EGG from Osmosys and STMicroelectronics graphics library for the blitter can both draw rotated images. Specifically images rotated about the Y axis and the X axis (not Z). This results in trapezoidal shapes like those below:
These trapezoids can be useful for user interfaces such as in the example below:
However, a blitter, on which they both rely, can only copy a block of memory and can only draw a scaled, flat, square image (not quite true but for simplification I will assume it). How is it possible to get 3D effects from a blitter? The trick is simple: use more than one blit to draw the angled surface and vary the scaling. For example in the case of a surface rotated about the Y-axis (as above), a number of thin vertical blits can be used to draw the resulting image. The scaling power of the blitter is used to simulate the decrease in size of the image as it gets further away. Thus a series of blits can be used to draw a single off angle surface in perspective.
The blitter on modern chips however is very efficient at drawing pixels but quite slow at setting up individual blits. The key then to performance is to reduce the number of the blits needed to draw such a surface.
So how many blits are required? The obvious (and patented technique at the time we created EGG) was one blit per line of the resulting image. This means drawing lots of one line wide blits. The scaling is calculated for each line and passed to the blitter. The drawing below illustrates this:
Incidentally, the part of the source image that fits into this destination line must also be calculated (to maintain correct perspective). However, the bottleneck is setting up the blitter and issuing a blit command. It should be obvious that reducing the number of blits is critical.
Osmosys have patented a more efficient technique, which is now public so I can blog about it.
Instead of blitting every verticla line with a new scaling factor, the technique idetifies the maximum size block of pixels with the same scaling factor that can be rendered using a single blit. In other words, the technique finds a series of rectangles in the destination image, often wider than one line. A diagram helps to explain this:
In the example above, the Osmosys technique uses 9 blits to draw the trapezoid, whereas the original technique uses 26. Thats 3x more efficient in this case. At smaller angles and big images the technique really wins as only a few blits are needed even for a large trapezoid, compared with many hundreds for the traditional technique. The worst case is the same with respect to the number of blits. Therefore the technique is of the order of a magnitude faster in the general case.
There are further optimisations possible but I'm not free to talk about those.
Of course, rotating about the X axis is similar but the blits are drawn horizontally, not vertically.
The technique could be taken further to draw triangles and allow true 3D shapes in their libraries or to rotate images about the Z axis. However, the number of blits could be very large and therefore probably not worth the effort. Its still remarkable however that a blitter, when used correctly can draw many 3D surfaces in HD resolution at frame rates of 20fps or more.
The biggest problem with the technique is aliasing. The edge of the image is very clear and presents so-called "jaggies". At IBC this year Sagem used the ST library to have a 3D interface. To cover up the jaggies, the background was a very strange grey-black grid of lines - highly unsuited to the TV screen but a clever way to hide the problems.
Incidentally, even though the technique developed at Osmosys is I believe optimal, the patent appears to be badly written (ie not by a graphics expert) and thus easy to work around but then again, I'm no lawyer.
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