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James Gregory
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35-07.md
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35-07.md
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@ -4,7 +4,7 @@
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**LISTING 35.3 L35-3.ASM**
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; Fast assembler implementation of Bresenham’s line-drawing algorithm
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; Fast assembler implementation of Bresenham's line-drawing algorithm
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; for the EGA and VGA. Works in modes 0Eh, 0Fh, 10h, and 12h.
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; Borland C++ near-callable.
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; Bit mask accumulation technique when |DeltaX| >= |DeltaY|
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@ -74,7 +74,7 @@
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local MoveToNextByte, ResetBitMaskAccumulator
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mov cx,bx ;# of pixels in line
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jcxz Line1End ;done if there are no more pixels
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; (there’s always at least the one pixel
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; (there's always at least the one pixel
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; at the start location)
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shl si,1 ;DeltaY * 2
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mov bp,si ;error term
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@ -87,7 +87,7 @@
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; for the initial pixel
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LineLoop:
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;
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; See if it’s time to advance the Y coordinate yet.
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; See if it's time to advance the Y coordinate yet.
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;
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and bp,bp ;see if error term is negative
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js MoveXCoord ;yes, stay at the same Y coordinate
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@ -101,7 +101,7 @@
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;load latches and write pixels, with bit mask
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; preserving other latched bits. Because
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; set/reset is enabled for all planes, the
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; value written actually doesn’t matter
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; value written actually doesn't matter
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add di,EVGA_SCREEN_WIDTH_IN_BYTES ;increment Y coordinate
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add bp,si ;adjust error term back down
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;
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@ -113,7 +113,7 @@
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else
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ror ah,1 ;move pixel mask 1 pixel to the right
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endif
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jnc ResetBitMaskAccumulator ;didn’t wrap to next byte
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jnc ResetBitMaskAccumulator ;didn't wrap to next byte
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jmp short MoveToNextByte ;did wrap to next byte
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;
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; Move pixel mask one pixel (either right or left, depending
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@ -134,7 +134,7 @@
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;load latches and write pixels, with bit mask
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; preserving other latched bits. Because
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; set/reset is enabled for all planes, the
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; value written actually doesn’t matter
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; value written actually doesn't matter
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MoveToNextByte:
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if MOVE_LEFT
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dec di ;next pixel is in byte to left
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@ -156,7 +156,7 @@
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;load latches and write pixels, with bit mask
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; preserving other latched bits. Because
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; set/reset is enabled for all planes, the
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; value written actually doesn’t matter
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; value written actually doesn't matter
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endm
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;
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@ -190,10 +190,10 @@
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;load latches and write pixel, with bit mask
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; preserving other latched bits. Because
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; set/reset is enabled for all planes, the
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; value written actually doesn’t matter
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; value written actually doesn't matter
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LineLoop:
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;
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; See if it’s time to advance the X coordinate yet.
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; See if it's time to advance the X coordinate yet.
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;
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and bp,bp ;see if error term is negative
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jns ETermAction ;no, advance X coordinate
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@ -225,7 +225,7 @@
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;load latches and write pixel, with bit mask
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; preserving other latched bits. Because
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; set/reset is enabled for all planes, the
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; value written actually doesn’t matter
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; value written actually doesn't matter
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;
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loop LineLoop
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Line2End:
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@ -270,9 +270,9 @@
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mov ax,[bp+Y0] ;line Y end, used later in
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;calculating the start address
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sub si,ax ;calculate DeltaY
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jns CalcStartAddress ;if positive, we’re set
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jns CalcStartAddress ;if positive, we're set
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;
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; DeltaY is negative — swap coordinates so we’re always working
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; DeltaY is negative — swap coordinates so we're always working
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; with a positive DeltaY.
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;
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mov ax,[bp+Y1] ;set line start to Y1, for use
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@ -385,7 +385,7 @@ which is well-commented, should speak for itself.
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One point I do want to make is that Listing 35.3 incorporates a clever
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notion for which credit is due Jim Mackraz, who described the notion in
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a letter written in response to an article I wrote long ago in the late
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and lamented *Programmer’s Journal*. Jim’s suggestion was that when
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and lamented *Programmer's Journal*. Jim's suggestion was that when
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drawing lines for which |**DeltaX**| is greater than |**DeltaY**|, bits
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set to 1 for each of the pixels controlled by a given byte can be
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accumulated in a register, rather than drawing each pixel individually.
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@ -393,16 +393,16 @@ All the pixels controlled by that byte can then be drawn at once, with a
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single access to display memory, when all pixel processing associated
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with that byte has been completed. This approach can save many **OUT**s
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and many display memory reads and writes when drawing nearly-horizontal
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lines, and that’s important because EGAs and VGAs hold the CPU up for a
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lines, and that's important because EGAs and VGAs hold the CPU up for a
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considerable period of time on each I/O operation and display memory
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access.
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All too many PC programmers fall into the high-level-language trap of
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thinking that a good algorithm guarantees good performance. Not so: As
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our two implementations of Bresenham’s algorithm graphically illustrate
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our two implementations of Bresenham's algorithm graphically illustrate
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(pun not originally intended, but allowed to stand once recognized),
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truly great PC code requires both a good algorithm *and* a good assembly
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implementation. In Listing 35.3, we’ve got y-oh-my, isn’t it fun?
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implementation. In Listing 35.3, we've got y-oh-my, isn't it fun?
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------------------------ --------------------------------- --------------------
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[Previous](35-06.html) [Table of Contents](index.html) [Next](36-01.html)
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