Resolution and Color

As Figure 23.2 shows, the Convert to Bitmap command offers several options, and the two key questions are resolution and color depth. Here, the ampersand is being converted into a 150dpi bitmap, meaning that each “screen dot” will be 1/150 of an inch tall and wide. This is in contrast to the “printer dot,” which is dependent upon the actual output device (1/300 for a standard laser printer, 1/600 for a modern laser printer, and 1/1200 and smaller for film-making equipment). This topic is a constant source of confusion for all but the more advanced users of graphics software. Suffice it to say that if an object is going to be rendered only in terms of dots, the software needs to know what size to make those dots. The larger each dot, the less opportunity there is to render detail; the smaller the dot, the more opportunity and the larger the file size.

The second question is the number of colors that will be used to render the object, and in the case of this rainbow-filled character, it is the more urgent one. Theoretically, millions of colors would be needed to traverse from the dark northern regions of this character to the lighter shades in the south. If converted to a 24-bit (or 16-million color) image, most human eyes would not be able to tell the difference between the original and the bitmap. But as soon as you drop down to 256 colors, you are asking DRAW to render this pattern with fewer colors than it needs to do the job correctly. And if you were to convert this to a 16-color bitmap, the results would be more hilarious than useful.

Dithering

If you intend to convert a full-color object into a bitmap with 256 colors or fewer, DRAW gives you the opportunity to cheat. In the world of color, cheating is referred to as dithering. It describes the process of approximating colors that don’t exist in the current palette. In the case of the ampersand, you would need more colors than 256 to smoothly depict the color shifts that take place. If you tried to render it at 256, the result would be a blunt transition between colors that would not do the image justice. Dithering is kind of like airbrushing—it glosses over the colors at their edges so that your eye is fooled into seeing a smoother transition. It is a mere illusion, and a close look will show how dithering is nothing more than a clever scrambling of dots.

Figure 23.3 illustrates this with two conversions of our ampersand to 16-color bitmaps. The same amount of colors are present in each, but by infusing the lower one with white pixels and interspersing the other 16 colors, the illusion exists of more colors present. The magnified portion shows how the trick is performed, with lighter and darker colored pixels sprinkled about.

FIGURE 23.3  When bitmaps do not contain enough colors to accurately portray an object, DRAW can resort to dithering, the computer equivalent of airbrushing.

We’re not trying to imply that either of these images looks very good—imposing a 16-color palette on this nice fountain is a misdemeanor at least. Dithering is a last resort, needed only when you are stuck with palettes well below 256 colors, which is, hopefully, very infrequent.

Backgrounds

The other option in the Color section determines how the background is to be handled. Normally, when DRAW converts an object to a bitmap, it defines a rectangular area (a “bounding box”) for that shape and treats that area as the object. As Figure 23.4 shows, this can create undesirable results. The top ampersand has been converted to a bitmap with a visible background. The lower one was converted with the Transparent Background option checked, producing much better results. We suspect that you will want your background transparent more often than not, and recommend turning this option on as your default. Once you check it, it remains checked until you say otherwise.

FIGURE 23.4  Transparent backgrounds are the ticket for getting good results with objects converted to bitmaps.

Anti-aliasing

A second cousin to dithering, the anti-aliasing process seeks to remove jagged edges from bitmaps, and is most noticeable at lower resolutions. Anti-aliasing is covered at length throughout Part V, “Drawing for Cyberspace.”

From Bitmap to Bitmap

Vector objects aren’t the only things that can be converted to bitmaps from within DRAW. You can also convert bitmaps to bitmaps. In other words, you can take an existing bitmap and change it into another kind of bitmap.

We found this to be extremely handy throughout the production of this book, as we took many pictures of the DRAW screen. Our video adapters are set for 24-bit color or higher, but the pages of this book are all black, white, and gray. So once we captured an image and pasted it into a drawing, we quickly converted it from 24-bit color to 8-bit grayscale. We estimate that this procedure saved over 500MB of storage space, without sacrificing quality at all. Here are the menu choices that come alive when you select a bitmap image.

Edit Bitmap

While DRAW can now wield considerable control over your bitmaps, it cannot actually add, remove, or change their pixels. For that, you would need to return to PHOTO-PAINT or your preferred image-editing program. This menu choice loads PAINT and opens the bitmap image in it.

Crop Bitmap

This takes the idea of cropping one step further than DRAW normally does. So for the sake of this discussion, there is cropping and there is CROPPING—the latter making the operation permanent, like the Trim command. As you may already know, you can crop a bitmap by using the Shape tool, but the area made invisible is still there, and you can bring it back by performing the reverse procedure with the Shape tool.

Once you crop a bitmap, you can CROP a bitmap by going to Bitmap Ø Crop Bitmap. It chops away the unseen parts, leaving you with a smaller bitmap.