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If you have version 5 or greater you have the ability to use Photoshop's "Save for Web" command. This handy feature gives you to ability to save a copy of your image that is optimized for use on the Web. By doing this your image file will be as small as possible, and that the image will use only Web-safe colors (if desired). The "Save for Web" feature can produce GIF, JPEG, or PNG format images.
Using Save for Web When your image is ready to be put onto your web page, select "Save for Web"... from the File menu. The "Save for Web" dialog appears:
This dialog box allows you to tell Photoshop how you'd like to optimize your graphic for the internet. You have the ability to select the file format to produce (GIF, JPEG or PNG), what size palette you'd like to use, how to cut down the colors to fit the chosen palette size, how much to dither the optimized image (if at all) and how much image quality to sacrifice in order to produce a smaller file size.
Image views The four tabs above the image view pane on the left allow you to view the original image, the optimized version (this is the default), the original and optimized versions side-by-side ("2-Up") and the original, optimized and two variations on the optimization all together ("4-Up"). You can also use the Hand tool top-left to move the view around (if the image is bigger than the view), the Zoom tool to zoom in and out of the image, and the Eyedropper tool to select a color from the image, to be used by various color options on the right of the dialog.
Presets There are many options in the Save for Web dialog, and the choices here can be quite bewildering! Fortunately, there are lists of presets you can choose from to make it easier. Select the Settings: drop-down list, which is located just below the Cancel button, to bring up the list of presets:
In the presets there are three basic image formats - GIF, JPEG and PNG. As a basic rule you use JPEG's for photos and GIF's for everything else. For a complete lesson on selecting the right image format, see Understanding image formats.
Optimizing GIFs If you're making a GIF, you will want to start off with a preset such as GIF 32 Dithered, which works well for most GIFs. You can then fine-tune the optimization to fit your needs. Some of the important optimization options are discussed below.
Color reduction method This drop-down box is located below the Image Format box. The commonly used options here include:
Perceptual: Creates the reduced color palette which favors colors that the human eye is more sensitive to. Selective: Like Perceptual, however better for Web work. Selective is the favored (and default) option. Adaptive: Creates the palette by taking an even sampling of colors from the image's color spectrum. This is generally not as good a choice as Selective. Web: Use this if you need a complete Web-safe color palette (Netscape 216-color palette). These days, this method usually isn't that important as most computer displays support at least 16 million colors. Dithering method This is located below the "color reduction method" box. Dithering involves adding patterned or random dots to the image to make it appear to contain more colors than are actually in the palette, allowing you to use a smaller palette size. The options are:
No Dither: No dithering is applied to the image. This is good if the dithering effect looks bad, but can produce severe "banding" of colors. Diffusion: Uses error diffusion dithering, which looks similar to Noise dithering and produces a random-looking pattern of dots. This method can produce seams when using ImageReady slices. Pattern: Uses a pattern for dithering, rather than random dots. Can work well for some images, but sometimes produces a rather unnatural look to the image. Noise: Similar to Diffusion, but does not create seams at the edges of slices in ImageReady. Transparency If your image does not have its Background layer turned on this checkbox will be available. It denotes that you wish for parts of the saved GIF to be translucent. If you uncheck this box, the transparent areas will instead be filled with the Matte color (or white if no matte is selected).
Interlaced A quick note about this: if checked it does increase the file size. This checkbox controls GIF Interlacing. If enabled, the GIF will appear gradually as interlaced horizontal lines as it is loaded on the Web page, which gives viewers something to look at while the full image appears.
Lossy If you wish to reduce the file size further this slider allows you to remove detail from the image. Use this only if you don't mind reducing the image quality quite severely! A value of 0 will not remove any detail; a value of 100 will remove the maximum amount of detail.
Colors - This is where you choose the size of your GIF palette. A palette of 32 colors is often sufficient for web images, but if your image has a lot of detail and looks too fuzzy/blurry/banded with 32, up it to 64, 128 or 256. If your image has very few colours in to start with, or doesn't look too bad with fewer colors, select 16, 8, 4 or even 2! This will make the GIF file size smaller.
Dither - If you have selected a dithering method (see above), this option lets you control how much the image is dithered - 0 means no dithering, 100 means lots of dithering.
Matte The matte is the background color you want to use for your image. If you've enabled Transparency then the foreground image will be faded at the edges to blend in with the matte color. If you choose the Matte setting of None, the GIF will have a "hard transparency" with no fading; this is great if you want to be able to use the image on any color background.
If you've not checked Transparency, the transparent areas of your image will be filled in with the matte color.
Web Snap If you want to use web-safe colors you will want to increase the Web Snap slider. Photoshop will bias your color palette more and more towards web-safe colors the higher the value of Web Snap.
Optimizing JPEGs If you're creating a JPEG you want to start off with a preset such as JPEG Medium, which works well for most JPEGs. Then you can fine-tune the optimization to suit your needs. Some of the important optimization choices are discussed below.
Quality Two ways to alter the compression quality include the Low/Medium/High/Maximum list (for quick access) and the Quality slider (for fine control). The lower the quality setting, the more blurry and bitty the JPEG will appear, but the resulting file size will be smaller.
Progressive A progressive JPEG is analogous to an interlaced GIF (see above). The image will load step by step on the Web page - a low resolution image first, then eventually the full, high-resolution image. Again this is great for stopping your viewers on slow modems from getting bored, but it does mean a slightly larger file size, and older browsers don't support progressive JPEGs.
Matte - If your Photoshop image has transparent areas, you can fill them with a specified matte color with this drop-down box.
Optimizing PNGs - The options for optimizing a PNG-24 is much the same as those for optimizing a JPEG. Similarly, the options for optimizing a PNG-8 are very similar to those for optimizing a GIF. See the GIF and JPEG sections above for details.
And, if you really can't be bothered... ...you can always allow Photoshop to optimize the image for you! Select the little arrow just to the right of the Settings... box and select Optimize to File Size...:
In the dialog that pops up, choose Auto Select GIF/JPEG then enter your desired file size. Click on OK and Photoshop will do the rest for you! If you're not pleased with the results you can tweak the settings as described in the sections above, or just choose a slightly larger file size and try again.
Saving the image Once you're happy with your optimized image, click OK to save it to disk. The file saved will be a copy of your original image, unless you specifically overwrite the original with the optimized file.
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