PC Plus HelpDesk Extra - issue 204

HelpDesk Extra - 
        Edge Detect.

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).

Paul Grosse


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.

Original Harrods photograph 1.

This is the original Harrods photograph.

Note the different saturation levels of the colours in the image.

Original Harrods photograph  
Colourized Hue Shift 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 ...
Saturation information still present Saturation information removed
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
Greyscale and then colourized 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  
This is why ...
original colour image greyscaled colour image
In the original colour image, the brickwork was red and the window blue.

Our eyes, see the colour green as brightest, followed by red and then blue. The greyscale process takes into account this in order to produce a halftone image like the one on the right with darkened blues and slightly brighter reds.

Red layer of colour image Green layer of colour image Blue layer of colour image
Red Green Blue
Original photographic negative material was sensitive to the blue part of the spectrum. These images are the RGB separations and the blue image is the one we are after.
Sepia image 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.



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.

Original Harrods photograph 1.

The original Harrods image.
Note how the bottom half of the picture is substantially darker than the rest.

Original Harrods photograph  
Solarization transform 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  
Conventionally solarized image 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  
Luminosity solarized 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.

Original Harrods photograph 1.

The original Harrods photograph showing plenty of detail around the windows and in the darker parts of the streets below.

Original Harrods photograph  
Image Processor Command Centre This is the electronic process that mimics the real-life tone-line effect.
  1. Copy the image to the clipboard ready to paste back.
  2. Blur the image like the upper image of the pair in the real process
  3. Make the blurred image negative
  4. Paste the original image back from the clipboard
  5. Set the pasted in image transparency to 50% so that its tone cancels out - note that in reality, steps 2,3, 4 and 5 would effectively be in a different order, it is just that here, the images are pasted on pixel to pixel without any physical gap as it is not a physical process.
  6. Combine the pasted-in object with the base.
  7. Use the solarization process to place the image tones, which are all distributed around the middle, throughout the tonal range.
Solarization transform The solarization transform puts the tones in the middle at the top of the luminosity range. Changing the gamma at this stange brings out various amounts of detail.
Harrods with tone-line using small amount of blurring 2.

This is the tone-line effect using a small gaussian blur. Note that you can also get this effect by using a different blurring method or even by sharpening methods such as sharpen or unsharp mask.

Moderate blurring producing a fairly fine edge effect  
Harrods with tone-line using larger amount of blurring 3.

This is the same as above but using a far larger gaussian blur. Images such as these can be used washed out as background images in presentations and so on.

Edge effect with larger blur  

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