File talk:Admiralty Chart No 1896. Entrances to Auckland Harbour nautical chart 55767, Published 1857.jpg

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Doubled Pixel Lines

[edit]

Probably the version from (02:28, 28. Feb. 2019) contains doubled pixel lines:

All pixels with coordinates n·256 resp. n·256+1 (both x and y coordinate) are copies of n·256−2 resp. n·256−1 for all n≥2 (disregarding JPEG artefacts). "≥2", because I already corrected doubled lines of the topmost and leftmost tile lines manually during composition.

For example, this is visible at top-right magnetic-declination-compass-rose, more exactly at the left seven rays of the "north star" above the rose (vertically). In addition at the magnetical east-west-connecting-line in the same rose, in top of the "17" (horizontally).

-- Pemu (talk) 11:02, 28 February 2019 (UTC), translated -- Pemu (talk) 11:39, 13 March 2019 (UTC)[reply]

Doubled pixel lines removed by means of a Python script for Gimp. As of nature of JPEG file format, this could not be done losslessly, because only translations in multiplies of 16 pixels can be done losslessly, AFAIR.
As source I used the version from 02:28, 28. Feb. 2019. Its topmost and leftmost line of tiles were already non-losslessly manipulated, therefore now they are again losslessly manipulated.
To yield version from 02:28, 28. Feb. 2019, I translated the topmost and leftmost tiles by 3 pixels to give a seamless joint to the losslessly used remaining tiles. In the current version (02:34, 19. Mär. 2019) I compensated for all 3 pixels, not only for 2 of them like in the version from 02:49, 13. Mär. 2019. The previous version’s got, in comparism to the source, an additional black 1-pixel margin on the top and on the left.
To summarize, the combination of the four versions between 02:12 and 02:28, 28. Feb. 2019 contains all tiles as harvested from the source, without generation loss.
-- Pemu (talk) 11:20, 14 March 2019 (UTC), updated -- Pemu (talk) 23:28, 19 March 2019 (UTC)[reply]
The Python script can be found under User:Pemu/CorrectDoubledLines. -- Pemu (talk) 22:28, 16 March 2019 (UTC)[reply]
Signal chain for current version (02:34, 19. Mär. 2019): Harvested tiles from source at Raremaps → stitched with jpegtran → doublings removed with Gimp and mentioned Python script → saved as JPEG (quality setting: 93). Exception: "Tilted L", constituted by topmost and leftmost 253 pixels, are subject to an additional JPEG generation loss. -- Pemu (talk) 22:23, 19 March 2019 (UTC)[reply]
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