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 dont 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 airbrushingit
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.
Were not trying to imply that either of these images looks very
goodimposing 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 arent 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 CROPPINGthe
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.
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