Hi all,
I post this at the gimp-dev list cause I think more people read here and
this is closely related to the progression of the Gimp Application
itself, even if the core of all is GEGL.
Being excited about the new high-bit depth capability I wrote an
exposure operation. It is somehow inspired by the equal-named photoshop
operation. A short explanation for anyone not that familiar with the use
of it: This operation is mainly important for manipulating hdr images,
i.e. floating point images containing values > 1. Working with such
images, most of the well-known operations (e.g. curves) doesn't work at
all or at least don't really make sense as they are based on the
assumption the image is in an absolute 0..1/black-white domain.
The "exposure" operation has three components, that are applied in the
following order:
Exposure: a multiplier, the usual way to manipulate a hdr images'
brightness. This is implemented as a relative change, i.e.in a
logarithmic way (like f-stops)
Offset: a constant value added, this can be used to shift the
black-level
Gamma: gamma-correction. In this case used to influence the
mid-range values
To summarize the components of the operation:
out_val = in_val * (2^exposure)
out_val +=offset
(clamp value to 0.0 because might have become negative)
out_val = out_val^(1/gamma)
Some more reading:
http://www.earthboundlight.com/phototips/photoshop-cs2-exposure-adjustments.html
http://www.francois-tarlier.com/blog/exposureoffsetgamma-photoshop-tool-to-shaders/
This is one of my first things in gegl, so some questions came up, also
some issues ihmo worth a discussion:
As result values are not clamped in the positive direction, some
thoughts about the parametersthat are worth thinking about:
Gamma parameter is restricted to a range of 0.01 to 10 for
now.Some confusion came up about gamma calculation, cause I am
sure gamma is pow(value, 1/gamma), but e.g.gegl:gamma operation
use it without the inversion, also some other applications.
Gain is not restricted for now, but cause it's exponential
values get pretty high, soon exceeding MAX_FLOAT. In Photoshop,
pixel values seem to be generally clamped to a value of 20 (or
is just the palette no able to display more?) which seem a
little bit to restricted to me. Especially in case of sunny day
images (e.g. HDRI Sphere for cgi rendering).
With negative offset values, negative pixel values may occur. This
is normally not wanted and in some cases the reason certain filters
are not suited well for hdr images. Values are getting clamped at
0.0 atm, but I cannot say that I am glad with that. Any suggestions
on that?
For my own interest I implemented this thing both as a gegl
meta-operation and as a GeglOperationPointFilter operation. Apart
from the fact, that doing things like pow(2, value) is already kinda
wicked in a GEGL graph, I was really asking myself if the meta
operation makes any senseat all speed-wise. Because this contains 5
to 6 nodes (without i/o).
Next thing for me is to make the main code use vectorization.
Codeis here:
http://pastebin.com/xYQYYXqX
http://pastebin.com/Rq5HWhbF
thanks for your interest
Felix(aka. kjaft)
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