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The information in this article applies to:
SUMMARY
Animation that uses a "brute-force" animation scheme, displaying
successive complete bitmaps, can be slow and choppy. The alternative
approach, displaying an initial frame and then modifying the elements
that change, can save a great deal of time, memory, and disk space.
MORE INFORMATION
Animation applications typically use a very simple algorithm to
display a sequence of bitmaps, or frames, one right over another, on
the same display surface. When the application stores an entire frame
in the device-independent bitmap (DIB) format, it suffers from three
particular bottlenecks:
- With large frames or long sequences, the storage file becomes quite
large.
- Loading an entire sequence requires a large amount of memory.
- Available Windows functions, such as BitBlt or SetDIBitsToDevice,
may be too slow to animate the sequence smoothly.
For an animation sequence of bitmaps with a consistent set of colors,
preprocessing the animation sequence and storing it in the run-length-
encoded (RLE) format enables an application to run faster with a
smaller storage file and memory requirement. Note that this RLE file
is different from an RLE compressed DIB. This RLE format consists of
several frames that are compressed according to the differences
between frames. This process is straightforward and consists of five
steps:
- Select the first frame in the sequence and store it in the DIB
format.
- Compare the DIB bitmaps of consecutive frames. Due to the nature of
animation sequences, the number of differences is often quite small
compared to the size of the entire frame.
- Encode the set of changed pixels into RLE format. The encoded frame
will contain information only on the pixels that change, the delta
records will skip the unchanged pixels. The RLE bitmap is stored
instead of the latter frame. For example, the RLE-encoded
difference between the first and second frames is stored as the
second frame.
- When the entire sequence is preprocessed, bring each frame into the
system as a BITMAPINFO structure and stream of bits. Since only the
first frame is in the DIB format, the memory requirement is quite
low. Moreover, frames that contain an identical set of colors can
share the BITMAPINFO structure, only the biCompression and
biSizeImage fields must be changed.
- At display time, load the first frame as a DIB. Then use the
SetDIBitsToDevice function to display subsequent frames in the
sequence. Because this function requires much less information,
the sequence can be animated much more quickly and smoothly.
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