wu'ed in haste; fried, stewed at leisure

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mov cx.8sub cx.Cbp1.IntensityBitsmov ch.byte ptr [bpl.NumLevelsdec chmov bp,BaseColor[bplxchg bp.ax: Draw all remaining pixelsXMajorLoop:add dx,bpjnc NoYAdvanceadd si.SCREEN_WIDTH-IN-BYTESNoYAdvance:add si .bx;CL - I/ of bits by which to shift: ErrorAcc to get intensity level (8: instead of 16 because we work only: with the high byte of ErrorAcc);mask used to flip all bits in an: intensity weighting, producing: result (1 - intensity weighting):***stack frame not available***:***fromnow on***:BP - ErrorAdj. AL - BaseColor.: AH - scratch register:calculate error for next pixel;not time to step in Y yet:the error accumulator turned over,: so advance the Y coord:advance Y:X-major. so add XDir to the pixel pointer: The Intensityeits most significant bits of ErrorAcc give us the intensity: weighting for this pixel, and the complement of the weighting for the: paired pixel.mov ah.dh :msb of ErrorAccshr ah.cl :Weighting - ErrorAcc >> IntensityShift:add ah,al :Basecolor + Weightingmov Csi1,ah :DrawPixel(X. Y. BaseColor + Weighting):mov ah.dh :msb of ErrorAccshr ah.cl :Weighting - ErrorAcc >> IntensityShift:xor ah.ch :Weighting A WeightingComplementMaskadd ah.al :Basecolor + (Weighting A WeightingComplementMask)mov [si+SCREEN-WIDTH-IN-BYTES].ah:DrawPixel(X. Y+SCREEN-WIDTH-IN-BYTES,: BaseColor + (Weighting A WeightingComplementMask)):dec di :--0eltaXjnz XMajorLoopDone:POP dspop dipop siPOP bPret-0rawWuLine endpend;we’re done with this line;restore C’s default data segment:restore C’s register variables:restore caller’s stack frame;doneNotes on Wu AntialiasingWu antialiasing can be applied to any curve for which it’s possible to calculate ateach step the positions and intensities of two bracketing pixels, although the implementationwill generally be nowhere near as efficient as it is for lines. However, Wu’sarticle in Computer Ofaphicsdoes describe an efficient algorithm for drawing antialiasedcircles. Wu also describes a technique for antialiasing solids, such as filled circles andpolygons. Wu’s approach biases the edges of filled objects outward. Although this isno good for adjacent polygons of the sort used in rendering, it’s certainly possible toWu‘ed in Haste; Fried, Stewed at Leisure 791
design a more accurate polygon-antialiasing approach around Wu’s basic weightingtechnique. The results would not be quite so good as more sophisticated antialiasingtechniques, but they would be much faster.In general, the results obtained by Wu antialiasing are only so-so, by theoreticalmeasures, Wu antialiasing amounts to a simple boxfilterplaced over a fuced-pointstep approximation of a line, and that process introduces a good deal of deviationfrom the ideal. On the other hand, Wu notes that even a IOpercent error in intensitydoesn ’t lead to noticeable loss of image quality, and for Wu-antialiased linesup to IKpixels in length, the error is under 1 Opercent. Ifit looks good, it is goodandit looks good.With a l6bit error accumulator, fixed-point inaccuracy becomes a problem for Wuantialiasedlines longer than 1K. For such lines, you should switch to using 32-biterror values, which would let you handle lines of any practical length.In the listings, I have chosen to truncate, rather than round, the error-adjust value.This increases the intensity error of the line but guarantees that fixed-point inaccuracywon’t cause the minor axis to advance past the endpoint. Overrunning theendpoint would result in the drawing of pixels outside the line’s bounding box, andpotentially even in an attempt to access pixels off the edge of the bitmap.Finally, I should mention that, as published, Wu’s algorithm draws lines symmetrically,from both ends at once. I haven’t done this for a number of reasons, not leastof which is that symmetric drawing is an inefficient way to draw lines that span bankson banked Super-VGAs. Banking aside, however, symmetric drawing is potentiallyfaster, because it eliminates half of all calculations; in so doing, it cuts cumulativeerror in half, as well.With or without symmetrical processing, Wu antialiasing beats fried, stewed chickenhands-down. Trust me on this one.792 Chapter 42
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wu'ed in haste; fried, stewed at leisure

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