Contrast and Saturation Enhancement—James Allan

Petrol reciept March 12Just this week I was doing my income tax and had to try and read some faded petrol dockets that had been sitting in my hot car for around 6 months.  The temperature of the car had caused the print on the heat sensitive paper to fade to white.   To my failing vision they were totally blank.  “Why don’t you photo enhance them” suggested my son Tom.  Not a bad idea that.  I took his advice and you can see the resulting image on the right.  Amazingly it is entirely legible.  Here’s what I did.  I scanned the dockets with the printer-scanner and then imported them into Photoshop.   I used essentially 2 tools to render an enhanced image.  Firstly I adjusted levels to maximise the contrast and then I used sharpening tools to increase the clarity of the text.(4-6px).

Suddenly it hit me that these were the same two processes that I found most useful in enhancing photographs.  I am only going to discuss the first of these two processes in this article.

There are many ways to maximise the tonal contrast of an image.  Many programs will have an “automatic” setting that will take a lot of the hard work out of the process.   The result is however a bit hit or miss.   Brightness and contrast can be adjusted (in the same way you adjust your television set), however they may result in loss of detail in the bright or dark parts of the image (clipping).  The “levels” tool will do the job without causing clipping.  However I like to use the “curves” tool as it gives a lot greater power and flexibility.  I will walk you through the steps using an image of a brown tree creeper (right).

Basic concepts

HistogramHistogram

Firstly I will explain some concepts:  The number of tones that an image file can contain is known as the gamut.  A particular image may use the entire available gamut, or only a portion of it.  You can easily visualise the amount of gamut utilised by displaying the histogram (right).  The histogram is a graph showing how often different tones are used in the image.  On the left are the darkest tones and on the right the lightest tones.  The higher the shaded area of the graph, the more that tone is in the image.  Unused gamut occurs where the graph is flat.

The Curves Tool

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The curve tool in photoshop (right) is also a graph.  On the x-axis are the tones of the input image and on the y axis are the tones of the output image.  The diagonal line shows how the tones of the input image are changed by the curves tool.  When this line is  straight  from the bottom left to the top right, this indicates there is no change at all.  You will note that the diagonal line of the curves is superimposed over the  histogram.  This is useful in knowing where to adjust the curve.

If you reverse the direction of this line (go from left top to right bottom) you can completely invert the image – or create a negative.  You have instructed the curves dialog to  make the dark tones light and the light tones dark. (Image Ib)

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Contrast Enhancement using the Curves Tool

Picture2Image 1 is the input image.  You can see that the curve is a straight line from bottom left to upper right. (no change)  The histogram does not extend all the way to the edges of the square.  This means that image is only using the central portion of the gamut.  That is why it looks a bit dowdy and unappealing.

Picture4In image 2 I have moved the black and white points up to the edge of the histogram.  It is important not to go beyond the edge of  the histogram as this will cause loss of detail in the dark or light areas (or clipping).  The effect is to stretch the existing histogram to cover the whole way across the graph, or put another way to eliminate the blank (unused gamut) in front and behind the histogram.  The image has changed so that there are now both darker and lighter tones in the image. The whites are whiter, the blacks blacker.  Unfortunately I am not happy, as I think the image still lacks a bit of punch.

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In image 3 I have created a handle in the middle of the line and dragged it downwards, giving the line a downward curve.  This has resulted in darkening of the whole image.  Conversely you could drag the line upwards to lighten the image.  This has the same effect as moving the gamma in the “levels” tool.  You can see that the image is a bit more interesting than the original.

Picture6

In image 4 I have created a second handle and dragged it upwards creating an “S” shape to the curve.  The overall effect of the S shape is to darken the darker tones and lighten the lighter tones, resulting in greater contrast and a bit more punch in the image.  To be more precise, the contrast is enhanced only in the central part of the image (where the line is steeper than the original straight line), and reduced in the very dark and very light areas where the line is less steep than the original.  The image has improved considerably.

Summary of steps

  • Move black and white sliders to the edge of the histogram to stretch the tones over the entire gamut
  • Move curve down to darken or up to lighten the image (Opposite in monochrome image)
  • Steepen the curve at the point where the histogram is tallest – create an S shape to the curve.

Maintain control when using the Curves Tool

Keeping in mind that the steepness of the line indicates how much contrast you create, it is possible to be selective and choose where you want to improve the contrast.  Sometimes too much contrast enhancement creates a cluttered busy image.  It may be you are enhancing detail in areas that are not important.  Look to see if you can identify the main subject of the image, and enhance mainly those tones.  For instance if all the detail is in the dark tones, make this end of the curve the steepest.  Experiment a bit until you are pleased with the result.  As a general rule however the best results usually come from placing the steepest part of the curve over where the highest peaks in the histogram occur.

You might also notice that the colour saturation has improved as a result of these changes.   You can further increase the saturation if desired using the HSU tool (Hue / Saturation / Brightness).  I suggest a subtle boost of contrast, say 5-10 %  is more pleasing than a strong boost  (20-25%)Do not touch the Hue or brightness controls if you want to maintain some authenticity to the image.

How much contrast or saturation enhancement should you provide?  That is entirely a matter of taste.  I often find that I  need different settings, depending upon whether I am viewing the image on a monitor, projected onto the wall or printed onto paper.  I hope that this walk through has been helpful and I have not confused you too much.  Curves is a very useful tool to get the most out of your images and certainly worth the effort learning to master it..

 

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