I. Simple black-white gra
dient image. The sample
point’s value is the same
going in and coming out.
II. Gradient image + HDR
Exposure (with a lowered
white point and a raised
black point). The sample
point value is the same,
but the upper and lower
limits (the white and black
points) have been pushed
towards the center, push
ing contrast. The flat parts
of this “curve” show areas
that will be clipped. Sam
pling further along the
curve would give you out-
put values that are differ-
ent from input values.
III. Gradient image + VDR
color preset (gradient isn’t
smooth and colors are
weighted according to the
film’s curves). In this case,
the single curve is broken
into different R, G, and B
curves. Notice that the val
ues on the output level
(the line to the left) are
staggered, indicating color
variation for that sample
B. Adjustment methods
I. More eloquence from Arnie regarding HDR Exposure: “The HDR
Exposure filter can select how to spend the limited output dynamic
range (typically 255:1) to best display the HDR data. Like gamma cor
rection, this process can bring up detail in shaded areas. HDR Exposure
rescales the image colors based on a black point, which is the highest
level that will be black in the output. This is expressed as a percentage
of the standard 24-bit black point (1/255). The white point is the lowest
level that will be white. The default value, 1.0, usually maps to 255 in
24-bit imaging. Raising the white point brings detail out of the bright
areas at the expense of the darker, while lowering the black point uses
more colors in the darker areas.” You can often achieve this type of
detail extraction with a gamma adjustment (see 2BIII), without clipping
your white and black extremes.
HDR Exposure provides options that are very similar to the Expo
sure settings in the Image Controls panel of the image viewer. This
allows for manual and automatic assignment of the white and black
points of the image. This can be used to bring the range of a render or
HDR image into something that fits into 24-bit space. The simplest uses
of this process would involve adjusting the black point to compensate for
having used too much ambient light, or adjusting the white point to bring
details out of a blown-out area. A more extreme example would involve
using it to pull a manageable image out of a completely unruly HDRI
map, as in some cases these images appear to only contain data in areas
showing windows or light sources, when they in fact are hiding entire
data in the dark areas. The following images demonstrate such a
Source HDR courtesy of Paul Debevec and Jitendra Malik
A. A raw HDR image, with
little visible detail (initially).
B. The original image with a
white point of 10.0. Notice
that details are being pulled
out of the blown-out areas.
C. The original image with a
white point of 10.0 and a
black point of 10% of the
original black value. There’s
a real image in there!
II. Regarding the Virtual Darkroom, here’s more from our friend Arnie:
“Making photographic prints from film also requires a restriction of an
image’s dynamic range. The original image, captured in the chemical
emulsion on film, goes through two exposure processes that re-map the
dynamic range. The first to create a negative, which is then used in the
second pass to make a positive print. The Virtual Darkroom image filter
simulates these two transformations using light response parameters
from actual black and white or color film to match the results of photo
graphic processing. This complex plug-in can be used to control the
exposure of HDR images, while adding some film artifacts like grain and
halo which may enhance the image’s apparent naturalism.”
The Virtual Darkroom (VDR) simulates the exposure and printing
processes for different types of film and paper. It will introduce color
shifting, representing the curves in the image in a way that more closely
represents specific film types, going as far as applying sepia tones when
the film stock has been designed to do so.
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