03 July 2012

Why Referendums?

UK Prime Minister David Cameron has raised again the possibility of a referendum on Britain's membership of the European Union (EU), but this begs the question: should we ever have a referendum on anything? Britain, in common with most western governements, is a representive democracy, that is people get the right to vote for people to run the country on their behalf - we are not a direct democracy, nor should we be.

Britain leaving the EU could have disastrous effects on our economy - or it might even be beneficial. I do not know, and I am sure that 99.9% of the population do not know either. Asking the general population their views on complex issues like this is madness, not least because voters will expect our elected officers to sort out the mess if it goes wrong. Whatever people think of govenment decisions, the people making them have to justify and implement them, and then deal with the consequences of their actions.

Thankfully referendums in Britain are rare - the last one we we had was on whether to adopt the Alternative Vote (AV) in general elections. The whole process was a shambles, not least because there is no rational argument against the adoption of AV, if you you accept that voting systems should be fair. The No Campaign was led by the Conservative Party, who knew full well that the current system was ludicrously unfair, but that crucially it favoured them. (Why some people in the Labour Part supported the No Campaign remains a mystery.)

In a general election we elect people to run the country for us - the goverment has an army of civil servants and others, to provide then with expert advice to guide them in their decisions. They should get on with the job and not ask the rest of us to make decisions for them.

29 June 2012

Photographic Reality

When people see a post-processed picture after using software such as Adobe Photoshop, or an HDR picture where tone mapping [1] has been used, a frequent comment is that they do not think the picture looks realistic - this begs the question of what people mean by realistic in this context. I believe that when people see a heavily post-processed picture they consider to be unrealistic, what they really mean is it does not look like a normal photograph.

Before the invention of photography, the only time people would see representations of scenes was in paintings and drawings. Landscape paintings by artists such as Constable do not look much like modern photographs of similar scenes. Interestingly when people see a tone-mapped high dynamic range picture they often say that it looks more like a painting than a photograph.

Superficially the human eye is similar to a camera in that it has a lens that focuses on a sensor, and it has aperture control and automatic focusing. However there are major differences: essentially a camera takes a snapshot with one focus setting and one aperture, and is a freeze of the picture taken in a fraction of a second. Conversely the brain/eye combination is more like a movie camera in that the eye is continuously scanning the image and the brain is composing a picture, so over a short period the aperture and focus are changing. Another significant difference is that we have two eyes, so depth perception is much better because of 3-D vision. Although 3-D cameras are available they are unlikely to become mainstream any time soon because a special viewer is need to see 3-D pictures.

When people have taken photographs in bright light conditions with dark shadows, they are often disappointed with the results, as often they do not think they represents what they actually saw at the time. The main reason for this is that human vision has a much greater dynamic range that that available on most modern cameras.

Dynamic range (or contrast ratio) is often measured by ratios such as 1:100,000. That is the brightest part of the picture is 100,000 times brighter than the darkest part of the picture. The darkest part is defined as the darkest colour above pure black that can be represented. These ratios can be difficult to understand, and I think it's better to use EV (Exposure Value) terminology, which is commonly used in photography. An increase of one EV is a doubling of the light received. Modern cameras can capture up to about 8 EVs if the picture is stored as a JPEG. In raw mode (usually only available on SLRs) more can theoretically be captured, but in practice most cameras cannot do more than about 12 EVs. Note that the dynamic range is more a function of the sensor than the bit depth. How human vision actually works is not well understood, but it is believed that the eye can capture at least 14 EVs. The fact that modern consumer technology cannot achieve the dynamic range of the eye is one of the reasons for many disappointing photographs. A modern display screen/display adapter can represent about 8 EVs, although in practice it is usually lower than that on LCD displays because of the backlight, which tends to turn black into dark grey. It is possible to get displays that represent about 17 EVs, but they are extremely expensive (around $50,000), so these are only really used in specialist applications like medical imaging. The situation is even worse when you try to print pictures, as it is very difficult to get printed material that contains much more than about 6 EVs.

Even if you take high dynamic range pictures using techniques such as overlaying multiple pictures taken at different EV levels (HDR), you still need to represent these on paper or an ordinary display. The standard way of doing this is to use tone-mapping, which essentially is a non-linear transformation of the large dynamic range back down to around 8 EVs. Generally when people refer to an HDR picture, what they are really are looking at is a tone-mapped version of the picture.

When JPEG (but not Raw) is the saved format, all cameras do some post-processing of the picture to produce a more balanced image. A gamma curve [5] is typically applied to produce a picture which is closer to normal human perception. Some cameras have optional facilities to lighten dark areas [6] and increasingly cameras are becoming available with simple built-in HDR processing [7].

I believe that tone-mapping is essentially what the eye/brain combination is doing, and also what landscape painters are doing naturally. I would contend that a realistic [2] tone-mapped version of a picture is probably a more accurate representation of a scene than a straight photograph. Note that not all scenes need a high dynamic range – on an overcast day a conventional picture is fine. But in bright sunshine I suspect that users will increasingly expect their camera to be able to provide an accurate representation of the scene as they perceived it at the time.

The second picture here is a tone-mapped HDR from 5 original photos taken at -2, -1, 0, +1, +2 EVs, and post-processed using special software (PhotomatixPro). The first picture is the middle picture of the sequence, and is what you would get with a straight photograph. Because of the very dark areas and the bright sunlight in this shot, it is not possible to capture the full dynamic range with one shot on a conventional camera. Most of the first picture is either under or over-exposed. The use of tone mapping enabled me to produce an image that was much closer to what I observed at the time.

Additional Notes

  1. For more information on HDR and tone-mapping see:
  2. A realistic tone-mapped image is where tone-mapping software is used to try and create a representation of the picture as the photographer saw it at the time. Examples can be seen in this Flickr group:
  3. Tone-mapping can also be used to produce exaggerated effects similar to surrealist or impressionist painters. This is moving into the realms of photography as art, and away from realism. As with all art, some people like the results and some people do not – but it is possible to produce some dramatic pictures from what would have been a dull single photograph. Pictures of dilapidated buildings or old vehicles are popular topics for this treatment. Various HDR groups on Flickr show examples of this, such as this one:
  4. New display technologies currently being developed hold out the prospect of affordable displays with improved dynamic range. However there are many technical difficulties and progress is slow. In particular OLED displays do not have a backlight and so, in theory, should be able to display pure black, and a much better dynamic range:
  5. More information on Gamma Correction can be found here: http://en.wikipedia.org/wiki/Gamma_correction
  6. The Dynamic Range (DR) facility on Sony Alpha cameras is on example of this. This allows for the lightening of dark shadow areas to bring out more detail. For JPEGs this is done in camera, but if Raw format is used additional control is available using Sony’s supplied software. One advantage with this is that it is done with a single shot and so can be used when there is movement in the subject.
  7. Manufacturers are now building-in simple HDR facilities to some of their cameras. Pentax and Sony have several models that support HDR, but most manufacturers have at least one camera that supports HDR/tone-mapping. The iPhone 4S has an HDR facility. Generally with the in-camera facility you have little or no control over the final appearance, but it is much easier than creating several images and post-processing them.

Photographs are of the Cloister at Iford Manor, Wiltshire, UK.