[orangepeel]

HDR Techniques
Introduction
What is a High-Dynamic Range (HDR) image?
The "dynamic range" of a scene is the contrast ratio between its brightest and darkest parts. A plate of evenly-lit mashed potatoes outside on a cloudy day is low-dynamic range. The interior of an ornate cathedral with light streaming in through its stained-glass windows is high dynamic range. In fact, any scene in which the light sources can be seen directly is high dynamic range.

A High-Dynamic Range image is an image that has a greater dynamic range than can be shown on a standard display device, or that can be captured with a standard camera with just a single exposure.

HDR images also have the important property that their pixel values are proportional to the amount of light in the world corresponding to that pixel, unlike most regular images whose pixel values are nonlinearly encoded.

HDR Images are typically generated by combining multiple normal images of the same scene taken with different intensity levels, or as the result of creating a global illumination rendering. In practice, high dynamic range pixels use floating-point numbers, capable of representing light quantities of one to a million and beyond. Low-dynamic range images usually represent pixels using eight bits per channel, with pixel values ranging as integers between 0 and 255.
Source.

Here we're going to talk about some of those techniques for creating HDR images then mapping them back to 8/16 bit for print or screen.




Preparation
I'm going to assume you shoot RAW. I'm also going to assume you shoot multiple exposures - These techniques can be used on a single RAW file processed at multiple EV levels but your mileage may vary.

Personally I use the bracketing function on my camera, expose my first frame as near to perfect as I can then shoot two others at +2ev and -2ev. It goes without saying, a tripod is essential.

Caveat: If you subject has movement (water, trees in a breeze etc) then this can have a negative affect on the final result.

I believe you'll need Photoshop CS2 or above for some of the techniques described.



Techniques
I've heard people say they don't like HDR, personally I tend to wonder if it's a specific technique they take issue with... It is possible, with any post processing technique, to take things to extremes. To them I'd say, well try some of the other methods. It may well work for you.

There are techniques I regularly use. These are by no means definitive but I will stick with the ones I employ - Notice I don't say "know". I'm learning here as much as the next person.

The techniques are: -

• Photomatrix Using Photomatrix to perform HDR generation and tone mapping.
• Photoshop Using Photoshop (Automate | Merge To HDR...) for HDR generation and tone mapping.
• Manual Manually blending layers in Photoshop

Photomatrix can easily create images which can look, arguably, unnatural as well as the dreaded halos you see so often.
Photoshop, for Photomatrix aficionados, can look a bit bland.
Manual can produce visible layering if not done with care.

Note - You could also use Photomatrix to do the HDR generation then perform the tone mapping in Photoshop. Not sure that this brings any benefits however.

[orangepeel]

Photomatrix
Pros -

• Very easy to do.
• Changes are instantly visible
• Can produce some simply amazing sky.
• Lots of controls.

Cons -

• Can look very artificial.
• Can produce a halo effect around high contrast areas
• Can produce strange lighting in areas which obviously shouldn't have as much light.
• Lots of controls.

There isn't much point in me repeating work already done... so why not take a look at this tutorial by GainfulShrimp. Remember and don't be shy about fiddling with the tone mapping controls. You can always redo the HDR.

[orangepeel]

Photoshop
Load your images

From the menu: -

File | Automate | Merge To HDR...


Select the source images as appropriate


Note About Aligning Source Images. If you shot on the tripod you shouldn't need this. If you shot handheld then this probably still won't help. You should have used a tripod!

Once you're selected your images, press the OK button to generate the HDR.


Near the end it'll show you a preview of the final HDR. The slider will let you change the white point. Don't worry about this, any change you do here will not affect the final result. The slider only adjusts the preview.

Once Photoshop has generated the HDR you should have an image on screen that's frankly pretty 'orrible. The next step is to do the tone mapping.

From the menu, select


Image | Mode | 16 Bits/Channel


This is where Photoshop get's a little complicated - so much so I'm going to quote from Mikkel Aalands' book "Photoshop CS2 RAW"
Links: -

• Amazon.com
• Amazon.co.uk
• Blackwell
• WHSmith
• O'Reilly

Highlight Compression and Equalise Histogram apply preset equations that may or may not be appropriate. There are no user controls for these options. I've found the Equalise Histogram works reasonably well on some merged images but haven't had much luck with Highlight Compression. Try for yourself.

Use Local Adaptation to gain real control. Local Adaptation and Photoshop's Shadow/Highlight image adjustment command work similarly: they both attempt to lighten or darken an image, based on surrounding pixels. With a localised approach, it is possible to lighten a dark area (or darken an area that's too light) without affecting other areas of the image.
In the case of Local Adaptation, this is achieved using a combination of Radius, Threshold and Toning Curve controls. There are no fast rules on hot to use these controls. What you do depends on the content of the image, resolution and tonal range.

As a very general rule of thumb:

• Pay particular attention to areas of detail and extreme tonal range as you adjust Radius and Threshold controls. Standard keyboard navigation commands are available. The Histogram palette isn't function but you can use the Eyedropper tool and info palette to confirm your highlights and shadow areas are within range.
• A high resolution image with fine detail requires an increased Radius setting. A low resolution image with little detail requires a very low radius setting. Keep in mind, higher Radius values require more processing power, and it'll take longer for your image to convert. (Simply put, Radius controls edge detail.)
• An image with high tonal values will require a higher Threshold setting. (The most useful Threshold settings, however, range from .5 to 2. Higher values really don't do much for the majority of HDR images.)
• Set your Radius and Threshold values first, then adjust the Toning Curve. (Some images benefit from a further tweaking of Radius and Threshold values after applying a new Toning Curve)

Once you are happy with the preview. Click the OK button. Now go get a cup of coffee. It can take a while.

With your 16 Bit image created you can now continue with the rest of your workflow as you see fit...

[orangepeel]

Manual
Manual HDR is simply about layering your individual exposures one on top of the other then masking out parts you want hidden.

This takes quite a bit of brush work but you do get very fine control over which exposure is visible where. This method is also useful for when there is movement between frames. With the fine control you can choose exactly which layer to use finely blend in highlight or shadow details.


In my Photoshop Layers tab you can see the three exposures I took,
Layer 0 : -2EV
Layer 1 : 0EV
Layer 2 : +2EV

If you know your masks then you'll be aware, from top to bottom (ignoring the adjustments), Layer 0 is mostly blacked out with only the very brightest part - the sun - allowed to show through.

With the Layer 1, all the sky is shown along with a touch of highlight on the water.

Layer 2 is completely visible but is occluded by the the relevant parts of Layers 0 & 1 above it. What you're left with is the foreground rocks and the water - but toned down slightly.


Now in this particular example, the waves have moved between frames. By very gently (Brush @ 1% flow) painting in Level 1 on top of the Layer 2 we can minimise the problems caused by this movement.

[orangepeel]

Results
I've made 3 images using each of the processes described above. Which is best is down to your own tastes really. For me there is aspects of each that I like. For that reason I wouldn't be particularly shy about pulling each of those aspects together into yet another image - which would then become my final result.

It's a lot of work... it's up to you as to whether or not it's worth it...


Photomatrix Processing


Photoshop Processing


Manual Processing

[Lone Buggin]

Giving this a go when I get home, on the sheepses that Dave posted in an earlier thread.

[Dave]

Excellent Tutorial OP Rep points added

[Steve]

Yup, its an excellent tutorial that I will give a go too

Thanks for the effort

[fionaB]

excellent stuff OP! very informative.

Fi

[j sotelo]

good tutorial !!

[Dabhand16]

Well done, OP. This is an area I'd like to get into and your tutorial will be of great help.

[GainfulShrimp]

Nice work, OP!

If it's not cheeky to mention, though: it's 'Photomatix' not 'Photomatrix' - it hardly matters really but it might ease confusion if people are Googling it or whatever.