Graphics Programs Reference
In-Depth Information
we want to look at it and likely encode it into an RGB color space. This
RAW data can be rendered to attempt to match the scene (as best the
technology will allow) or the image can be rendered to create a pleasing
reproduction of the scene. There's a big difference between the two and
it's important to understand the differences. The actual scene we attempt
to record is very often far beyond the scale of color, luminance, satura-
tion (you name it), that our devices can record, and certainly is beyond
the ability of output devices to reproduce. In most cases, when the ren-
dering is such that we attempt to reproduce these items as closely as pos-
sible to the scene colorimetry, the measured color of the scene, we end
up with an image that's not very pleasing when viewed on a display (we
have to view the digital image on something). This scene colorimetry is
related to a term called
input-
or
scene-referred
.
Since we need to view this image on something like a display or a
print, which has a far more limited range than the scene, it's necessary
to make the image appear more pleasing on the output device and to
produce the desired color appearance the image creator wishes to express
and reproduce. This is known as
output-referred.
The need to fit the color
gamut and dynamic range of the scene-referred data to output-referred
data is called rendering. The camera usually performs this rendering
when you select a color matrix setting such as sRGB or Adobe RGB
(1998). If the camera is set to capture just RAW data, the rendering
becomes the job of the image creator—you, the photographer. The
creator of an image expresses their idea of how the scene should be repro-
duced on an idealized output device such as a display or printer. The
desired color appearance of an image you are editing is dependent on the
output medium. This is not the measured color of the scene itself (scene-
referred). An example of how a user would handle this output-referred
process would be using a RAW converter to produce the appearance they
prefer from the RAW data.
When you set your digital camera controls to capture an image (which
is initially a RAW data file), yet request a color space (let's use sRGB in
this example), there are two fundamental parts to this process:
rendering
the data and then
encoding
the data. In creating an output-referred image,
the camera or computer system has to perform the color rendering pro-
cessing before it can encode the result of the processing into sRGB.
Because sRGB is rendered to output-referred data, it cannot be used
to accurately represent the scene appearance or what some would call
the colorimetry (measured color) of the scene. Therefore, first the data
is rendered, based upon how a camera manufacturer feels they will
produce the most pleasing image appearance for their customers. As such,
this rending varies from different camera manufacturers and perhaps
even different models of camera from the same manufacturer; the ren-
dering is not standardized. Think of this rendering process as a percep-
tual rendering of sorts; the rendering is that which the manufacturer feels
produces visually pleasing color, not generally the colorimetrically correct
