the slightly blurred lines in the warped image. In reality this is not necessarily a bad thing given that the lines are not inherently thicker, in computer graphics speak it is known as antialiasing and usually considered desirable. Indeed in my opinion the quality of both images would be improved if Stellarium performed antialiasing itself.

• It goes without saying that in order to get an exact image with a spherical mirror (the same as image on the dame as with a fisheye) a precise warping mesh is required. That is, one created specifically for the particular hardware being used.

Photograph of Stellarium image in the iDome (Photographic difficulties accepted)