How is an HDRI created?
A series of photographs are needed for an HDRI. The camera stays at the same place and one of the four parameters are varied:
- the diaphragm by a full stop (f/32 - f/16 - f/8 - f/4),
- the exposure time (shutter speed) is doubled or halved (1/4 - 1/8 s - 1/16s - 1/32s),
- different neutral grey-filters are screwed in front of the lens,
- the filmspeed is changed (200 ASA - 400 ASA - 800 ASA - 1600 ASA).
- Waiting for the passing of time, until the sky gets darker.
A camera is needed that offers at least one of the features listed above. It is absolutely possible to create HDRI's from traditional chemical film. But this is tedious because it must be acertained that no brightness corrections are performed when the film is processed and a print made. Also, when the prints are scanned, the original brightness must be retained. A digital camera makes life much more easier in these respects.
Stability
Preferably, the camera is mounted on a stable tripod and if the shutter can be triggered using a remote
control, the much the better. The pictures must fit one above the other. Shifting the camera ever so
slightly between the individual exposures asks for a lot of additional work afterwards. It pays off
to handle things in a very careful way when the f-stop or shutter speed is changed for the next shot.
Exposure
SLR (Single Lens Reflex) and DSLR cameras feature the so-called «Bracketing».
The camera finds the best settings, takes a shot, dims by 1/3 to 1 step, takes another shot and brightens
by the same amount and takes a third shot. This is already enough to create an HDRI.
The serious HDRI photographer, however, does not consider this sufficient even though a series of pictures ranging over 32 f-stops is not striven for. Best practice is to aim for 8 to 15 f-stops for excellent results.
f-stops
In HDRI photography, we speak of f-stops. However, this does not mean that only the diaphragm
may be varied. Opening the diaphragm by a full stop, e.g. f/16 to f/8, corresponds to doubling the
exposure time, e.g. from 1/500 second to 1/250 second or a doubling of the film speed, for example
from 400 ASA to 800 ASA (which can be adjusted on the fly on digital cameras and actually just
changes the amplification factor of the readout electronics). The easiest way is changing the
exposure time because there is the widest range readily available — typically from 30 seconds to
1/4000 second (18 steps) while the diaphragm can be varied by 6 stops (f/64 to f/2) in the best case
and the film speed has an even narrower range of 4 steps (200 ASA to 1600 ASA). As for the filters:
their range is limitted by your budget to purchase filters and fix them in front of the lens.
Animated Objects
It is obvious that you cannot create HDRI's from racing cars streaking past the lens of the camera.
People usually move around and will appear as smears or ghosts. HDRI photography is limitted to
still objects. Clouds may perhaps move too fast over the sky and photographing vegetation —
trees and flowers — should not be attempted on a windy day. A soft breeze may be too much
already. Wideangle photos are less delicate than zoomed-in ones; objects far away apear not to move
so fast as near ones.
Example
Now to illustrate what has been elaborated, a series of photographs that could be assembled successfully
to an HDRI.
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| 8 s | 4 s | 2 s |
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| 1 s | 1/2 s | 1/4 s |
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| 1/60 s | 1/125 s | 1/250 s |
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| 1/500 s | 1/1000 s | Linear tone-mapped |
The last picture is the HDRI, combining the 14 pictures before it. The exposure times used correspond to 13 f-stops, calculated were 17. The dynamic range is 158'873:1 and the maximum radiosity is 276 (radiosity is a parameter that is important if an HDRI is to be used as a light source for image based light for 3D computer graphics).
The HDRI was linear tone-mapped for display. This means that the dynamic range was linearly squeezed down from 158'873:1 to 256:1. Even though the picture does not appear particularly successful we have to admit that there are neither too dark nor overly bright parts in it (look at the fluorescent tubes and the blue sky outside). Nevertheless, there is still a lot of potential to extract from this picture as will be demonstrated under the topic «Tone-mapping».
How are pictures assembled to an HDRI?
A program to do so is needed. It reads the series of pictures and adds up the brightness and
colour values for each pixel and displays the result (usually) linear tone-mapped. The programs differ
considerably in what features they offer. Some are able to adjust slightly shifted pictures
automatically, others permit manual alignment and still others do not have such a feature. Some
programs read the EXIF data from the JPG photographs, others demand that the exposure values are
entered manually, still others have a tool to calculate the camera response and save it. This curve
can be used as a reference for the HDRI creation. Not all programs are freeware. The following list
must not be considered as conclusive.
| Program | Align | Reads EXIF | Tone-mapper | Op-sys | Price |
|---|---|---|---|---|---|
| HDRShop v.1 | no | camera response | 0; 10 with Plugins | Windows >= 98SE |
0.00 |
| HDRShop v.2 | ? | ? | ? | Win32 | US$ 400.00 |
| Picturenaut 2.12 | automatical | yes | 2 + 10 with Plugins | Win32 | 0.00 |
| Artizen 2.5x | automatical + manual | ? | 12 | Win32 | CND$ 35.99 |
| Photomatix Basic 1.1 | ? | ? | no | Win32 OS-X |
0.00 |
| Photomatix Pro 2.4 | ? | ? | 1 | Win32 OS-X |
US$ 99.00 |
| Photosop CS2 | ? | ? | Plugins | Win32 OS-X |
€ ~200.00 |
| Photosop CS3 | ? | ? | Plugins | Win32 OS-X |
€ ~800.00 |
| Qtpfsgui | automatical | ? | 8 | Win32 OS-X Linux |
0.00 |
Remarks to the programs
HDRShop v.1 is free only for non-commercial use — if you read the licence agreement carefully, it is
only free for HDRI research. Maybe it is not available anymore. Link:
HDRShop v.1 und v.2. Tone-mapping is
supported through Francesco Banterle's HDRShop 1.03 Plug-ins.
Picturenaut features 2 own tone-mappers and additionally supports Francesco Banterle's free HDRShop 1.03 Plug-ins. Alignment of input picture is automatic, but can be switched off.
Artizen HDR is not free but still affordable and it is quite complete. If you pay for it, you are entitled for free updates. Since version 2.4.4 many tone-mappers have been added, version 2.5.14 from 14. December 2007 has less but the Old Tone Mapping Operators can still be downloaded. Input pictures can be aligned manually and automatically. Artizen HDR supports dual core processors.
I do not know Photomatix 2.4 Pro. Photomatix Basic 1.0 only assembles bracketted photographs to an HDRI and does not feature any tone-mappers. Version Basic 1.1 can still be downloaded for free.
Photoshop CS2 (Version 9) can read HDRI and permits image processing. Artizen offers tone-mapper plugins for this program. Meanwhile, the successor CS3 is in the shops, but I do not know it. CS2 is about ten times slower for loading a finished HDRI than HDRShop v.1, Picturenaut or Artizen.
Gtpfsgui is quite complete and freely available for three operating systems. To align large pictures takes a long while but the option can be untagged.
Convenient accessory
A stable tripod is mandatory if you wish to make HDRI photographs. Another, rather unconventional,
accessory is the Gorillapad «Joby».
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Less laborious and bulky to carry around than a tripod. Simple to fix on a tree or a lamppost. However, not cheaper than a tripod – particularly if the Joby is equipped with a fast camera mounting plate.
Waiting for the time to pass …
This undocumented method cannot be found in any textbook about HDRI and is, of course, a joke. Nevertheless,
it works, but only during the periods of dawn and dusk. Study from the balcony with a fisheye lens …
The picture on top left was taken with automatic exposure: f-4, 1/250 second at ASA 200. Then, I waited until it grew so dark that I would have to use about the double exposure time at the same f-stop, but took a photograph with 1/250 second nevertheless. I continued this. Here are the values:
| 3. August 2008. f-4, 1/250 s, 200 ASA | |||||||
|---|---|---|---|---|---|---|---|
| Time (CEST) | Exposition ought to be (s) | HDRShop (s) | Time (CEST) | Exposition ought to be (s) | HDRShop (s) | Time (CEST) | Exposition ought to be (s) | HDRShop (s) | ||
| 20:40 | 1/250 | 250 | 20:58 | 1/80 | 80 | 21:02 | 1/50 | 50 | ||
| 21:06 | 1/25 | 25 | 21:09 | 1/15 | 15 | 21:14 | 1/8 | 8 | ||
| 21:18 | 1/4 | 4 | 21:23 | 1/2 | 2 | 21:26 | 1 | 1 | ||
The pictures were loaded into HDRShop and as absolute scale I gave the inverse values; e.g. for 1/250 second 250 seconds (see table). The HDRI got a dynamic of 136'203'908 : 1.
The pixel values were multiplied by 10 and the tone-mapping done as follows:
- 1. In Picturenaut 2.12 with the Photoreceptor Physiology Operator
- Exposure adjustment: 3.00,
- Dynamic compression: 0.00,
- Saturation: 0.500,
- Contrast: 0.500,
- Gamma 2.500, Rec. 709 not ticked.
- 2. In Artizen 2.60 beta2, with the Dramatic Operator
- PreGamma 1.50,
- PreGamma 1.50,
- Brightness -4.00,
- Saturation 0.25,
- Strength 0.25,
- Soften Detail 0.50,
- Fine Detail 0.50,
- Contrast 0.00,
- Highlights 0.00,
- Shadows 0.50,
- Midtones 0.50.
Both pictures were then loaded into HDRShop and added. Finally, the picture was "defished" using PTLens. I sat there and I had this impression when I shot the first picture. A huge success ;-)