ConvImgs user guide

More on histograms


More on histograms

Most users of digital cameras will be familiar with the idea of an intensity histogram. They are often used to check whether an image has been 'correctly' exposed. This essentially shows the number of pixels in the image with a colour intensity value corresponding to each of a number of colour value levels, from zero colour intensity to the maximum possible. This may be thought of as from pure black, through all shades of grey, to full white, with the image being treated as a grey-scale image.

The image below shows a picture with a wide range of tones and pixel intensities.

The histogram for this image is shown below.

This shows that there is already a full range of intensities in the image and there would be no advantage in processing the pixel intensity range any further.

On the other hand, the image below was taken with a telephoto lens (300mm focal length on a DSLR with an APCS size sensor, equivalent to a 480mm focal length on a full frame camera) on a morning described by the 'weatherman' as misty and murky. The image is very much in shades of mid-grey.

The corresponding histogram shows that only the mid part is populated with pixel intensities. There are no pixels approaching black or white.

The histogram diagrams used here for illustration are produced using DPlngScan. This application has the facility of dragging around the two arrows, below the histogram. In use it is usual to drag them to the points in the histogram where pixel intensity is just observed, as shown below.

You can then 'expand' the intensity range so that the intensities between the two sliders are adjusted to cover the complete range from black to white, everything outside the sliders becoming either pure black or pure white. The histogram of the resulting intensities is shown below.

This is essentially what the Histogram option in ConvImgs does.

  1. It calculates the histogram for the image
  2. It steps through the histogram from the full black level, until the pixel intensity becomes greater than a threshold value (referred to as a percentage of the maximum in the histogram). This becomes 'the position of the left slider'
  3. It then steps through the histogram from the full white level, until the the number of pixels becomes greater than the threshold value. This becomes 'the position of the right slider'
  4. It then processes the intensity values so they cover the full range from black to white

In ConvImgs it is possible to set the intensity threshold level, and the number of intensity levels used in the calculation.

The image below is the result of allowing ConvImgs to process the misty image shown above.

The histogram facility was added to ConvImgs because I had about 70 images of the RFA Argus leaving Falmouth for Sierra Leone, taken from the end of the Roseland peninsula across the Fal estuary from Falmouth. The image shown above was one of the lowest contrast ones in the set, taken when the ship had only just left Falmouth harbour.

Histogram expansion can be very useful when the pixel intensities in an image cover only a limited range. It is also sometimes of help in recovering images that have been a little over or under exposed, although it obviously cannot ever put back detail that has been lost.

The bottom line is - use with care and keep backups of the original.


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ConvImgs is © Chris Johnson, 2013
Email:chris@chris-johnson.org.uk