Browser Preview
Web Posterize
Web Scrubber
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A primer on making great web graphics
There is an art to making small web graphics that look great.
The filters in this series help you master that art.
To see the images in this manual properly, your display should
be running in millions-of-colors mode.
Browser Preview
The first tool we'll look at is Browser Preview. It helps you
decide how to prepare your graphics for presentation on the web.
Let's take an image of some fruit (from Photoshop's Samples folder):
Looks great in thousands of colors. But what about in 256 colors?
We could save the file, switch our monitor settings and open up
the graphic in a web browser. Or just click on Browser Preview's
"256 colors" checkbox:

And the rest of our display has not been subjected to 256 colors.
Cool!
We may be designing on a Mac -- not the machine that most people
use to view the web. What do all those Windows users see? Click
on the gamma popup menu to find out.

Hey! Why'd the image get dark?
Monitors connectedf to PCs running Windows may do no gamma correction
to present a more even range of colors, or it may do the correction
differently from a Mac. Because there are so many types of uncorrected
monitors on PCs, you can select the radio buttons to sample a
range of hypothetical monitors.
Conversely, we could be designing on Windows, and choose one of
the bright gamma options to see what the image will look like
on a Mac.
Now let's see what happens when we start to compress the image.
First, let's take a look at JPEG. Images compressed with JPEG
rely on a mathematical technique called the Discrete Cosine Transformation
(DCT). Explaining the math behind the algorithm is hard. Explaining
effect is simple: your image gets more blocky as you lower the
quality and increase the compression. The image also loses some
color resolution.
The following image is about 2K bytes (quality of 1, based on
image):
Compare it with Photoshop's default (quality of 8, not based on
image):

The second image's quality is much higher. It's also more than
5 times the size of the first (at about 11K bytes).
The trick with JPEG is to lower the quality to the point where
you can't see the blocks. Browser Preview lets you see the effects
of the compression as you adjust the quality.
Another issue when using JPEG are it's colors. It uses millions
of colors to store the image. However, you don't have any control
over which colors it uses; could be 216 colors, could be 16 million.
Browser Preview will show you how the millions of colors will
look on a display with 256 colors.
The other format for web graphics is GIF. A GIF file can have
up to 256 colors. And you can choose whatever colors you want
for the graphic.
Colors in a GIF can either be "web safe" or not. Colors that are
not web safe will dither on a display with 256 colors. That is
because the color in the GIF file can not be reproduced exactly.
Let's look at the fruit in 256 non-web safe colors on a browser
that can display thousands of colors:

Now, let's look at the same GIF file on a browser with a 256 color
display:

This is called "browser dithering". It happens when you use colors
that are not web safe. Sometimes it looks right, sometimes it
doesn't. With Browser Preview, you can check the graphic without
saving it and opening it up in a browser running on a 256 color
display.
A GIF file that uses web safe colors will look the same on any
browser, regardless of it's display setting. Here's an example
of the graphic with web safe colors:

It looks worse, but at least it's consistent.
One common technique used to make the web safe colors more palatable
is to add dithering to the file (as opposed to the dithering in
the browser, discussed above):

Looks much better.
So why not dither all your images? Because it increases the file
size. The non-dithered image is about 7K bytes. The dithered image
is about 10K bytes. Browser Preview tells you how much the dithering
costs in terms of file size.
As you're probably beginning to realize, there's a lot of factors
involved in getting the graphics to look good. But what are the
factors for getting the files small so they load quickly?
The biggest factor is reducing the number of colors. Fewer colors
allows more information to be packed into each byte of the file.
For example, if you are using 256 (8-bit) colors, then only one
pixel can be put into each byte of the file. If you are using
16 (4-bit) colors, then you can put two pixels into each byte
of the file. And it's half as big.
Another factor in reducing the size of the file is to maximize
the areas of solid color. The GIF compression algorithm can record
these areas of solid color much more efficiently than areas of
constantly changing colors. That's why dithering adds so much
to the file size (30% in the example above).
Browser Preview makes it easy to check out different scenarios
and find the compression scheme that looks the best and creates
the smallest file.
There's a separate manual that explains Browser Preview's controls.
Web Posterize
Now we've seen that using the web safe palette makes the most
consistent graphics. What if we could play around with this palette?
Enter the filter: Web Posterize
Here's the fruit with increased contrast and a yellow tint:
The graphic can be stylized even further by removing the color
and tinting the foreground and background:

Both of the images above are completely Web Safe. And both will
compress well; there are large areas of flat color and small palette
can be used for those colors.
Web Posterize is most effective when you aren't concerned with
the fidelity of the original. Try it when you are concerned about
grabbing eyes.
There's a separate manual that explains Web Posterize's controls.
Web Scrubber
The final filter we're going to look at is Web Scrubber.
This filter helps you control which areas of the image will be
dithered by the browser. Remember that a browser will dither a
GIF file when it can't reproduce a color exactly.
Scrubbing an image shifts colors in the image towards the 216
web safe colors. You control how many colors get shifted. As more
colors shift, fewer colors dither.
The following image was scrubbed into a 128 color (7-bit) GIF;
about half the colors were shifted. On a display with thousands
of colors, it will look like this:

On a 256 color display, it will look like this:

Note how some colors of the image don't dither (like the greens
and browns on the rim of the plate). Other colors, such as orange
tones, do dither. Both images look acceptable, even though they
look different on the different platforms.
Note: This example is a bit contrived. This technique usually
works best on navigational and banner graphics. Places where you
want to mix dithering with areas of flat color.
There's a separate manual that explains Web Scrubber's controls.
We hope that this has helped you understand web graphics a little
better. It's not an easy thing to learn; we have spent many hours
scratching our heads. Hopefully, you will benefit from our trials
and tools.
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