Editing Digital Photographs

In a traditional darkroom, you control images with choices of films, chemicals, papers and processes. In the digital equivalent of the darkroom, you do so with a computer system and a photo-editing program. Just as in a traditional darkroom, the quality of your digital tools has an impact on the quality of your images.

However, a digital darkroom isn't like a traditional darkroom. For one thing, the lights are on. For another there is no prep time, so you can work only as long as you want. Costs are also lower because you can experiment and try things without wasting time and materials. If you make any really serious mistakes, you can always use the undo command.

Digital images can be edited in a wide variety of ways using photo-editing software. In some cases an editing program is used to "improve" an image by eliminating or reducing its flaws. In other cases it is used to take an image to a new place, making it something it never was.

Saving Images

Most digital cameras store images in one of three formats: JPEG, TIFF, or CCD RAW. When you open these images to work on them, you can, and often should, save them in another format.

Source Formats

When you take photos, they are stored on your camera's storage device in one of the following formats.

JPEG

The JPEG (Joint Photographic Experts Group) format, pronounced "jay-peg," is by far the most popular format for display of photographic images on the Web. The term "JPEG" is often used to describe the JFIF file format (JPEG File Interchange Format). JFIF is the actual file format that contains an image compressed with the JPEG method. These newer JFIF files originally used the JPG extension, however, the latest standard calls for using a JIF extension instead. The format is optimized for the display of photographs and doesn't work as well as GIF for type or line drawings (GIF is optimized for those). JPEG images have two distinctive features:

• JPEG uses a compression scheme but you can vary the amount of compression and hence trade off file size for image quality, even making extremely small files with poor quality.
• JPEG supports 24-bit color. GIF, the other format widely used on the Web supports only 8-bits.
  

Compression is performed on blocks of pixels eight on a side. You can see these blocks when you use the highest levels of compression or greatly enlarge the image. JPEG is a two pass compression and de-compression algorithm. This means it take longer to load and display than a gif file. You can save images in a progressive JPEG format that works somewhat like an interlaced GIF. While a standard JPEG loads from top to bottom, a progressive JPEG displays the entire image starting with the largest blocks. This allows the image to be displayed first in low-resolution and then filled in as more data arrives. When you save an image in this format, you can specify the number of progressive scans. Don't use JPEG to save original images you expect to modify later. Every time you open one of these files, and then save it again, the image is compressed. As you go through a series of saves, the image become increasingly degraded. Be sure to save your originals in a loss-free format such as TIFF or BMP at maximum color depth. Also, when you save an image as a JPEG, the image on the screen won't reflect the compression unless you load the saved version.

TIFF

TIFF (Tag Image File Format), pronounced, "tiff," was originally developed by Aldus Corporation to save images created by scanners, frame grabbers, and photo editing programs. This format has been widely accepted and widely supported as an image transfer format not tied to specific scanners, printers, or computer display hardware. TIFF is also a popular format for desktop publishing applications. There are several variations of the format, called extensions, so you may have occasional problems opening one from another source. Some versions are compressed using the LZW or other methods. TIFF files support up to 24-bit colors.

CCD RAW

When an image sensor captures data for an image, some cameras allow you to save the raw, unprocessed data in a format called CCD RAW (.CRW). This data contains everything captured by the camera. Instead of being processed in the camera, where computing power and workspace is limited, the raw data can be processed into a final image on a powerful desktop computer. The increased power and workspace can make a significant difference in the results. In addition, you can save the raw data and process it with other software or in different ways. When the raw data is processed in the computer into a JPEG or other image, it's a "one size fits all" form of processing and the RAW data is discarded. In the final file, not only has some of the original data been changed, some has also been deleted.

Some of the main benefits of CCD RAW files are their smaller file size written by the camera (approximately 60% smaller than an uncompressed RGB TIFF file of equal resolution), a potentially shorter interval between exposures, and optimum image quality due to superior image processing algorithms made possible by an external PC. Of course, one of the main reasons for shooting CCD RAW in the first place is to eliminate the clarity-robbing "artifacts" created by JPEG compression. As you know, Canon was among the first manufacturers to offer an uncompressed file format as far back as 1996 with the PowerShot 600; it's only very recently that other camera makers have begun to offer uncompressed file formats as well. Some makers still don't.

In addition to the digitized raw sensor data, which results in 1 byte per pixel, the .CRW format also records white balance data, contrast mapping, etc. This information is applied during signal processing to enhance color accuracy and other aspects of image quality.

It should also be noted that current PowerShot cameras initially capture image data at 10 bits per channel in 4 channels (C-M-Y-G). Whereas current signal-processing algorithms reduce this to 24-bit RGB output (8-bits per channel), it's conceivable that, with .CRW files at least, a revised algorithm could preserve the 10-bit input data intact with no increase in initial file size. In fact, future cameras could increase the digital capture gradation from 10-bit to 12-bit or even higher, resulting in even further improvements in color quality.

To summarize, the .CRW format already offers many advantages compared to other file formats. There's also a lot of room for further improvements over time."

Working Formats

As new programs are introduced, developers have a tendency to create proprietary, or native formats that can be read only by their programs. Part of this desire is to have a competitive advantage. But there is also a need sometimes to design a new format to accommodate new procedures or possibilities. However, native formats present serious problems for users who want to transfer image files among programs and share them with others. They are often not readable by other programs.

When editing photos, you usually preserve the original file and save the one on which you're working in an uncompressed format. One such format, TIFF has already been discussed. Here is a brief discussion of some other leading file formats. None of these formats are compressed, so their file sizes are quite large compared to JPEG images.

Photoshop (.PSD)

When working on images in Photoshop, there are many features, such as layers, that serve a purpose only when editing. For this reason, Photoshop has its own native format you use to save file while working on them. This format saves everything you've done to the image so you can just reopen the file and continue working. When finished, you usually save the image in another, more common format such as TIF, JPEG, or BMP.

PICT (.PIC)

The PICT format, pronounced, "pick," was introduced along with MacDraw software for the Macintosh. It has since become a Macintosh standard.

BMP (.BMP)

BMP, pronounced a letter at a time "B-M-P," files use a Windows bitmap format. These images are stored in a device-independent bitmap (DIB) format that allows Windows to display the bitmap on any type of display device. The term "device independent" means that the bitmap specifies pixel color in a form independent of the method used by a display to represent color.

Output Formats

Many digital photos end up on the Web or attached to e-mail so they are viewed on the screen. For these purposes, small files that are easy to send over the Internet are favored. The leading format for images used in these ways is JPEG, the format most often used to store captured images. However, rival formats have been developed to improve on JPEGs or to serve other purposes. Here are some of them.

PNG (.PNG)

PNG (Portable Network Graphics), pronounced, "ping," was developed to replace the aging GIF format and is supported by both Microsoft Internet Explorer and Netscape Navigator. PNG, like GIF is a lossless format, but it has some features that the GIF format doesn't. These include 254 levels of transparency (GIF supports only one), more control over image brightness, and support for more than 48 bits per pixel. (GIF supports 8 for 256 colors). PNG also supports progressive rendering, as interlaced GIFs do, and tends to compress better than a GIF. The format has never caught on and remains a curiosity on the Web.

EPS (.EPS)

EPS (Encapsulated PostScript) files, pronounced a letter at a time "E-P-S," use a format developed by Adobe for PostScript printers. These files generally have two parts. The first is a text description that tells a PostScript printer how to output the image. The second is an optionally bit-mapped PICT image for on-screen previews. Once an image has been saved in the EPS format, you can import it into other programs and scale and crop it. However, its contents are often no longer editable except by a few programs such as Adobe Illustrator. For this reason, these files are generally created at the end of the process when they are about to be incorporated into a printed publication.

GIFs (.GIF)

GIF (Graphics Interchange Format) format images, pronounced "jiff," are widely used on the Web but mostly for line art, not for photographic images. This format stores up to 256 colors from an image in a table called a palette. Since images have millions of colors, a program such as Photoshop selects the best ones to represent the whole when you save the image in this format. When displayed, each pixel in the image is then displayed as one of the colors from the table, much like painting by numbers.

There are two versions of GIF in use on the Web; the original GIF 87a and a newer GIF 89a. Both versions can use interlacing; storing images using four passes instead of one. Normally, when an image is displayed in a browser, it is transmitted a row at a time starting at the top row and filling in down the page. When saved as an interlaced GIF, it is first sent at its full size but with a very low resolution. This allows a person to get some idea of all of the contents of the image file before it is completely transmitted. As more pixels are sent in the next three passes the image fills in and eventually reaches its full resolution. The newer GIF 89a version adds some additional capabilities that include the following:

• Image backgrounds can be made transparent. To do so, you specify which color in the table is to be transparent. When viewed with a Web browser, the browser replaces every pixel in the image that is this color with a pixel from the web page's background. This allows the background to show through the image in those areas. You have to choose the transparent color carefully. If you select one that occurs anywhere in the image besides the background, your image will appear to have "holes" in it.
• Images can be animated. By rapidly "flipping" through a series of images, objects can be animated much as a movie simulates motion using a series of still images. This works best with line drawings but can also be done with photographs. Depending on bandwidth, the animation may not work the first time. However, once it's stored in cache and replayed, it will work fine.
  

GIF images are limited to a maximum of 256 colors. These colors, stored in a table, index, or palette, are often referred to as indexed colors. When you convert a photograph to GIF format, most graphics programs will allow you to dither it. This replaces lost colors with patterns of those available in the palette. Dithering improves the appearance of the image, but it also increases the size of the file. Although GIF photographs often look OK on-screen, they suffer if compared side-by-side with images saved in JPEG and other formats. The GIF format is best used for line art such as cartoons, graphs, schematics, logos, and text that have a limited number of colors and distinct boundaries between color regions. GIF images are compressed using a "lossless" form of compression called LZW (Lempel-Ziv-Welch). The amount of compression achieved depends on the frequency of color changes in each pixel row. This is because when two or more pixels in a row have the same color, they are recorded as a single block. Hence, a picture of horizontal stripes will compress more than one of vertical stripes, because the horizontal lines would be each stored as a single block. Photographs with large areas of identical colors such as skies, snow, clouds, and so on, will compress more than images with lots of colors and patterns. To save a 24-bit image as a GIF, you must reduce the bit depth down to 8 bits. To reduce file sizes in GIF format, you can further reduce the number of colors in the image. This is difficult with most photographs, but not with line art. For example, if your image has 16 or fewer colors, you can convert it to a 4-bit (16-color) palette. Most graphics programs will allow you to do this. Even with photographs you can sometimes reduce the image to fewer colors than actually exist without noticeable loss. The discarded colors are those that are seldom-used or transitional colors between more frequent colors. When working with grayscale images, GIF works as well as JPEG because almost all programs use 8-bits (256 colors) for gray scale images.

Re-sizing images

The number of pixels it contains measures the size of an image. To reduce an image, you reduce the number of pixels in it. To enlarge and image, you add pixels. Both of these processes are referred to as resizing the image. Reducing an image almost always has less affect on its appearance than does enlarging one. This is because; enlarging requires the program to add pixels. To do so, the computer analyzes adjoining pixels to determine the color of any new ones it inserts. Normally, you can double the size of an image without effects showing. However, trial and error is usually the only way to see because images vary so much.

Cropping Images

Great photographers try to compose, capture, and then print images full-frame. However, editors and others love to crop images to fit unusual shapes in layouts, or to make the image more dramatic. It's like looking for a better photograph within a photograph. At other times, you crop to straighten a horizon line, remove distracting elements, or enlarge small portions.
To evaluate an image for cropping, make a printout. Then cut two "L" shapes from a piece of paper or cardboard large enough to cover the photograph you're evaluating. By arranging the L's in various configurations, and size and shape rectangle can be formed to isolate a portion of the image. When you decide how you want to crop, mark straight lines onto the print to use as a guide when cropping in your photo-editing program.

As with film photography, cropping reduces the area of the image to be enlarged. The greater the enlargement, the more blur, grain, and other faults show. Try to shoot images full-frame so you don't have to crop them later. Professionals have always tried to so this, and it's only in photojournalism and the graphic arts where a lot of cropping goes on as editors and designers look for images within images.

Rotating Images

If an image becomes rotated in the frame during capture or scanning, you can rotate it back to a vertical or horizontal position. When you use this command, you'll always loose a part of the image when you crop the edges to make the edges of the rotated image square or rectangular again.

Changing Brightness and Contrast

In a darkroom, you control for brightness by changing exposure times and contrast by your selection of papers or filters. In digital photography dragging slider bars usually controls these. To understand what effects these have, just change the brightness and contrast settings on your monitor.
Saturation refers to the intensity of a color. This feature is often used with color balance to vary the intensity of specific color levels to get those color photos looking just right.

Using Filters

Since one of the most interesting things to do with digital images is to manipulate them, be sure to look into filters. Many of this like the unsharp filter are used to make your images look more like the original scene. But many others can make photos looks like paintings, stained glass windows, or hundreds of other objects. The can emboss images, add textures, twist and twirl them, and create all kinds of creative havoc. They are easy to use, you just select the command and sit back and watch your computer go to work. If you don't like the result, just click the undo command and try another filter. And don't stop with just one filter, use two or more in succession on the same image and watch as it moves further and further from reality. Many photo editing programs come with filters built-in, but some programs allow you to purchase separate plug-ins to expand your library of effects from which to choose.