TechNote (Archived)

What is an anti-aliased bitmap?

How a computer determines anti-aliasing
Anti-aliasing examines the vector graphics on the screen. It looks at neighboring colors, then adds tinted pixels similar to those colors. These pixels are added in a pattern around the original pixels. When you see the final effect from a distance it looks as if the jagged edges have been smoothed over.What is happening is that your eye is averaging the solid colors that were there before the anti-aliasing with the tinted colors in between them to produce a blurring effect which makes the transition between pixels appear smoother.

How FreeHand handles anti-aliasing
It is not possible to draw a smooth curve on a piece of graph paper. The computer screen is divided into squares or pixels similar to the graph paper. For curved lines on the computer it has to fill in the squares of the graph in a way that makes the illusion of a curved line.If you draw a circle in FreeHand you'll see this. The edges of the circle aren't smooth, as they would be if you traced a circle using pencil and paper. Because the computer can only draw pixels on the diagonal to simulate a curve, curved surfaces in FreeHand appear as jagged stair-steps.When exporting FreeHand information to a PostScript format such as EPS this isn't a problem. The PostScript language handles the creation of smooth curves. But when exporting a FreeHand file to a bitmap format such as TIFF or GIF these jagged edges become an issue.To help eliminate these jaggy edges FreeHand uses anti-aliasing.

Try this
To see this effect, draw a black circle in FreeHand. Then export a tiff of the circle at 8-bit color depth, 72-dpi resolution and an anti-aliasing value of three. Then import the tiff back into FreeHand and place it next to the original square and circle. The effect is easier to see if you align the left edge of the TIFF with the right edge of the original circle. Use the magnifying glass to zoom in on the edges of the two images.If you examine the edges of the black FreeHand circle you'll see black diagonal pixels in a stair-step pattern. But if you look at the edges of the TIFF file you'll see that some of the pixels are black with intermediate gray pixels around the edges. This provides the smoother effect often seen in Web graphics and print material.