[Elphel-support] JPEG quantisation table

[Sebastian Pichelhofer]

*Problem Definition*

Chart showing resulting file-size depending on JPEG quality and coring
index:
http://community.elphel.com/files/jpeg-filesize-chart.jpg

Buffer-Monitor is a software that attempts to automatically adjust image
compression ratio (JPEG Quality 0-100%) based on the data-rate of the
current image stream and the amount/ratio of free/used circular-buffer
memory. The goal of the Buffer-Monitor software is to keep JPEG quality at a
highest possible value (best image quality) but preventing the buffer from
overflowing (dropping frames). The chart above shows that the effects of
changing JPEG quality on resulting file-size increase exponentially.

Image size is mainly affected by 2 parameters:
a. JPEG quality (where the area of primary interest is > 85%)
b. actual image content (eg. changing lighting conditions have drastic
effects on file-size)

So this narrows down the control that Buffer-Monitor has over image quality
to basically 15 discrete values. The closer to 100 the value gets the higher
the impact of a 1% increase/decrease is on the resulting file-size.

I think this is not optimal to ensure continuous high image quality but at
the same have time decent room for corrections when reacting to changing
image content.


*Ideas for Solutions/Improvements*

The DCT quantisation table is the only lossy step in JPEG compression and
controls which frequency ranges are stored with which accuracy. (higher
accuracy -> higher filesize). Higher frequencies are in general reduced
stronger.

a.
Altering DCT quantisation tables to shift focus on higher
data-rate/frequency image content.
This would mean that lower JPEG quality ranges would have higher frame
sizes.
Stretching the >85% (X-Axis) area of the above chart over a wider area
(0-100%)

b.
Changing how the JPEG quality value (0-100) affects the DCT quantisation
table to achieve the same effect.


*Questions*

Where are the current quantisation tables stored in the camera and how can
they be read/changed?

How does JPEG quality affect the quantisation table?


Regards Sebastian
Credits also go to Oleg for helping understand the topic!
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