Adobe Photoshop CS5 : Specialized Processes - Save For Web & Devices, Convert to CMYK, Add an Alpha Channel

10/4/2012 4:05:47 AM
Creating files for special uses often requires special processing. The techniques discussed in this section are fairly elaborate, so the short overviews are meant for a clearer understanding of possibilities. The creation of specialized formats for the Internet, professional printing, or video requires a mastery of several interconnected skills. Let’s take a quick look at converting to special purpose files.

Save For Web & Devices

Preparing images for the Web or mobile devices is all about compromise. You must learn to balance appearance with file size. If a Web page takes too long to load, people will leave—which defeats the purpose of running the site. Fortunately, Photoshop provides a powerful command for compressing images and previewing the results: the Save For Web & Devices command.

Let’s give the Save For Web & Devices command a try:

Open the file Ch16_Surfboards.tif
Choose File > Save For Web & Devices.

The Save For Web & Devices dialog box offers several important options for optimization and preview:

  • Tools. If you can’t see the entire image, you can use the Zoom tool to make the image more visible. Additionally, you can use the Hand tool (or hold down the spacebar) to drag and navigate around the image. Alternatively, you can click the Zoom Level menu in the lower-left corner and choose a magnification level.

  • Optimization tabs. By clicking the four tabs at the top, you can choose to view the Original image, an Optimized view, 2-Up for two versions of the image side by side, or 4-Up for four versions of the image side by side. Being able to compare optimized images helps you choose the right format and compression settings. For this image, choose 2-Up.

  • Image Optimization Info. The area below each image in the Save For Web & Devices dialog box gives you optimization information. You can see the current optimization applied, the projected file size, and the estimated download time based on a selected modem connection speed. Choose the JPEG High preset, and you’ll notice that the file has been reduced from 32 MB to 1.055 MB (a significant savings). However, the download time is 196 seconds on a 56K modem (you can right-click the time to choose another speed).

You need to further reduce the file size for Internet delivery. The first area to tackle is the actual image size in pixels. In the Image Size field you’ll see that the image is more than 4,000 pixels wide (which is much taller than a typical Web page that can be displayed on most monitors). Type in a Height of 600 pixels, so the image can integrate easily into the Web page (even with a screen resolution of 1024 × 768, a height of 600 would allow the image to display without scrolling up and down). Press the Tab key to exit the file and apply the resize value.

The file size has been significantly reduced, but it’s hard to see the effects of the compression. Set the image magnification view to 100%.

Change the amount of Compression by either changing the preset (from High to Medium, for example) or adjusting the Quality amount. You can manually enter a number or click to access a slider (you will need to release the slider for the image to refresh). Try a setting of 45 to see the results. The image is now at just over 32K, which is more than a 99.9 percent reduction in file size and a fundamental change for Web delivery.

Toward the lower-right corner you have the ability to choose to preview the image in a Web browser. If you don’t see your browser of choice, just choose Edit List, and then choose Find All to add all Web browsers on your computer.

Click Save to specify a location for the saved file. Choose your desktop and click Save in the new dialog box to process the image and save a compressed Web-ready version. The original file will remain untouched, and its resolution and quality will be identical to its state when you launched the Save For Web & Devices command.

Experiment with other file formats such as GIF and PNG to see their benefits and limitations.

Convert to CMYK

Although CMYK conversion is an everyday process for many users, several authors and trainers have developed some useful techniques. What I offer here is a proper workflow that will work for most users, on most images, in most environments. I encourage you to continue to explore prepress production through further reading. CMYK conversion can be a very tricky process, and it is essential that you have access to the color profile used by your output device. Additionally, be sure to discuss the process with your service bureau that will do the professional printing. With all of these caveats said, let’s take a look at the process:

Check your color management settings by choosing Edit > Color Settings or by pressing Shift+Command+K (Shift+Ctrl+K). Choose North America General Purpose 2.

Open the file Ch16_Parrots.tif
Choose View > Gamut Warning or press Shift+Command+Y (Shift+Ctrl+Y). Areas that are too bright or saturated for CMYK printing will be highlighted in gray. This is because the RGB space can represent a wider ranger of visible colors based on the additive method of color. CMYK printing instead uses the subtractive model, and it has a narrower range. The warning is useful because it lets you identify areas that are subject to color shifting when printing or converting to the CMYK color mode.

Select the Sponge tool (O) from the Tools panel. Adjust the brush to a large size with soft edges. Set the flow to a lower value such as 20% and the mode to Desaturate. Deselect the Vibrance option to have greater impact on the saturated color areas. These settings will gently soak up the color in the oversaturated areas.

Carefully paint over the oversaturated areas with the Sponge tool. It may take multiple strokes, but you’ll see the gamut warning go away as you reduce the oversaturated areas. Repeat for other problem areas in the photo.

When all of the gamut warning has been removed, choose Image > Mode > CMYK. There should be no visible color shifting. By taking the time to manually touch up the out of gamut areas, you’ll get a better CMYK conversion without any posterized edges or color clipping.

Save the image in a print-ready format such as TIFF.

Add an Alpha Channel

The alpha channel can be used to store transparency information, and it is particularly useful for video and multimedia users. In Photoshop’s Actions panel, you’ll find the Video actions that I co-wrote with Daniel Brown. These can speed up certain tasks for a video workflow. Two of these actions can create an alpha channel for multilayered graphics with transparency.

Make sure the Logo layer is selected in the Layers panel.

Call up the Actions panel and load the Video Actions by clicking the submenu. Choose the Video Actions set.

Choose the Create Alpha Channels from Visible Layers action. You must see Photoshop’s transparency grid for it to work.

Click the Play Selection button to run the action. A dialog box appears with instructions. Read it and click Continue. A new alpha channel is added to the document.

Choose File > Save As and save the file as a PSD, TIFF, or Targa file, and then choose to embed the transparency by including the alpha channel.

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