Rendered File Formats
Rendered file formats are defined as those formats ready to be edited in a bitmap graphics editor such as Photoshop. Here we provide an in-depth discussion of rendered file formats, including the elements of the file and what it's well suited for.
PSD (Photoshop document) format was developed to support complex image editing within the Adobe Photoshop application. This includes layers with masks, color spaces, ICC profiles, transparency, text, annotations, alpha channels and spot colors, clipping paths, and duotone settings. It was never intended to be a delivery or interchange format and, although widely supported, it remains an Adobe proprietary format and is not as universally supported as TIFF and JPEG.
PSD is no longer the clear choice for creating and saving layered masterfiles since TIFF can now save everything that can be saved in PSD format. There is another problem with using PSD for layered masterfiles. The RLE (run length encoding) compression the PSD format uses can make for slightly smaller saved layered files if you turn off the maximum file compatibility function.Turning on maximum file compatability saves a flattened version of the file. Unfortunately turning off maximum compatibility will make PSD files unusable by other applications, especially Lightroom and Media Pro, since they depend on being able to read the flattened version of the layered file. However, when the maximize file compatibility is turned on, layered PSD files can end up twice as large as without that feature enabled.
PSD does have two advantages over TIFF for creating and saving master files: layered PSD files are somewhat quicker and more efficient to open and save compared to layered TIFF files and, for some, use of the PSD format for master files and TIFF for flattened output or delivery files can be used as an at-a-glance organizational tool.
PSB (large document format) is a new Photoshop native format with all the attributes of PSD plus the added capability of supporting file sizes larger than the 30,000 x 30,000 pixel limit of PSD.
TIFF (tagged image file format) is an open standard file format specifically designed for images. The TIFF format supports all the same image editing features as PSD including layers with masks, color spaces, ICC profiles, transparency, text, annotations, alpha channels and spot colors, clipping paths, and duotone settings. The current TIFF specification supports files sizes up to 4GB. A big TIFF proposal would substantially increase the supported size of TIFF without breaking TIFF compatibility with other applications.
Although Adobe Systems owns the copyright for the TIFF specification, they maintain the format as an open standard, currently in revision 6. TIFF can incorporate several types of compression if desired, including LZW, JPEG, and ZIP. The format is particularly well suited for the storage of high-quality, archive images.
|Figure 1 TIFF is a container format designed for storing image data. Ironically, it was created as a means to get desktop scanner vendors to agree on a common scanned image format in the mid 1980s, instead of creating their own proprietary formats. We have the same problem today with camera vendors creating their own formats instead of using a standardized raw format such as DNG.|
JPEG is the standard created by the Joint Photographic Experts Group for the compression of photographic or photo realistic images and the accompanying file format. It employs a lossy compression algorithm that can significantly reduce file size but at the expense of image quality and detail.
Caution should be exercised when using JPEG to re-save original JPEG files because the cumulative effect of compression artifacts can significantly degrade the image quality. JPEG files saved at Photoshop 12 quality (least compression) are visually indistinguishable from uncompressed TIFF or PSD files and will not visually deteriorate after a small number of re-saves.
The standard JPEG format does not support the same image editing features as TIFF, PSD and PSB, such as layers, alpha channels, or greater than 8-bit depth. JPEG format does support color spaces and embedded ICC profiles. Because of its efficient (albeit) lossy compression feature, JPEG is widely used as a delivery format.
We recommend not going below Photoshop 8 quality for delivery files, although we consider 10-12 quality to be more appropriate compression choices. JPEG is also widely used as a capture format choice for digital cameras since it makes very efficient use of memory chips.
|Figure 2 JPEG is actually the name of an image compression method used in a whole variety of image formats that are all called JPEG. While most JPEG formats feature lossy compression and 8-bit depth, there are some new variants which are described below.|
JPEG 2000 (.jp2, .jpx, .jpf) was created by the Joint Photographic Experts Group committee in 2000 and offers a compression performance gain of around 20 percent compared to the standard JPEG. JPEG2000 improves on standard JPEG by having a fully lossless option.
The main benefit of JPEG2000 is "smart decoding." A single JPEG2000 image can supply multiple, reduced-resolution versions of the original, which might include specific file sizes, and/or a high-quality, high-resolution view of a specific portion of the image. This makes JPEG2000 an excellent format if you require the ability to smoothly zoom, pan and rotate images.
Creating compressed images that contain different quality levels allow master images in an archive to supply multiple derivatives, saving time and bandwidth. This makes an image archive much more efficient. In addition to this array of output options, JPEG2000 can handle very large images, at least up to a terabyte.
JPEG2000, unlike standard baseline JPEG, supports high-bit-depth (up to 16 bits per channel vs. 8 for standard JPEG) and high dynamic-range images. JPEG2000 also underpins the MJ2 and JPM formats for motion images (each frame is a JPEG2000-compressed image) and compound images (images, graphics and text).
These additional features make the JPEG2000 format a potentially valuable option for archiving film, video and historical materials.
JPEG XR, originally called Windows Media Photo, then renamed HD Photo Microsoft, is now an approved ISO standard format. JPEG XR offers more efficient compression with less problematic compression artifacts than standard JPEG. In addition, the new format offers additional types of compression including visually lossless and mathematically lossless compression.
The format was originally called HD photo because of its ability to store pixel values in up to 32 bit-depth, and store as integers, fixed point, or floating point values in order to support a range of high dynamic range imaging schemes.
JPEG XR can potentially replace a number of other specialized formats due to its support for monochrome, CMYK, up to 16 n-channel formats (some with alpha channels), as well as standard three-channel RGB.
JPEG XR is also the basis for the Microsoft Seadragon project and the related Photosynth photo viewing technology. These new technologies use JPEG XR's ability to decode only the needed portions of an image, allowing for rapid screen draws as images are combined, zoomed, and panned. The latest Windows operating systems include full support for JPEG XR. Although Photoshop does not yet incorporate JPEG XR support, plug-ins are readily available.
The future of JPEG XR depends to a large degree on whether camera makers will offer it as a capture format. If and when this happens, JPEG XR would become a potential archival format as well.
Graphics Interchange Format is a bitmap image format, but it only supports 8 bits per pixel, so it is not really suitable for color photographic images unless they are used in small animations or low resolution film clips. Photographers seldom need to delivery images as GIF files.
Portable Network Graphics format was designed to improve on GIF, and it does in many ways, supporting full-color RGB images for instance. PNG, however, doesn't support embedded color profiles or EXIF data, so its use is mostly limited to the web. Even then, JPEG is a more efficient format in terms of maintaining high quality at a small file size.
PNG does support transparency, which JPEG doesn't, and PNG is a better choice for images that contain text or line art, as it handles sharp transitions better than JPEG does. PNG uses non-lossy compression so it doesn't suffer from increasing artifacts with subsequent edits and re-saves, as JPEG does.