[COG] Mixed Raw and Overview data compression ?

[Kirill Kouzoubov]

This would be very useful, reducing storage costs while simultaneously
allowing faster preview functionality. One issue to consider is `dtype`
support/conversion. My understanding is that JPEG is best supported at 8
bit, and can not go beyond 12, while many sensors have more than 8 bits
worth of data per band, and even more than 12. So one would need bit-depth
reduction as well. To complicated things further, one might have 14-bit
data stored in 16-bit image, so bit-depth reduction is not a trivial
operation. Also `nodata` comes into play as well.


Kirill

On Mon, Sep 17, 2018 at 10:39 AM Vincent Sarago <vincent.sarago at gmail.com>
wrote:

> What do I mean about `mixed` compression ?
> JPEG (or WEBP) compression are great for storage saving (and also for
> small http request) but they are `bad` because they are not lossless (WEBP
> can be set to be lossless but compression is then not as great) so my idea
> is to package both lossless and non-lossless in one COG. e.g keeping high
> resolution data to a ZSTD/DEFLATE compression but the overview in JPEG.
>
> Why ?
> For web application we mostly want fast tile serving while for processing
> we will mostly want to read full resolution lossless data, a file with
> mixed compression could make both world happy ?
>
> The gain won’t be astonishing but might be worth a try ?
>
> Vincent
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