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<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
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<html>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<meta name="GENERATOR" content="Mozilla/4.78 [en] (X11; U; Linux 2.4.18-19.7.x+obaq i686) [Netscape]">
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<meta name="Author" content="Paul Schneider">
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<meta name="Description" content="Describes how to use the OpenEXR file format plug-in for Photoshop
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">
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<title>OpenEXR for Adobe Photoshop</title>
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<body>
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<h1>
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OpenEXR for Adobe Photoshop</h1>
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This document describes how to use the "EXRFormat" plug-in to read and
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write OpenEXR images using Adobe Photoshop. For more information
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about the OpenEXR format, please visit <a href="http://www.openexr.com">www.openexr.com</a>.
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<br>
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<h2>
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Supported Operating Systems</h2>
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This plug-in has only been tested on MacOS 9.2.2, Mac OS X 10.2.4, and
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Windows 2000. It should work on MacOS 8.1 and higher, and Windows
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98 and higher, but this has not been verified.
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<br>
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<h2>
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Supported Host Applications</h2>
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This plug-in has only been tested with Adobe Photoshop 7.0. It should
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work with Photoshop 5.5 and higher, but this has not been verified.
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This plug-in will not work with versions of Photoshop earlier than 5.5.
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<p>This plug-in will not work correctly with other applications which support
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some Photoshop file format plug-ins, such as combustion from Discreet,
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or Commotion from Pinnacle Systems.
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<p>This plug-in will also not work correctly with Adobe After Effects.
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An After Effects-specific plug-in may be included in a future release of
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OpenEXR.
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<br>
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<h2>
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Installation</h2>
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To install the plug-in, copy it to your Photoshop plug-ins folder.
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If Photoshop is running, you will have to quit it and launch it again.
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Here is a typical Macintosh installation:
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<p><img SRC="images/Picture00.jpg" NOSAVE height=717 width=453>
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<p>You could also place the EXRFormat plug-in in the "File Formats" folder,
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or in any folder inside the "Plug-Ins" folder.
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<br>
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<br>
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<h2>
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Opening an OpenEXR image</h2>
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Once you have installed the plug-in, you will be able to open images in
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the OpenEXR format. Select "Open..." from Photoshop's File menu:
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<p><img SRC="images/Picture01.jpg" NOSAVE height=100 width=212>
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<p>and navigate to a folder containing OpenEXR images. This example
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uses the images distributed with the OpenEXR source code release.
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<p><img SRC="images/Picture02.jpg" NOSAVE height=434 width=561>
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<p>Note that when you select an OpenEXR image, the "Format:" menu in the
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Open dialog reads "OpenEXR". This lets you know that the plug-in
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is installed correctly, and that Photoshop has detected that the file is
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in the OpenEXR format.
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<p>When you click Open in the Open dialog, you will see a new dialog which
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allows you to specify how the image is interpreted in Photoshop.
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<p><img SRC="images/Picture03.jpg" NOSAVE height=323 width=494>
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<p>This dialog is necessary because an OpenEXR image can contain many more
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colors than you can work with in Photoshop, even in Photoshop's "16 Bits/Channel"
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mode. OpenEXR uses floating-point (real number) pixels, and supports
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a high dynamic range of pixel values - that is, colors which are brighter
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than white. Photoshop uses integer (whole number) pixels, and does
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not support colors brighter than white. Because of this, not all
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of the pixels in the OpenEXR image will be preserved when the image is
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loaded into Photoshop. This dialog lets you control which pixels
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are preserved, so that you can work with the image in Photoshop with a
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minimal loss of quality.
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<p>Here is a quick explanation of what these controls do:
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<ul>
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<li>
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Exposure: this controls the apparent exposure of the image. An exposure
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of 0 (the default) will apply no change to the image. An exposure
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of 1 will make the image twice as bright, revealing more detail in the
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low end. An exposure of -1 will make the image twice as dark, revealing
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highlights that may have been clipped at higher exposures.</li>
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<li>
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Gamma: this controls the gamma correction that is applied to the image.
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Most images, such as JPEGs or TIFFs, are stored with an implicit gamma
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encoding of 2.2, the standard for video images. OpenEXR images are
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stored with no gamma correction. By default, the EXRFormat plug-in
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applies a gamma correction of 2.2 to the image as it is read into Photoshop,
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so that it will match the color space of other images, and display correctly
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on a computer monitor. If you do not need to color-match the EXR
|
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image with an image in another, gamma-corrected format, you might find
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other gamma values more visually appealing. For example, most Macintosh
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monitors have a display gamma of 1.8, not 2.2. (Microsoft Windows
|
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computers generally have a display gamma of 2.2).</li>
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<li>
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Un-Premultiply: by convention, OpenEXR images are "premultiplied" - the
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color channel values are already matted against black using the alpha channel.
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In Photoshop, it's more convenient to work with unmatted images.
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It's important to use this option rather than un-premultiplying the image
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within Photoshop, because the plug-in will un-premultiply before applying
|
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exposure and gamma correction. This option will have no affect if
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your image does not contain an alpha channel.</li>
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</ul>
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The dialog also contains a preview area, which lets you see the effect
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your settings will have on the image. Click in the black area of
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the dialog to load the preview:
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<p><img SRC="images/Picture04.jpg" NOSAVE height=324 width=494>
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<p>In this preview, we can see that this image has a fairly broad dynamic
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range. Much of the image is too dark to see clearly, but the lamps
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are so bright that they have been clipped at the default settings.
|
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If we darken our exposure, however, we can see that the image does contain
|
||||
data for those bright areas. Setting the exposure to -2 makes the
|
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image four times darker - the pixels that now appear white are actually
|
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four times brighter than "white".
|
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<p><img SRC="images/Picture05.jpg" NOSAVE height=324 width=494>
|
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<p>We could reduce the exposure even further, and reveal more detail in
|
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the highlights, but we would start to lose detail in the dark areas.
|
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If we click "OK" at this point, the image will be opened in Photoshop at
|
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an exposure of -2, and will appear very dark (as we saw in the preview):
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<p><img SRC="images/Picture06.jpg" NOSAVE height=461 width=636>
|
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<p>It appears that we have lost a large amount of detail in the image,
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but there is actually much more detail here than can be displayed on your
|
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computer screen. Checking the "Image->Mode" menu in Photoshop reveals
|
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that this image is 16 bits per channel, and a computer monitor can only
|
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display 8 bits of information per channel.
|
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<p><img SRC="images/Picture07.jpg" NOSAVE height=315 width=337>
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<p>To see the hidden detail, we can use the preview feature of Photoshop's
|
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Levels command:
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<p><img SRC="images/Picture08.jpg" NOSAVE height=99 width=367>
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<p>This will create a dialog that will show us the histogram of the image.
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Note that there is a large gap between the brightest pixels (the blown-out
|
||||
highlights) and the majority of the colors in the image.
|
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<p><img SRC="images/Picture09.jpg" NOSAVE height=465 width=926>
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<p>If we adjust the white point so that the majority of the pixels define
|
||||
the full range of the image, the highlights will be blown out, but more
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low-end detail will be revealed.
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<p><img SRC="images/Picture10.jpg" NOSAVE height=467 width=926>
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<p>Note that if you click OK in the Levels dialog, the range adjustment
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will be applied to the actual image, and all of the overrange pixel values
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will be clipped to white. When working with OpenEXR image in Photoshop,
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It's important to be careful that the details you care about are preserved.
|
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<p>Also, note that when working on a 16 bit image in Photoshop, many features
|
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such as painting and layers are unavailable. If you wish to use these
|
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tools, you must convert the image to 8 bits per channel using the "Image->Mode"
|
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menu, which will result in an even greater loss of data. Cautious
|
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use of the exposure and gamma controls in the EXR Import dialog, along
|
||||
with applying the Levels command before converting to 8 bits, will help
|
||||
you use all of Photoshop's powerful tools while preserving the colors that
|
||||
are most important in your image. Of course, some images will simply
|
||||
contain too much data to be manipulated safely in Photoshop - it's up to
|
||||
you to ensure that the image you end up with is correct and visually appealing.
|
||||
<br>
|
||||
<h2>
|
||||
Saving an OpenEXR image</h2>
|
||||
Saving an OpenEXR image is similar to opening one. When you choose
|
||||
to save an image in OpenEXR format, a dialog will appear, allowing you
|
||||
to specify how the image should be saved. Many of these settings
|
||||
are similar to the controls provided by the Import dialog.
|
||||
<p><img SRC="images/Picture11.jpg" NOSAVE height=301 width=363>
|
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<p>Note that the Exposure and Gamma settings mean something different in
|
||||
this dialog - they specify the settings that currently apply to the image.
|
||||
For example, when the image opened in the previous example is saved, this
|
||||
dialog states that the image has been gamma corrected and darkened.
|
||||
The plug-in will undo both of these settings before saving the OpenEXR
|
||||
file. This means that if you use the same settings when you save
|
||||
an OpenEXR image as when you opened it, the color space of the image will
|
||||
stay the same. However, any colors which were lost when the image
|
||||
was opened (clipped highlights or crushed low-end values) will not be restored.
|
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If you are saving a new image, or an image originally in another format,
|
||||
as an OpenEXR image, the default settings will result in a valid image.
|
||||
<p>This dialog also allows you to specify the compression scheme used to
|
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save the OpenEXR image. The OpenEXR format supports several methods
|
||||
of compression, all of them lossless. For details about which method
|
||||
best suits your needs, see <a href="http://www.openexr.com">www.openexr.com.</a>
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<p><img SRC="images/Picture12.jpg" NOSAVE height=302 width=362>
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<br>
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<br>
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<br>
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<br>
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