[gdal-dev] What is lost when converting 12 bit imagery to 8 bit?

[Frank Warmerdam]

Patrick,

FWIW, Rob's post is on the process he uses in Photoshop to prepare images
for various venues.  For imagery published through the platform we (Planet)
do not use per-image white-point and black-point (or we would not have day
to day, and scene to scene consistency).  We do apply color curves Rob
prepared in our automated process but with "fixed" black/white point which
results in an automated 8bit RGB product that tends to be very suboptimal
in dark or bright areas.     The imagery Planet shows in our web-explorer
interface is served from highly compressed JPEG-in-TIFF adding an
additional layer of image damage. :-)  While that pains me, we are keeping
around nearly 3 billion scenes online at nearly full resolution for fast
visualization so some compromises have to be made.

Beyond nit-picking, I think my point is:
 - given 12bit "rawer" data you have the opportunity to do careful scene
dependent conversion to 8bit in a way that best brings out the details
available in the source data if you have the time and patience.
 - having this process done for you in advance by a skilled supplier
(perhaps in such a way as to maintain reasonable consistency for large area
coverages) may actually save you a lot of work if you mostly just want to
fairly generic visualization - and it might even be a better visualization
than you would do yourself if you aren't going to do a lot of work.

Best regards,
Frank

On Wed, Mar 17, 2021 at 11:13 PM Patrick Young <
patrick.mckendree.young at gmail.com> wrote:

> I would guess you usually see 8bit RGB images because that is what your
> monitor can display.   What is lost is a deeper question, per channel you
> have to squeeze the original [0 - 4095] pixel value range per channel down
> to [0 -255], and there are lots of ways to do it.  The problem is sometimes
> called tone mapping
> <https://en.wikipedia.org/wiki/Tone_mapping#:~:text=Tone%20mapping%20is%20a%20technique,a%20more%20limited%20dynamic%20range.>.
> Planet had a nice blog post describing how they manually convert their
> imagery to 8bit RGB here <https://www.planet.com/pulse/color-correction/>.
> If you were using the imagery for analytic things (e.g. converting pixel
> values to reflectance) you'd probably not want the 8bit product.
>
> To get GDAL in the mix, note that gdal_translate can do simple tone
> mapping for you:
> https://gdal.org/programs/gdal_translate.html#cmdoption-gdal_translate-scale
>
> Patrick
>
>
>
>
>
> On Wed, Mar 17, 2021 at 3:23 PM <Matt.Wilkie at yukon.ca> wrote:
>
>> SPOT 6/7 satellite imagery is captured with a dynamic range of 12 bits
>> per pixel per channel (ref <https://eos.com/spot-6-and-7/>). However
>> almost all of the SPOT imagery I have seen in use has been 8 bits per
>> channel, and split into RGB natural colour (Bands-321) and Near-infrared-RG
>> false colour (Bands-432). What information is lost in this 12 to 8 bits
>> conversion?
>>
>> I'm wondering if we should be altering our request for purchase
>> specifications to deliver the full bit depth.
>>
>> *Although I'm referencing SPOT imagery specifically the question is
>> general and really applies to any satellite or sensor system.*
>>
>> Cross-post:
>> https://gis.stackexchange.com/questions/390315/what-is-lost-when-converting-12-bit-imagery-to-8-bit
>>
>>
>>
>>
>>
>> *Matt Wilkie*
>>
>> Geomatics Analyst
>>
>> Environment | Technology, Innovation and Mapping
>>
>> T 867-667-8133 | *Yukon.ca <http://yukon.ca/>*
>>
>> *Hours: 08:30-16:30, Tue-Wed: Office, Thu-Fri: Remote.*
>>
>>
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