How Aperture Converts a Raw Image


Aperture can tell if a file is a raw file by reading the format information stored in the image's EXIF header. From this data, Aperture can also determine the make and model of the camera that was used to shoot the image. If Aperture finds that the file was created using an unsupported camera, it won't convert it. The image will be displayed in the Import dialog box as a gray square (Figure 6.5).

Figure 6.5. An images that is stored in an unsupported raw format appears as a gray box in Aperture's Import dialog box.


Note

The raw conversion features of Aperture are built on top of the raw conversion capability that's included in Mac OS X itself. Consequently, Aperture supports all the same cameras that are supported by the Finder, Preview, and iPhoto. For a complete list of supported cameras, see www.apple.com/aperture.


When processing a raw image, the first thing Aperture does is demosaic the file. Though this processing yields a legible image, the results are far from finished, and a number of additional processing steps are required.

Aperture includes a profile of every supported raw-capable camera. Created by Apple's engineers, these profiles contain, among other things, colorimetric information about the color filters that cover the photosites of the camera's image sensor: Aperture may know that a particular pixel has a red filter, but what exactly does "red" mean? The colorimetric data stored in each camera profile contains detailed descriptions of the color characteristics of the sensor's color filters.

After demosaicing, Aperture uses the camera's colorimetric data to adjust the color of the image. Unlike the color adjustments that you performed in the last chapter, this type of adjustment does not involve the remapping of tones in the image. Instead, the underlying definitions of the primary colors themselves are changed. Thus, this is a "free" edit that doesn't use up any of your editing latitude.

The resulting data is mapped into a color space. A color space is a mathematical model that describes the range and extent of the colors that your image can contain. You may know that a pixel is "100 percent red," but exactly what "red" that 100 percent is, is defined by the color space that the image is in.

As you probably know, white balance is the process of calibrating your camera for the particular light source in which you're shooting. After its colorimetric adjustment and color space conversion, Aperture white balances your image. As with the colorimetric conversion, this is also a "free" change.

The camera profiles that Apple creates also contain a gamma correction curve. This curve is applied to your image to make the linear image data appear like that of the nonlinear images that we're used to seeing.

With all of these adjustments made, your image is ready for display.

If you are shooting in JPEG mode, these same adjustments are made inside your camera. However, they are then followed up by saturation, tone, and contrast adjustments (according to settings that are usually defined in the camera's menu) and then sharpening and possibly some noise reduction. Finally, the image is JPEG compressed and stored. All of these operations are destructive; they use up some of your editing latitude.

The Aperture Raw Advantage

Photoshop and most other raw converters require you first to process your raw file, save it in a standard file format such as TIFF or PSD, and then open that new file in your image editor for additional adjustments.

However, while performing those adjustments, if you find that your original raw conversion wasn't quite right, or if you want to change directions with your edits and need different raw conversion parameters, you'll have to throw out your converted file and start over, reconverting it in your raw converter and then saving a new file.

In Aperture, raw conversion is performed in real time on the fly. This means that you don't have a separate raw processing step. All of your raw processing parameters sit side by side with your normal editing parameters. In Aperture, you don't have to divide your editing workflow into "raw processing" and "post-raw conversion edits." You can freely move back and forth between adjusting raw parameters and making other adjustments.

If you're used to separate conversion and editing steps, then you'll want to spend some time experimenting with Aperture's more flexible editing workflow.





Real World(c) Aperture
Real World Aperture
ISBN: 0321441931
EAN: 2147483647
Year: 2007
Pages: 106
Authors: Ben Long

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