1 Overview

• with RAW images and JPEG images:
  • Illumination of shadowy areas
  • Enhancement of contrast
• with HDR (High Dynamic Range) images:
  • Dynamic Range Compression
  • Visualisation for normal media like monitor or print
  • Conservation of contrast and details

	Fig. 1: FDRCompressor Plug-In

This tutorial shows the usage of FDRCompressor Plug-In for the above mentioned applications. Using the Plug-In is simple and intuitive. However, when processing RAW images a few specialties need to be considered. Before starting with your own images, you should read section System requirements and performance at any rate!

2 Call FDRCompressor

To call FDRCompressor use the script Call FDRCompressor available from the File - Scripts menu. This script also serves as a sample for calls on FDRCompressor from your own scripts.

The reason for cutting off the Filter menu item is: In order to achieve an optimal result FDRCompressor needs to know which color profile is assigned to the image. Otherwise the result could show color deviations or it could be that the achieved contrast improvement is less than optimal.

Not all of the necessary information is accessible from a Filter Plug-In. Hence the script retrieves the color profile and hands it to the Filter Plug-In via a parameter. Should there no profile be assigned to the image then the working profile is used.

3 Tonal Range Compression with RAW images

The following image comparison illustrates this with an example of a partly sunlit flower. The RAW image comverted like described in section 3.1 is compared to the results achieved with Adobe Camera Raw Defaults and FDRCompressor:

	RAW image Photoshop CS2 Camera RAW Defaults FDRCompressor

Please execute the following steps to reproduce the FDRCompressor result:

1. Download HibiskusRot.jpg (906 KB).
   • Due to the large file size this is not the original RAW image. However, the image was created with Adobe Camera Raw from the original as described in section 3.1, converted to 8-Bit and finally saved as JPEG with embedded ICC profile.
2. Load the image to Photoshop.
3. Start the Plug-In with File - Scripts - Call FDRCompressor.
4. Adjust the FDRCompressor parameters as ahown in Fig. 3.
5. Start the calculation with OK.
   • The calculations take some time. This is an opportunity to read section System requirements and performance, if you haven't done that before.
6. Finally adjust black and white point accurately by eye with Image - Adjustments - Levels..., q.v. section 5.

	Fig. 3: The FDRCompressor parameters

3.1 Open image with Adobe Camera Raw

Some of the possible adjustments in Adobe Camera Raw disturb the optimal function of FDRCompressor. FDRCompressor gives optimal results with unmodified, especially linear image data. In order to minimise the perturbings the following settings should be carried out in Adobe Camera Raw:

Fig. 1 shows the Settings within Adobe Camera Raw. Within tab Adjust all sliders from Exposure to Saturation should be set to 0. Within tab Curve the Tone Curve should be set to Linear.

	Fig. 4: The Settings in Adobe Camera Raw

Within Workflow Options choose your preferred color profile, here ProPhoto RGB was chosen:

	Fig. 5: The Workflow Options in Adobe Camera Raw

Adjust Depth to 16 Bits/channel. Close Adobe Camera Raw by opening the image.

4 Tonal Range Compression with HDR images

The difference between largest and smallest exposure value is enormous: about 19 EV. Without compressing the dynamic range this image can not be displayed properly on a monitor.

Hence the uncompressed HDR image is partially black and partially white. The other versions show a comparison between Photoshop CS2 Local Adaptation and FDRCompressor:

	HDR image Photoshop CS2 Local Adaptation FDRCompressor

Please execute the following steps to reproduce the FDRCompressor result:

1. Download AltesRathaus.hdr (2,5 MB).
2. Load it to Photoshop.
3. Assign the image the Wide Gamut RGB color profile with Edit - Assign Profile....
   • The image was created and saved in Wide Gamut RGB color space. This must be assigned manually here because the used file format Radiance RGBE does not allow to embed an ICC profile.
4. Start the Plug-In with File - Scripts - Call FDRCompressor.
5. Adjust the FDRCompressor parameters as shown in figure 7.
   • Black and white point are set with the two sliders below the histogram. They should be reasonably well adjusted now. A precise adjustment is done in the final step.
6. Start the calculation with OK.
7. Finally adjust black and white point accurately by eye with Image - Adjustments - Levels..., q.v. section 5.

	Fig. 7: The FDRCompressor parameters

5 Correction of black and white point

If the image is in 32 Bits/channel mode the color depth must be reduced to 16 Bits/channel prior to further processing. This happens within HDR Conversion, Navigation Image - Mode - 16 Bits/channel. Do not change the default values - just confirm with OK.

Now the values for black point and white point are adjusted with Image - Adjustments - Levels... The HDR image from section 4 has the following values:

	Fig.  8: Black/white point correction within the Levels dialog

6 The FDRCompressor parameters

• The sliders below the histogram adjust black point and white point. The settings should be approximately right. You should avoid overexposure because that can't be corrected later on. Fine adjustment can be performed following the compression with the Photoshop Levels dialog.
• The settings for the three panorama switches 360° Pano, Zenith and Nadir is explained in section 7.
• Compression regulates the strength of tonal range compression. Higher dynamic range values require more compression. For Raw images the value range from 1 to 5 is suitable, HDR scenes require values up to 10.
• Contrast substantially determines image contrast. Small values result in smooth transitions while large values yield strong contrasts. Should the image look dirty or should undesired seams show up (blue skies are especially delicate) you should lower the value.
• Gamma allows to lighten or darken the image easily. Normally this is not necessary but may make sense sometimes, e.g. in a night scene like that of section 4.
• Saturation regulates the color saturation and is not changed normally.

7 Handling of 360° panoramas

Because of their large field of vision extreme contrasts are nearly unavoidable in panoramas. Often the sun and/or other strong sources of light show up in the image and lead to severe exposure problems. Capturing the full dynamic range of a scene by means of exposure series regularly results in HDR-Panoramas that need to be further processed so that they can be displayed on traditional media.

FDRCompressor handles cylindrical as well as spherical panoramas in equirectangular projection. This form of projection is quite common in panorama photography; for more information see Panorama Tools.

The panorama geometry can be adjusted with three check boxes: 360° Pano, Zenith and Nadir. The following list explains how to check the boxes against the panorama form:

cylindrical partly spherical, with Zenith, no Nadir partly spherical, with Nadir, no Zenith spherical

8 System requirements and performance

The following table sums up the numbers that were measured for the processing of two HDR images (32 bits/channel) of different size. The measure was done with a PC, equipped with an AMD Athlon XP 1800 Processor (clock speed 1.53 GHz) and 1.5 Gigabytes of RAM. On the computer only Windows XP and Photoshop CS2 were running. This makes a system memory load of about 230 MB. The memory numbers given in the table name the overall system memory load.

	Image size	2 Megapixels	8 Megapixels
	Image loaded into Photoshop	280 MB	460 MB
	Processing with FDRCompressor	400 MB	960 MB
	Computing time	0:30 min	1:50 min

End.