PC Plus HelpDesk Extra - issue 204
 |
This month, Paul Grosse shows you
more detail of how some effects work and how to modify
others.
We look at: Sepia; Solarisation; and, Tone-Line Effect
(Edge Detect).
|
 |
Sepia
Sepia was originally created when the silver in black and
white images (halftone or greyscale) was allowed to react with
sulphides in the air. The silver sulphide produced gives a brown
pigment instead of a black one - the original silver is finely
divided and scatters the light making it look black.
Early photographic films also only "saw" blue light.
It was much later on that additional chemicals were used to make
the orthochromatic and panchromatic black and white films that we
see today.
 |
1. This is the original Harrods photograph.
Note the different saturation levels of the colours in
the image.
|
| Original Harrods photograph |
|
 |
2. Here is the original (1.) after it has had
"Colourize" selected when going through the hue
shift. Doing this sets the hue the same, regardless of
what it started out as. Here it has been set to 36
degrees and the saturation shift adjusted to give an
acceptable level of saturation.
The image however, contains elements with different
levels of saturation and where the luminosity is the same
but the saturation less - the lower floors and the
building on the right - the image is greyer.
|
| Colourized Hue Shift |
|
| This
is why ... |
 |
 |
| When colourizing,
the saturation information is retained therefore
the amount of sepia toning depends upon the level
of saturation. |
In real sepia,
silver is silver regardless of how saturated the
original image colours were. The silver reacts
with the sulphides in the air and makes sepia as
above |
|
 |
3. This is the original which has been greyscaled
so that all colour information has been lost, then turned
back into a colour image (so that it can take colour
imformation again) and then colourized to the sepia hue
(36 degrees) with the saturation and brightness being
adjusted to give the desired effect.
However, the bottom half of the window at the top of
the corner of the building is too dark and the building
itself is a little pale.
|
| Greyscale and then colourized |
|
|
 |
4. Here we have a convincing sepia image.
The original colour image has been split into its RGB
channels, the blue channel then converted back into an
RGB colour image which has then been colourised,
producing the image on the left.
|
| Sepia |
|
Solarization
It can be argued that as the original solarization seen when
images were over exposed such as in atomic bomb footage, that
solarization should only be applied to greyscale (halftone)
images. Here, however, we look at what happens when it is applied
to colour images.
|
1. The original Harrods image.
Note how the bottom half of the picture is substantially
darker than the rest.
|
| Original Harrods photograph |
|
 |
This transform extends the tonal range of the
darker half of the image and also does the same
with the lighter half but inverting that, thus
creating a photograph that looks part positive
and part negative. Look in the two images below
at teh two people crossing teh road. In the
original, one has black trousers and the other,
white.
|
| Solarization
transform |
|
|
 |
2. This is the result. Note that the colours become
negative to their opposites but as the density of each
colour layer goes through the transistion (at 50%), the
resulting hue flicks through the primary and secondary
colours.
Thus, on the front of the building, we go from the
orange brickwork, through green and cyan to blue - the
opposite colour to red - instead of going straight from
red to blue.
In the darker areas, the green canopies remain green.
|
| Conventionally solarized image |
|
 |
3. Here, the image is split into Hue, Saturation
and Luminosity (HSL) and the luminosity is then
transformed using the solarization tranform as above.
Then, the image is recombined.
In this case, the hues have stayed the same as that
layer remained unalterd. Note that there is no rainbow
effect in this image
|
| Luminosity solarized. |
|
Tone-Line Effect (Edge Detect)
Originally, this was done by making a 100% negative of the
image and then laying the two on top of each other such that
there was a thickness of transparent material between them (the
film base) and then shining a light through at a range of angles
onto an unexposed negative so that the lower image was
essentially a contact print and the upport image was blurred.
The image densities cancelled each other out but where there
was any localised change of density, such as at an edge, light
shone through.
Here, we recreate this using an image editing package showing
how, by understanding what is going on, the effect can be
controlled and manipulated more effectively.
|
