I recently made a post in the pbi forum which mentioned that the 
bump_map{} code has a seam bug. See:

http://news.povray.org/povray.binaries.images/thread/%3C67a154c1%241%40news.povray.org%3E/

Bald Eagle posted a link to: https://iquilezles.org/articles/tunnel/ 
which is a cousin to one of two issues found with the current POV-Ray 
code. (Thanks Bill)

Details...

There are many factors which contribute to the vibrancy of the issues 
found in any given render result. Very generally it's worse with the 
bump_map due how it takes multiple samples from the image to implement 
the normal perturbation. Yes, the issues exist with image_map{}, 
image_pattern{} and image_indexed_textures{} (formally material_map{}) 
too - but mostly in muted / harder to see ways.

With respect to bump_map{}, the first root issue is due the multiple 
samples sometimes spanning the seam in a given mapping type. The second 
root issue - sitting smack on top of the first - is that games are being 
played internal to all interpolations which introduce / mangle the 
values in the pixels at the images edges.

Attached is a v3.8 beta 2 compatible scene which shows the issues and 
two images. A triangle_wave gradient of x*pi is used to create the input 
image for the bump_map{} where the mapping is spherical. The camera is 
zoomed in on a smaller part of the sphere's surface containing the seam.

Both images show No-interpolation and Bi-Linear interpolation in the top 
row and Bi-Cubic and Normalized-distance in the bottom row.

The image bump_map_state.png shows the current state with both issues 
present and overlapping.

The image bump_map_intrpIssues.png shows the interpolation issues in 
isolation - with code where the first issue is fixed.

Bill P.

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Attachments:
Download 'bump_map_intrpissues.png' (23 KB) Download 'bump_map_state.png' (18 KB)

Preview of image 'bump_map_intrpissues.png'

Preview of image 'bump_map_state.png'