A.D.A. Amiga Demoscene Archive

I was reading about Amiga hardware in Wikipedia the other day and noticed this line about the Blitters line drawing hardware:-

"The line mode can also be used to rotate bobs: the image data of the bob is used as line pattern while the algorithm scans the bob position line by line."

The blitter could draw lines at 1 million pixels a second with 1 bitplane and a pattern was only 16 pixels (1 register). Surley this algorithm isn't to usefull ? Are there any well known implementations of this in demos ? I don't think I've ever seen any.

When Chaos first said his 3d rotator in Roots had a limitation of 16 pixels my intial thought was that he'd use pattend blitter lines sowehow but it was always fairly obvious it would be far to slow.

http://en.wikipedia.org/wiki/Original_Amiga_chipse t

Another problem you would face if you were to rotate bobs by drawing blitter-lines, is that it will be difficult to hit every pixel in the final image.

This is why many image processing operations (such as rotation) walk linearly through the output dataset, and determine, for each output pixel: "what is the final colour of this pixel?"

(Whereas the blitter-line method would be more akin to going linearly through the input dataset, and determining, for each pixel: "what effect does this pixel have on the output image?")

Another problem would be that the blitter counts in pixels not length units. If you for instance wanted to render the top line of your bob in 45 degrees on screen, that rendered line would have a length of 16 pixels, but that's around 23 l.u. because it's tilted.

Thus, the output graphics will vary in size along with the rotation... you'd have to compensate using several scaled blobs.

The result would not be pretty.