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The Digital Darkroom
part 2: scanning, cleaning and grading
By Dr J Floor Anthoni (2003-2006)
www.seafriends.org.nz/phgraph/darkroom.htm
The darkroom is back with a vast array of sophisticated tools and techniques to improve your images. In part 2 you'll learn how to scan your images, how to clean them and grade them by adjusting colour and contrast. This chapter is also important for movie makers and people with digital cameras.

 
The first step is to obtain a digital image from a negative or slide, but it is fraught with problems and pitfalls.
The digital darkroom has been in existence for over a decade, first practised on Mac computers. But even today, much is left to be desired, scanners being no exception.
After having worked in the digital darkroom for a while, a preferred sequence becomes evident. The remainder of this page follows the steps in this sequence.
One of the most powerful editing tool is the clone tool which allows one to rid the image of unwanted blemishes. However, it requires skilful use to remain invisible.
It is surprising how much of influence colour correction is to the final look and feel of the photograph. Learn how to correct colours for fidelity in order to be able to add your own flavour to the end result. Also auto adjust and chroma/hue.
Adjusting contrast is definitely the most important component in the digital darkroom toolbox and it is complicated if one wishes to fully exploit its possibilities. Also using the gamma curve.
go to part 1.  go to part 3.
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For suggestions, comments and corrections, please e-mail the author Floor Anthoni.
-- Seafriends home -- underwater photography index -- Seafriends site map --
Rev 20031022,20031113,20040226,20051010,20070712,

scanning
Some photographers operate bulk scanners. They insert a film and let the scanner software scan it at high speed, producing a folder full of images. The photographer then deletes the ones that fail, retaining the most promising ones, which is similar to discarding the bad slides, but it is not intelligent since it negates the digital darkroom possibilities right at the point of scanning. Something similar happens when you use a bureau for your scans.
Why is it important to optimise the scan at the very beginning? Most scanners do not scan with 8 bit precision but with 10 or 12 bits, which means that they are capable of producing much finer colour distinctions, particularly in the shades. Doing their pixel calculations with more precision they suffer less from rounding or truncation errors (llike 1.7x1.5= 2.55 rounded to 3.0 or truncated to 2.0). Thus the darks can be enhanced without adding to quantising (truncation) or noise errors. Serious colour or density corrections must be done at the scanner stage before the image is saved in RGB. This is where the RAW format comes in handy when you use a digital camera. If your digital camera's output is JPEG sRGB only, you must take care that the image is optimally exposed, particularly in the shades.

Some scanners such as those of Nikon provide additional features:

grey scale calibration patternCalibrating the scanner
You have to calibrate the scanner for each type of film you are using. Click on the calibration sheet to print a larger version and keep it in your camera box. Make one of the black boxes even blacker by sticking some black velvet or other very black material on (or shading it with your hand when photographing it). Stick the sheet on non-transparent cardboard. For each film type take a photo of this test pattern. Blur the image by disabling autofocus and defocusing it. In this manner the graininess of the paper won't show through. the idea behind this is that all black to white neutral densities have equal values for RGB. Thus (40,40,40) is neutral dark grey; (120,120,120) is neutral mid grey; (240,240,240) is neutral almost white.
In the scanner software this image shows 8 distinct peaks in the histogram of the curve tool. Use the Green curve as reference by not altering it. Adjust the Red curve to produce equal results to the Green curve, for each peak. Often more than 3 nodes must be placed to achieve this. Do the same for the Blue curve. This calibration takes much of the guesswork out of later colour correction.
If you like, you can produce a calibration curve for incandescent light, fluorescent light and so on. You can include a test pattern at the beginning of every shoot or when the light changes. Save these calibrations and back them up.
 
Tip: most beginners will use the automatic settings of the scanner like auto-adjust. My advice: disable all the automatics and calibrate your scanner as shown above. It is the only way to obtain consistent results.
Tip: for negative scans the preferrred way is to calibrate using analog gain settings. Negative films have enough linearity to obtain consistent scans for underexposed to overexposed film.
Tip: a new scanner software has appeared on the market, VueScan from www.hamrick.com (US$90). This scanning software also works for over 500 flatbed scanners and film scanners of all makes. It uses advanced features like autofocus and the infra-red channel with scratch removal. It has become my preferred choice as results obtained are better than I've ever seen. VueScan also works perfectly on Linux.

 
 
The scanning
Scan your images at the highest resolution available since this makes sharpening and blurring easier. It also retains the most colour information since several pixels together carry a finer degree of colour than a single one. Most importantly, do not create blacks at the scanning stage because what is black has lost its colour information. Also for cleaning the image, one must be able to see what one does, and there must be some light coming from the dark passages in the image.

Last but not least, print the scanner manual and reread it several times. It contains good information and you must become a confident and professional scanner operator - the best.



 
scanner problems
In the past 25 years colour technology has made huge strides with improved film, such as better colour fidelity and longevity. Yet why can one find such large differences between the various brands and types of film found today? Above we have described a way to calibrate your scanner for your film at the time you shot your images. But for negative film the situation can be more complicated.

 
major colour corrections while scanningScanner manufacturers must provide calibration curves for each brand and type of negative film. Instead, they often apply an automatically adjusting logic whereby the lightest point in the photo is assumed to be white and the darkest black. Between these two points the colour curves are 'calibrated' automatically. Although this often produces near-satisfactory results in snappies, it is a pain for those photos that have no black or white points (like underwater). Then the scanned result is nasty, requiring extensive colour correction before and afterwards.
The diagram shows how to correct such problems. Use the (calibrated by you) tone curves for each colour until highlight white and/or mid tone greys are balanced. Since these are major signal corrections, they must be done during scanning and not later on.

Note that this is not a disadvantage of negative film but a shortcoming of the scanner's logic, its inbuilt software. For the time being it appears something one has to live with, a symptom of colour technology's infancy. However, users of negative film will find the digital darkroom techniques of more value than those shooting slides.

A major colour shift may also occur in case some of the scanned image does not cover the film base which is coloured for negative films. Make sure the scanner does not see the cut edges.

Scanners need maintenance too because atmospheric dust and grime settle onto optical surfaces, causing unwanted blurring. Once every two years the unit must be serviced and cleaned. Make sure you buy a scanner with good service backup. Scanners with electrical ventilators for internal cooling particularly suffer from this problem. With modern technology it is not necessary for a scanner to have a cooling fan.
 
 

Tip: make sure you have tried to disable all automatic features of your scanner before giving in to extensive colour corrections.
Tip: sliding the base of the RGB curve up is necessary where images need to be brightened in their shades, thus where the deep shadow is not black (as in the background of an underwater photo). It is always advisable to brighten the shadows, so that blemishes can be seen and corrected, but most importantly that they can be darkened with an S-curve which softens the shadows and compresses the image where no colour can be expected but where detail needs to be retained. In this manner, there remains more tone space for the mid-tones, resulting in a brighter and more colourful image which still shows soft shadows and highlights and which has not lost any colour information. This method also makes the shadow signal softer, resulting in less graininess and noise.



 
editing sequence
In theory you could go about the editing process in the digital darkroom in whichever way you like, but there are good reasons for following the steps in the order as presented here.
  1. mounting and cleaning: start with a clean image. Buy some isopropyl alcohol (50ml lasts a long time) from the paint shop or local chemist because it dissolves grease and acts like alcohol without affecting the emulsion layer. Get some white gloves from the photographic supplier. Gently wipe the image clean with the glove or your oldest handkerchief. Mount it in the slide/neg holder. Use a manual blower brush to puff-wipe it again. Use isopropyl only as a last resort!
  2. scanning with major corrections: in the scanner software use the tone curve tool to approach the log-lin curve (depending on the subject). Produce a soft image for highest colour fidelity and in order to be able to correct the density of bright objects (the sky) by masking. Save the image in a systematical way. See filing and backup below. From here on, use your photo editing software. Enable the scratch removal option.
  3. cropping: Where an image needs to be cropped, do it now, since this requires less cleaning. Look carefully at the edges for any black or white intrusion. Crop them off with the image/paper size or canvas tool. Use the crop tool or rectangular mask (followed by crop to mask) to crop off large parts.
  4. cleaning with the clone tool: your image is a bit bleak at this stage, which shows blemishes clearly. Set the enlargement scale to 1:1 for one pixel on screen = 1 image pixel or better (for fine scratches 2:1 or 200%). Scan from left to right and top to bottom and remove impurities. See the use of the clone tool below. Save the image to be able to fall back to in case further editing ends in disaster.
  5. colour/density correction with mask: this advanced technique will be discussed in the chapter on masking techniques. The idea is to do it here since the image has enough colour fidelity to allow for partial density corrections. Use the tone curve tool to adjust partial density. The brightness/contrast tool requires more care for this.
  6. colour correction: the main colour corrections are done now although fine tuning may occur later on. See the chapter on colour correction below. If the image looks very weak, do some density correction first.
  7. density correction: the whole image is now adjusted for the final look. Blacks are created, but preferably near-blacks for your archival storage. There's no way back.

  8. See the chapter on density correction below. It pays to make a checkpoint (temporary backup) now in case of later editing mistakes.
  9. chroma correction: weak images improve by intensifying colour chroma slightly (5-10%). For slides that have lost chroma in the darks and the highlights, chroma correction with a density dependent mask may be necessary. Use chroma correction with care. It is also provided in the Brightness/Contrast/Intensity (BCI) tool as intensity.
  10. gamut test: just to make sure, apply the gamut test to see whether any colours have been pushed outside the printable range. Culprits to watch are chroma correction and density correction.
  11. blurring and resampling: so far, the image has benefited from maximal noise which renders colours more truly, as multiple pixels average out to represent true colour. Now the noise needs to be reduced as at the same time sharpness is increased. These are two opposing requirements which must be handled with skill and care. A mask may be necessary. Resampling may be needed to arrive at the minimal allowable file size. See the chapter on blurring and sharpening.
  12. sharpening: several sharpening tools exist but two have most use: the unsharp mask and the adaptive sharpening. Skilful use of these leads to largely improved results. See the chapter on blurring and sharpening.
  13. web pictures and screen images: once the master image has been made and saved, it can be reduced to much smaller sizes and compacted further than an original. See the chapter on resampling.


 
cleaning and cloning
The cloning tool is perhaps the most beneficial tool in the photo editing toolkit, but perhaps also the most maligned. It takes quite a while before one can remove blemishes without creating new ones. However, by reading this chapter you don't need to reinvent the wheel.
The cloning tool is not a magical tool that makes blemishes disappear. It does what it says: it copies (clones) one part of the image over another part while mixing the result depending on the opacity (opposite of transparency) setting. If that part is not precisely what was blemished, an artifact will result. In fact, it is never possible to achieve exact replacement. So let's think carefully about what we want to achieve: The cloning tool is a brush and those with a good feel for painting will be more competent. Stroke is also an important ingredient, as is brush size and its soft edge. So how should we use it?

In principle, the cloning must be done with the smallest brush, approximately three times the size of the width of a scratch or half the size of a blotch. Begin cloning all the small spots and skip the larger ones to be done with a larger brush, which also needs already cleaned territory and a less detailed view. A brush size of 15-20 pixels is quite useful at the beginning. Set opacity to 75% (transparency 25%) and soft edge to no more than 50%. This is necessary to retain graininess. Otherwise cloning becomes more like polishing and the polished patches stand out like a sore thumb. Select the nearest similar patch to clone from. Brush across the line between the two patches. If you move towards, you will clone a repetitive pattern becoming smoother as you go, which is wrong. If you move away, something similar but less disturbing happens. Change direction frequently and for larger patches, nibble away at their edges working from the outside inward.

During the second pass with the larger brush, set brush size to match the background sharpness or unsharpness rather. This will make it easier to tackle the larger blemishes.

To replace part of an image with something totally different, be artistic and inventive. Set brush size to the largest affordable, about 1/4-1/3 of the patch. Now brush the main bits in without overstepping boundaries. Then decrease the brush size without changing the distance to the cloned patch and work on the details of the outline. Make sure graininess is as expected.
 
 
 

Tip: to see blemishes more clearly in even shapes, slide the image slowly left-rigth with the window slider at the bottom of the window.

Tip: work in a darkened room to make blemishes on screen stand out.

Tip: you will save much time by adding a graphic tablet with pen to your computer, like an Aiptek HyperPen or a Wacom stylus. It needs a while to get used to, but it improves your work considerably while saving much time as well. A graphic tablet does not have the stickiness of a mouse and you can swipe it like a brush.

Tip: learn the shortcut keys for undoing the last brush stroke. Set your program parameters to undo at least 6 previous strokes.

Tip: After all the cleaning is done, save the image or make a checkpoint to be able to fall back to should later editing be unsatisfactory.


 
The magical clone brush
Wouldn't it be nice to have a clone brush which knows what the blemishes are and pastes over the scratch only and not outside it? There is something like this. For instance, if you need to correct white scratches and white scatter (lighter coloured blemishes), set your clone brush mode from Normal to If Darker. The brush will then paint only if its colour is darker than the canvas, thus it will paint over the light spots while leaving the darker surround untouched. If you have dark blemishes, set your brush mode to If Lighter.
Have you ever tried to repair blemishes in gradual colour gradients? You'll never find the right colour and density to do this. Here is where the If Darker (or If Lighter) brush mode comes in handy. Place the origin of the brush to a slightly lighter place and clone this over the lighter blemish. Because the brush won't be darker than the surround of the blemish, it will not cause an artifact. In theory it will not remove the blemish completely, but this is not necessary, as long as it is not visible or distracting.
Tip: set transparency to 50% for difficult colour gradients.

 
 
Avoiding scratches
Film scratches are a photographer's worst nightmare, and digital photography should be welcomed if only for the reason that its images cannot be scratched. I have been fighting scratches for over 40 years and frustration has increased rather than subsided. Today, processing labs make extensive use of scratch removal technology, which is fine for snapshots but is inadequate for poster-size magnification. As a result, operators have become less careful than ever before.

Film processors' first reaction to complaints is that they never caused the scratches, but that they are the fault of you and your camera. Nobody ever complains. Don't believe it. Here is how they make all the scratches on your films:
The film is yanked out of the cassette and taped onto the processing conveyor. This threatens your first frames (and at the end of the film a few more). As it moves through the processor, a chance of scratching is minimal but new threats appear in the form of dust and even lint, when the filters are not cleaned regularly. The processing chemicals can also leave streaks on both sides of the film. But the main problems occur when the 'dried' film is handled.
When the 'dried' film exits the developer, the emulsion is still soft. It takes in fact over 24 hours for it to harden sufficiently to stand handling. Worse still, when the film is printed immediately and then sleeved, the emulsion is still able to fuse with the sleeving material, bonding bits of paper to the emulsion. Needless to say that this destroys your film. Professional laboratories do not use paper sleeves but transparent plastic sleeves which do not have this problem.
So, most labs handle the 'dry' film when its emulsion is still unfit for handling. Operators are always in a hurry and the film is rough-handled to meet time schedules. Worse still, most machines have not been polished such that all sharp edges on chromium-plated guides and pathways have been securely smoothed. As a result, the film gathers numerous scratches from printing alone. Ironically, the prints do not show these scratches but reprints do.
Then the film is sleeved by a sleeving machine. You would not believe that any film handling equipment has rollers touching the image rather than the sprocket path, but sleeving machines do. They have rollers that engage with the images and spew the film four or six frames at a time, but not precisely. So the operator pulls back on the film now and then, which causes short deep scratches at each fourth frame or so.

Here is what you should have your laboratory agree to, in order to prevent scratching:

And here is what you must do to prevent scratches at your end:




 
colour correction
Once you start using your density calibration curves for slides, colour corrections become minimal. But even so, older slides have lost some of their colours and negatives usually need more correcting as they also present a richer scale of 5-7 f-stops. In general, colour correction has a major influence on the final look and feel of the photograph.
If every image had a black, a grey and a white point, colour correction would be straightforward. Still, it is surprising how many objects provide a reliable reference: white in house paint, clouds, grey in tar seal, rock, deep shade, light bounced off shiny objects (direct light), wavewash, froth, and so on. Learn the typical RGB values for a blue sky and a green leaf and a white man's face.

Find out what the short-cut key is to sample the RGB of a square of pixels because you will use it a lot. Try to memorise the values for RGB, but particularly how many points they differ. This forms the basis for your colour correction. The colour correction has a number of components:

The colour scale should be memorised as follows:
 
more Red = less Cyan
more Green = less Magenta
more Blue = less Yellow

So the best way of adding yellow is by reducing blue, because red + green = yellow. The best way of adding cyan is by reducing red because cyan = green + blue. It sounds a little confusing but this is how additive colours works. Note also that red can make a landscape picture somber but it makes skin tones glow. Yellow makes a picture look dirty.
 
 
 

Colour correction under water
Having such powerful colour correction tools at one's disposal as provided by the digital darkroom, one may think that the use of filters under water is no longer needed. This is true to some extent and certainly for small corrections and where super wide lenses are used which do not fit filters easily. A red filter (heavy warming filter) under water blocks blue light while letting the warm colours pass. It works only over a limited distance due to the filtering effect of the water and it does not affect the colour of distant blue (it deepens/ darkens it). However, in the digital darkroom, colour corrections apply equally to 'distant' parts in the image as it does to the 'foreground'. So it does not have the same effect. 
Note that it is often possible to make a neutral gradient filter in the photo editing program (paint on mask) and if this co-incides with, say a wall that disappears in the distance, then application of a warming effect, works like having used a filter under water.

Underwater photographers with digital still or video cameras have an option to change the white balance to filter colours. The advantage of this method is that it can be very precise and it can be correcting for any kind of colour. For instance when presenting the (underwater) camera with a white slate underwater, and allowing it to balance for pure white, it creates the perfect colour correction filter for that depth. But our eyes may want a less precise solution and manufacturers have limited white balance correction to within reasonable limits. For professional results, disable the automatic white balance feature, and exert tight control over a manual way of doing it. 


 
auto-adjust
Most photo editing software have an auto-adjust tool which automatically stretches the histogram between the extremes of black and white while doing the same for each of the three colours. It unites both colour correction and maximum contrast in a single operation. This works for simple photos but very often does not produce the right result and it NEVER produces the best result (see tone curve tool below). Use it to tentatively see what it does and then use your good judgement whether it is of value. Later you will hardly ever use it.

 
Tip: an auto-adjust usually results in harsh shadows and benefits from a gamma adjustment of 1.1 or 1.2 afterwards, or a mid-tone correction using the tone tool.

Tip: when the main part of the image appears bleached, darken the entire image while reducing contrast, such that highlights become bright colours. This can be done by sliding the tip of the tone curve tool down its vertical axis.


 
hue and chroma
The image can be edited in its entirety using the LCH tone curve tool (Luminance Chromaticity and Hue). It is another way of representing colour. The Luminance component can be edited exactly like the tone curve tool with identical results. But the Chromaticity or colour intensity gives a totally new perspective.
With it you can de-saturate colour, turning an image effectively into grey scale or any flavour in between. For some photographs this creates attractive effects. If the subject is masked, then its surroundings can be turned into gray scale, making the subject stand out. Many other applications can be found, but here we are interested in restoring the original colour.

Remember that photography is a multiple stage process unlike our eyes which convert the real world directly into an image in our brains. By comparison, a photo usually takes another step (slide) or two steps (photo) or three steps (neg/slide, scan, print). In the process some of the original colour intensity may get lost, and this happens also when slides and negatives age or when they are underexposed. Use the chromaticity editor with care to restore colour. It can easily be overdone and when gamut is exceeded, your image remains damaged.

The hue editor is of little practical value, except that it can be used to warm or cool the image somewhat. Used to excess, it becomes a powerful creative tool. You may find it useful in creating sunset effects.
 
 
 

Tip: to emulate a warming filter, set red to 10 and blue -5 for yellow (or 6,-3 etc.). A warming filter 'adds' some red and yellow. For cooling, reverse the values.


 
adjusting contrast
The way contrast can be adjusted in the modern digital darkroom has no equivalent in the darkroom of photographic paper and chemical baths. It is a completely new set of tools that you must learn to master like nothing else because it allows you to grade the image precisely for the purpose intended, to give it your personal touch and to emphasise that part of the image deserving interest and colour.

You have to learn to understand the histogram to see how the luminance is spread between black and white. It is somehow a photographic sport to push a photograph out between these two margins without losing detail, at least that was the game of black and white photography. However, in colour photography we are guided more by how to make the image look like the real thing. It pays to try to make your photo look real so that you can adapt it later for creative reasons.
 
 
 

Tip: the histogram often does not show how saturated a pixel is. for instance, a pure red colour of (255,0,0) may not show at the very bright side of the histogram, since white light (255,255,255) has higher luminosity. So it is easy to clip bright colours without noticing. Make it a habit to look at the individual histograms for R G and B, particularly when reaching optimal densities. Once a colour clips, your image loses colour and detail information forever.

Note: contrast is a very important property of your image. It can easily be under- or overdone but it can be readjusted at any later stage using the tone curve tool.

Note: by increasing contrast, you also increase the noise and graininess of your image.


 
standard contrast & brightness toolThe standard brightness/contrast tool is important for using the available colour space (256 levels) and to come close to what the image should look like. However, this tool keeps the same contrast level throughout the range (straight line) and is not suitable for softening or hardening one part of the scale. In the diagram the effect of these tools is shown. The brightness tool shifts the whole histogram left or right even when it clips. In the example the highlights will be affected by the black line. The contrast tool rotates the curve, which has the effect of widening the histogram, darkening the dark parts while equally brightening the bright parts. In the example some highlights will be clipped. Note that in some editing software the tone curve tool can be used for this purpose by placing the straight lines visually as desired.

 
Tip: in order to prevent clipping towards the highlights, use brightness -5% at the same time as contrast is increased by 10%, because the histogram widens +5% to the highlights (as it also widens 5% to the shadows). Likewise, to prevent clipping the darks, increase contrast by 10 while at the same time brightening by 5.

 
versatilty of the tone curve toolThe tone curve tool is the most versatile tool in the tool box and you will use it already when scanning. As the above example shows, the classical contrast and brightness tools can be done with this tool too. But you can do more. 
  • linear: the linear 'curve' is there for reference as it forms the starting position and has no effect. You can slide the top node of this 'curve' down its vertical axis, thereby reducing contrast.
  • log-lin: the theoretical density correction to make densities look more natural, but consider this curve as the envelope beyond which one seldom goes. It is done intuitively by placing two nodes on the linear curve and pulling both up and sideways. This curve will make the image rather washed and dull and needs to be followed up with the s-curve, allowing one to shift humps in the histogram to the density of maximal interest while making the image peppy again.
Important note! Always remember that the top half of the curve represents but one f-stop whereas the bottom part 3-5 f-stops. Thus little compression can be achieved in the top half. Compression here serves to bring more emphasis and colour to the mid tones, without losing detail too much.

Phew! That was an extensive treatise on this powerful tool, but it has more tricks in store!
What did I say before? Don't make blacks until at the very end of the process. Once a pixel becomes black or near-black, it loses all colour information and there is no way back.
 
 
 

slide linear scan
A Fujichrome 50 slide scanned without correction using a linear tone curve, as is customary.
slide loglin scan
The same slide scanned with the log-lin curve. Shadows and darks show detail but the image looks dull, yet all colour information is there. Notice the halos in the blacks.
slide corrected
Colour corrections applied and contrast placed where it matters, using the s-curve. The image looks natural and sunny with soft shadows and highlights.

 
slide linear scan
A Fujichrome 50 slide scanned without correction. The image would improve considerably from brightening, but shadow detail becomes gritty.
slide loglin scan
The same slide scanned with the log-lin curve looks dull but the detail in the shadows looks smooth.
slide corrected
After colour correction and contrast adjustment with the tone curve tool, the image looks natural and sunny.

 
Note! Some (or all?) reversal films produce a wide halo in the dark colours around bright areas.

Note! The blacks from a scanned slide contain no colour information and must be turned into blacks again after colour correction and contrast adjustment.

Note! a transparency slide cannot be corrected as much as a negative.


 
 
gamma
The gamma curve comes from television technology, representing the number of electrons freed from a heated cathode as a voltage difference is applied. At first electrons are unwilling to escape but later they do so in an almost linear relationship to the applied voltage. The gamma is the greek symbol for c. Depending on the value of c, the curve is either exponential (c>1) or logarithmic (c<1). What you have to know is that the gamma curve affects the shadows more than the highlights and it runs steeper than the darken and brighten curves discussed above. Use it successfully to make a difference in contrast in the dark regions but remember that it can also be done with two nodes in the tone curve tool, as in the log-lin curve, which is what you will end up doing.

 
creative tone curves
creative tone curves; solarisationThe tone curve tool can be used to create stunning colour effects similar to solarisation in the darkroom. Solarisation happens when a photographic print is exposed again during development. This darkens and inverts the highlights while also creating edge effects. The solarisation curves shown here do not precisely achieve this but they achieve what cannot be done in the darkroom, see photographs below.
original
The original image, a diver coming down a fresh water cave. The image consists almost entirely of two colours.
Inverse: the image becomes a negative. Cyans become reds.

Solarise1: a strong s-curve which inverts highlights and shadows somewhat.blacks, reds and blues.
Solarise2: inverts the mid tones. New colours appear.
Solarise3: inverts the highlights and shadows.
Solarise4: inverting the mid tones to highlights.
solarise5: inverts the shadows to the mid tones.
Mid-tones: a density-dependent grey scale mask for blurring and sharpening. See use of masks below.
edge detect for smart sharpening
Edge detect and invert creates the mask for smart sharpening.

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