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@ -12,24 +12,24 @@ As I write this, the wife, the kid, and I are in the throes of yet
another lightning-quick transcontinental move, this time to Redmond,
Washington, to work for You Know Who. Moving is never fun, but what
makes it worse for us is the pets. Getting them into kennels and to the
airport is hard; there’s always the possibility that they might not be
airport is hard; there's always the possibility that they might not be
allowed to fly because of the weather; and, worst of all, they might not
make it. Animals don’t usually end up injured or dead, but it does
make it. Animals don't usually end up injured or dead, but it does
happen.
In a (not notably successful) effort to cheer me up about the prospect
of shipping my animals, a friend told me the following story, which he
swears actually happened to a friend of his. I don’t know—to me, it has
swears actually happened to a friend of his. I don't know—to me, it has
the ring of an urban legend, which is to say it makes a good story, but
you can never track down the person it really happened to; it’s always a
friend of a friend. But maybe it is true, and anyway, it’s a good story.
you can never track down the person it really happened to; it's always a
friend of a friend. But maybe it is true, and anyway, it's a good story.
This friend of a friend (henceforth referred to as FOF), worked in an
air-freight terminal. Consequently, he handled a lot of animals, which
was fine by him, because he liked animals; in fact, he had quite a few
cats at home. You can imagine his dismay when, one day, he took a kennel
off the plane to find that the cat it carried was quite thoroughly dead.
(No, it wasn’t resting, nor pining for the fjords; this cat was bloody
(No, it wasn't resting, nor pining for the fjords; this cat was bloody
*deceased*.)
FOF knew how upset the owner would be, and came up with a plan to make
@ -38,29 +38,29 @@ markings. He would substitute that cat, and since all cats treat all
humans with equal disdain, the owner would never know the difference,
and would never suffer the trauma of the loss of her cat. So FOF drove
home, got his cat, put it in the kennel, and waited for the owner to
show up—at which point, she took one look at the kennel and said, “This
isn’t my cat. My cat is dead.”
show up—at which point, she took one look at the kennel and said, "This
isn't my cat. My cat is dead."
As it turned out, she had shipped her recently deceased feline home to
be buried. History does not record how our FOF dug himself out of this
one.
Okay, but what’s the point? The point is, if it isn’t broken, don’t fix
it. And if it is broken, maybe that’s all right, too. Which brings us,
Okay, but what's the point? The point is, if it isn't broken, don't fix
it. And if it is broken, maybe that's all right, too. Which brings us,
neat as a pin, to the topic of drawing lines in a serious hurry.
### Fast Run-Length Slice Line Drawing {#Heading3}
In the last chapter, we examined the principles of run-length slice line
drawing, which draws lines a run at a time rather than a pixel at a
time, a run being a series of pixels along the major (longer) axis. It’s
time, a run being a series of pixels along the major (longer) axis. It's
time to turn theory into useful practice by developing a fast assembly
version. Listing 37.1 is the assembly version, in a form that’s
version. Listing 37.1 is the assembly version, in a form that's
plug-compatible with the C code from the previous chapter.
**LISTING 37.1 L37-1.ASM**
; Fast run-length slice line drawing implementation for mode 0x13, the VGA’s
; Fast run-length slice line drawing implementation for mode 0x13, the VGA's
; 320x200 256-color mode.
; Draws a line between the specified endpoints in color Color.
; C near-callable as:
@ -93,13 +93,13 @@ plug-compatible with the C code from the previous chapter.
public _LineDraw
_LineDraw proc near
cld
push bp ;preserve caller’s stack frame
push bp ;preserve caller's stack frame
mov bp,sp ;point to our stack frame
sub sp, LOCAL_SIZE ;allocate space for local variables
push si ;preserve C register variables
push di
push ds ;preserve caller’s DS
; We’ll draw top to bottom, to reduce the number of cases we have to handle,
push ds ;preserve caller's DS
; We'll draw top to bottom, to reduce the number of cases we have to handle,
; and to make lines between the same endpoints always draw the same pixels.
mov ax,[bp].YStart
cmp ax,[bp].YEnd
@ -117,10 +117,10 @@ plug-compatible with the C code from the previous chapter.
mov di,si
add di,ax ;DI = YStart * SCREEN_WIDTH + XStart
; = offset of initial pixel
; Figure out how far we’re going vertically (guaranteed to be positive).
; Figure out how far we're going vertically (guaranteed to be positive).
mov cx,[bp].YEnd
sub cx,[bp].YStart ;CX = YDelta
; Figure out whether we’re going left or right, and how far we’re going
; Figure out whether we're going left or right, and how far we're going
; horizontally. In the process, special-case vertical lines, for speed and
; to avoid nasty boundary conditions and division by 0.
mov dx,[bp].XEnd
@ -229,11 +229,11 @@ plug-compatible with the C code from the previous chapter.
; (may be adjusted later). This is also the
; final run pixel count
push cx ;remember final run pixel count for later
; If the basic run length is even and there’s no fractional advance, we have
; If the basic run length is even and there's no fractional advance, we have
; one pixel that could go to either the initial or last partial run, which
; we’ll arbitrarily allocate to the last run.
; If there is an odd number of pixels per run, we have one pixel that can’t
; be allocated to either the initial or last partial run, so we’ll add 0.5 to
; we'll arbitrarily allocate to the last run.
; If there is an odd number of pixels per run, we have one pixel that can't
; be allocated to either the initial or last partial run, so we'll add 0.5 to
; the error term so this pixel will be handled by the normal full-run loop.
add dx,si ;assume odd length, add YDelta to error term
; (add 0.5 of a pixel to the error term)
@ -241,7 +241,7 @@ plug-compatible with the C code from the previous chapter.
jnz XMajorAdjustDone ;no, already did work for odd case, all set
sub dx,si ;length is even, undo odd stuff we just did
and bx,bx ;is the adjust up equal to 0?
jnz XMajorAdjustDone ;no (don’t need to check for odd length,
jnz XMajorAdjustDone ;no (don't need to check for odd length,
; because of the above test)
dec cx ;both conditions met; make initial run 1
; shorter
@ -267,7 +267,7 @@ plug-compatible with the C code from the previous chapter.
inc cx ;one extra pixel in run
sub dx,[bp].AdjDown ;reset the error term
XMajorNoExtra:
rep stosb ;draw this scan line’s run
rep stosb ;draw this scan line's run
add di,SCREEN_WIDTH ;advance along the minor axis (Y)
XMajorFullRunsOddEntry: ;enter loop here if there is an odd number
; of full runs
@ -277,7 +277,7 @@ plug-compatible with the C code from the previous chapter.
inc cx ;one extra pixel in run
sub dx,[bp].AdjDown ;reset the error term
XMajorNoExtra2:
rep stosb ;draw this scan line’s run
rep stosb ;draw this scan line's run
add di,SCREEN_WIDTH ;advance along the minor axis (Y)
dec si
@ -323,18 +323,18 @@ plug-compatible with the C code from the previous chapter.
; (may be adjusted later)
push cx ;remember final run pixel count for later
; If the basic run length is even and there’s no fractional advance, we have
; If the basic run length is even and there's no fractional advance, we have
; one pixel that could go to either the initial or last partial run, which
; we’ll arbitrarily allocate to the last run.
; If there is an odd number of pixels per run, we have one pixel that can’t
; be allocated to either the initial or last partial run, so we’ll add 0.5 to
; we'll arbitrarily allocate to the last run.
; If there is an odd number of pixels per run, we have one pixel that can't
; be allocated to either the initial or last partial run, so we'll add 0.5 to
; the error term so this pixel will be handled by the normal full-run loop.
add dx,si ;assume odd length, add XDelta to error term
test al,1 ;is run length even?
jnz YMajorAdjustDone ;no, already did work for odd case, all set
sub dx,si ;length is even, undo odd stuff we just did
and bx,bx ;is the adjust up equal to 0?
jnz YMajorAdjustDone ;no (don’t need to check for odd length,
jnz YMajorAdjustDone ;no (don't need to check for odd length,
; because of the above test)
dec cx ;both conditions met; make initial run 1
; shorter
@ -400,11 +400,11 @@ plug-compatible with the C code from the previous chapter.
dec cx
jnz YMajorLastLoop
Done:
pop ds ;restore caller’s DS
pop ds ;restore caller's DS
pop di
pop si ;restore C register variables
mov sp,bp ;deallocate local variables
pop bp ;restore caller’s stack frame
pop bp ;restore caller's stack frame
ret
_LineDraw endp
end