On 11/01/18 10:47, Tom Hughes wrote:
On 11/01/18 10:38, Bjoern Hassler wrote:
(1) DPI on exported images
Actually isn't the real problem here just needing to know what DPI
is being being assumed by mapnik when rendering?
IIRC it's 96dpi?
As you say (and also in the link Yves posted) it's about 90.7dpi,
standard pixel size of 0.28 millimeters as defined by the OGC (Open
Geospatial Consortium) SLD (Styled Layer Descriptor). However, the
images downloaded are claiming 72dpi. I need to check whether they
don't have dpi metadata (and therefore my OS assumes 72) or whether
they are tagged incorrectly.
Either way, that sounds like a discrepancy though, right?
If you're fetching PNG then does PNG even have a way of specifying the
DPI in the format?
Per
https://en.wikipedia.org/wiki/Portable_Network_Graphics#Ancillary_chunks
I think that a pHYs chunk could store it but our images don't appear to
have that:
% ./pngcheck -v map.png
File: map.png (205670 bytes)
chunk IHDR at offset 0x0000c, length 13
1192 x 1018 image, 32-bit RGB+alpha, non-interlaced
chunk IDAT at offset 0x00025, length 32768
zlib: deflated, 32K window, default compression
chunk IDAT at offset 0x08031, length 32768
chunk IDAT at offset 0x1003d, length 32768
chunk IDAT at offset 0x18049, length 32768
chunk IDAT at offset 0x20055, length 32768
chunk IDAT at offset 0x28061, length 32768
chunk IDAT at offset 0x3006d, length 8933
chunk IEND at offset 0x3235e, length 0
No errors detected in map.png (9 chunks, 95.8% compression).
I suspect 72 is just an OS default.
Tom
--
Tom Hughes ([email protected])
http://compton.nu/
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