43 KiB
| title | author | date | identifier | publisher | category | chapter | pages | |||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Michael Abrash's Graphics Programming Black Book, Special Edition | Michael Abrash | 1997-07-01 |
|
The Coriolis Group | Web and Software Development: Game Development,Web and Software Development: Graphics and Multimedia Development | 49 | 913-930 |
Chapter 49 -- Mode X 256-Color Animation
How to Make the VGA Really Get up and Dance
Okay—no amusing stories or informative anecdotes to kick off this chapter; lotta ground to cover, gotta hurry—you're impatient, I can smell it. I won't talk about the time a friend made the mistake of loudly saying "$100 bill" during an animated discussion while walking among the bums on Market Street in San Francisco one night, thereby graphically illustrating that context is everything. I can't spare a word about how my daughter thinks my 11-year-old floppy-disk-based CP/M machine is more powerful than my 386 with its 100-MB hard disk because the CP/M machine's word processor loads and runs twice as fast as the 386's Windows-based word processor, demonstrating that progress is not the neat exponential curve we'd like to think it is, and that features and performance are often conflicting notions. And, lord knows, I can't take the time to discuss the habits of small white dogs, notwithstanding that such dogs seem to be relevant to just about every aspect of computing, as Jeff Duntemann's writings make manifest. No lighthearted fluff for us; we have real work to do, for today we animate with 256 colors in Mode X.
Masked Copying
Over the past two chapters, we've put together most of the tools needed to implement animation in the VGA's undocumented 320x240 256-color Mode X. We now have mode set code, solid and 4x4 pattern fills, system memory-to-display memory block copies, and display memory-to-display memory block copies. The final piece of the puzzle is the ability to copy a nonrectangular image to display memory. I call this masked copying.
Masked copying is sort of like drawing through a stencil, in that only certain pixels within the destination rectangle are drawn. The objective is to fit the image seamlessly into the background, without the rectangular fringe that results when nonrectangular images are drawn by block copying their bounding rectangle. This is accomplished by using a second rectangular bitmap, separate from the image but corresponding to it on a pixel-by-pixel basis, to control which destination pixels are set from the source and which are left unchanged. With a masked copy, only those pixels properly belonging to an image are drawn, and the image fits perfectly into the background, with no rectangular border. In fact, masked copying even makes it possible to have transparent areas within images.
Note that another way to achieve this effect is to implement copying code that supports a transparent color; that is, a color that doesn't get copied but rather leaves the destination unchanged. Transparent copying makes for more compact images, because no separate mask is needed, and is generally faster in a software-only implementation. However, Mode X supports masked copying but not transparent copying in hardware, so we'll use masked copying in this chapter.
The system memory to display memory masked copy routine in Listing 49.1 implements masked copying in a straightforward fashion. In the main drawing loop, the corresponding mask byte is consulted as each image pixel is encountered, and the image pixel is copied only if the mask byte is nonzero. As with most of the system-to-display code I've presented, Listing 49.1 is not heavily optimized, because it's inherently slow; there's a better way to go when performance matters, and that's to use the VGA's hardware.
LISTING 49.1 L49-1.ASM
; Mode X (320x240, 256 colors) system memory-to-display memory masked copy
; routine. Not particularly fast; images for which performance is critical
; should be stored in off-screen memory and copied to screen via latches. Works
; on all VGAs. Copies up to but not including column at SourceEndX and row at
; SourceEndY. No clipping is performed. Mask and source image are both byte-
; per-pixel, and must be of same widths and reside at same coordinates in their
; respective bitmaps. Assembly code tested with TASM C near-callable as:
;
; void CopySystemToScreenMaskedX(int SourceStartX,
; int SourceStartY, int SourceEndX, int SourceEndY,
; int DestStartX, int DestStartY, char * SourcePtr,
; unsigned int DestPageBase, int SourceBitmapWidth,
; int DestBitmapWidth, char * MaskPtr);
SC_INDEX equ 03c4h ;Sequence Controller Index register port
MAP_MASK equ 02h ;index in SC of Map Mask register
SCREEN_SEG equ 0a000h ;segment of display memory in mode X
parms struc
dw 2 dup (?) ;pushed BP and return address
SourceStartX dw ? ;X coordinate of upper left corner of source
; (source is in system memory)
SourceStartY dw ? ;Y coordinate of upper left corner of source
SourceEndX dw ? ;X coordinate of lower right corner of source
; (the column at EndX is not copied)
SourceEndY dw ? ;Y coordinate of lower right corner of source
; (the row at EndY is not copied)
DestStartX dw ? ;X coordinate of upper left corner of dest
; (destination is in display memory)
DestStartY dw ? ;Y coordinate of upper left corner of dest
SourcePtr dw ? ;pointer in DS to start of bitmap which source resides
DestPageBase dw ? ;base offset in display memory of page in
; which dest resides
SourceBitmapWidth dw ? ;# of pixels across source bitmap (also must
; be width across the mask)
DestBitmapWidth dw ? ;# of pixels across dest bitmap (must be multiple of 4)
MaskPtr dw ? ;pointer in DS to start of bitmap in which mask
; resides (byte-per-pixel format, just like the source
; image; 0-bytes mean don't copy corresponding source
; pixel, 1-bytes mean do copy)
parms ends
RectWidth equ -2 ;local storage for width of rectangle
RectHeight equ -4 ;local storage for height of rectangle
LeftMask equ -6 ;local storage for left rect edge plane mask
STACK_FRAME_SIZE equ 6
.model small
.code
public _CopySystemToScreenMaskedX
_CopySystemToScreenMaskedX proc near
push bp ;preserve caller's stack frame
mov bp,sp ;point to local stack frame
sub sp,STACK_FRAME_SIZE ;allocate space for local vars
push si ;preserve caller's register variables
push di
mov ax,SCREEN_SEG ;point ES to display memory
mov es,ax
mov ax,[bp+SourceBitmapWidth]
mul [bp+SourceStartY] ;top source rect scan line
add ax,[bp+SourceStartX]
mov bx,ax
add ax,[bp+SourcePtr] ;offset of first source rect pixel
mov si,ax ; in DS
add bx,[bp+MaskPtr] ;offset of first mask pixel in DS
mov ax,[bp+DestBitmapWidth]
shr ax,1 ;convert to width in addresses
shr ax,1
mov [bp+DestBitmapWidth],ax ;remember address width
mul [bp+DestStartY] ;top dest rect scan line
mov di,[bp+DestStartX]
mov cx,di
shr di,1 ;X/4 = offset of first dest rect pixel in
shr di,1 ; scan line
add di,ax ;offset of first dest rect pixel in page
add di,[bp+DestPageBase] ;offset of first dest rect pixel
; in display memory
and cl,011b ;CL = first dest pixel's plane
mov al,11h ;upper nibble comes into play when plane wraps
; from 3 back to 0
shl al,cl ;set the bit for the first dest pixel's plane
mov [bp+LeftMask],al ; in each nibble to 1
mov ax,[bp+SourceEndX] ;calculate # of pixels across
sub ax,[bp+SourceStartX] ; rect
jle CopyDone ;skip if 0 or negative width
mov [bp+RectWidth],ax
sub word ptr [bp+SourceBitmapWidth],ax
;distance from end of one source scan line
to start of next
mov ax,[bp+SourceEndY]
sub ax,[bp+SourceStartY] ;height of rectangle
jle CopyDone ;skip if 0 or negative height
mov [bp+RectHeight],ax
mov dx,SC_INDEX ;point to SC Index register
mov al,MAP_MASK
out dx,al ;point SC Index reg to the Map Mask
inc dx ;point DX to SC Data reg
CopyRowsLoop:
mov al,[bp+LeftMask]
mov cx,[bp+RectWidth]
push di ;remember the start offset in the dest
CopyScanLineLoop:
cmp byte ptr [bx],0 ;is this pixel mask-enabled?
jz MaskOff ;no, so don't draw it
;yes, draw the pixel
out dx,al ;set the plane for this pixel
mov ah,[si] ;get the pixel from the source
mov es:[di],ah ;copy the pixel to the screen
MaskOff:
inc bx ;advance the mask pointer
inc si ;advance the source pointer
rol al,1 ;set mask for next pixel's plane
adc di,0 ;advance destination address only when
;wrapping from plane 3 to plane 0
loop CopyScanLineLoop
pop di ;retrieve the dest start offset
add di,[bp+DestBitmapWidth];point to the start of the
; next scan line of the dest
add si,[bp+SourceBitmapWidth] ;point to the start of the
;next scan line of the source
add bx,[bp+SourceBitmapWidth] ;point to the start of the
;next scan line of the mask
dec word ptr [bp+RectHeight] ;count down scan lines
jnz CopyRowsLoop
CopyDone:
pop di ;restore caller's register variables
pop si
mov sp,bp ;discard storage for local variables
pop bp ;restore caller's stack frame
ret
_CopySystemToScreenMaskedX endp
end
Faster Masked Copying
In the previous chapter we saw how the VGA's latches can be used to copy four pixels at a time from one area of display memory to another in Mode X. We've further seen that in Mode X the Map Mask register can be used to select which planes are copied. That's all we need to know to be able to perform fast masked copies; we can store an image in off-screen display memory, and set the Map Mask to the appropriate mask value as up to four pixels at a time are copied.
There's a slight hitch, though. The latches can only be used when the source and destination left edge coordinates, modulo four, are the same, as explained in the previous chapter. The solution is to copy all four possible alignments of each image to display memory, each properly positioned for one of the four possible destination-left-edge-modulo-four cases. These aligned images must be accompanied by the four possible alignments of the image mask, stored in system memory. Given all four image and mask alignments, masked copying is a simple matter of selecting the alignment that's appropriate for the destination's left edge, then setting the Map Mask with the 4-bit mask corresponding to each four-pixel set as we copy four pixels at a time via the latches.
Listing 49.2 performs fast masked copying. This code expects to receive
a pointer to a MaskedImage structure, which in turn points to four
AlignedMaskedImage structures that describe the four possible image
and mask alignments. The aligned images are already stored in display
memory, and the aligned masks are already stored in system memory;
further, the masks are predigested into Map Mask register-compatible
form. Given all that ready-to-use data, Listing 49.2 selects and works
with the appropriate image-mask pair for the destination's left edge
alignment.
LISTING 49.2 L49-2.ASM
; Mode X (320x240, 256 colors) display memory to display memory masked copy
; routine. Works on all VGAs. Uses approach of reading 4 pixels at a time from
; source into latches, then writing latches to destination, using Map Mask
; register to perform masking. Copies up to but not including column at
; SourceEndX and row at SourceEndY. No clipping is performed. Results are not
; guaranteed if source and destination overlap. C near-callable as:
;
; void CopyScreenToScreenMaskedX(int SourceStartX,
; int SourceStartY, int SourceEndX, int SourceEndY,
; int DestStartX, int DestStartY, MaskedImage * Source,
; unsigned int DestPageBase, int DestBitmapWidth);
SC_INDEX equ 03c4h ;Sequence Controller Index register port
MAP_MASK equ 02h ;index in SC of Map Mask register
GC_INDEX equ 03ceh ;Graphics Controller Index register port
BIT_MASK equ 08h ;index in GC of Bit Mask register
SCREEN_SEG equ 0a000h ;segment of display memory in mode X
parms struc
dw 2 dup (?) ;pushed BP and return address
SourceStartX dw ? ;X coordinate of upper left corner of source
SourceStartY dw ? ;Y coordinate of upper left corner of source
SourceEndX dw ? ;X coordinate of lower right corner of source
; (the column at SourceEndX is not copied)
SourceEndY dw ? ;Y coordinate of lower right corner of source
; (the row at SourceEndY is not copied)
DestStartX dw ? ;X coordinate of upper left corner of dest
DestStartY dw ? ;Y coordinate of upper left corner of dest
Source dw ? ;pointer to MaskedImage struct for source
; which source resides
DestPageBase dw ? ;base offset in display memory of page in
; which dest resides
DestBitmapWidth dw ? ;# of pixels across dest bitmap (must be multiple of 4)
parms ends
SourceNextScanOffset equ -2 ;local storage for distance from end of
; one source scan line to start of next
DestNextScanOffset equ -4 ;local storage for distance from end of
; one dest scan line to start of next
RectAddrWidth equ -6 ;local storage for address width of rectangle
RectHeight equ -8 ;local storage for height of rectangle
SourceBitmapWidth equ -10 ;local storage for width of source bitmap
; (in addresses)
STACK_FRAME_SIZE equ 10
MaskedImage struc
Alignments dw 4 dup(?) ;pointers to AlignedMaskedImages for the
; 4 possible destination image alignments
MaskedImage ends
AlignedMaskedImage struc
ImageWidth dw ? ;image width in addresses (also mask width in bytes)
ImagePtr dw ? ;offset of image bitmap in display memory
MaskPtr dw ? ;pointer to mask bitmap in DS
AlignedMaskedImage ends
.model small
.code
public _CopyScreenToScreenMaskedX
_CopyScreenToScreenMaskedX proc near
push bp ;preserve caller's stack frame
mov bp,sp ;point to local stack frame
sub sp,STACK_FRAME_SIZE ;allocate space for local vars
push si ;preserve caller's register variables
push di
cld
mov dx,GC_INDEX ;set the bit mask to select all bits
mov ax,00000h+BIT_MASK ; from the latches and none from
out dx,ax ; the CPU, so that we can write the
; latch contents directly to memory
mov ax,SCREEN_SEG ;point ES to display memory
mov es,ax
mov ax,[bp+DestBitmapWidth]
shr ax,1 ;convert to width in addresses
shr ax,1
mul [bp+DestStartY] ;top dest rect scan line
mov di,[bp+DestStartX]
mov si,di
shr di,1 ;X/4 = offset of first dest rect pixel in
shr di,1 ; scan line
add di,ax ;offset of first dest rect pixel in page
add di,[bp+DestPageBase] ;offset of first dest rect pixel in display
; memory. now look up the image that's
; aligned to match left-edge alignment
; of destination
and si,3 ;DestStartX modulo 4
mov cx,si ;set aside alignment for later
shl si,1 ;prepare for word look-up
mov bx,[bp+Source] ;point to source MaskedImage structure
mov bx,[bx+Alignments+si] ;point to AlignedMaskedImage
; struc for current left edge alignment
mov ax,[bx+ImageWidth] ;image width in addresses
mov [bp+SourceBitmapWidth],ax ;remember image width in addresses
mul [bp+SourceStartY] ;top source rect scan line
mov si,[bp+SourceStartX]
shr si,1 ;X/4 = address of first source rect pixel in
shr si,1 ; scan line
add si,ax ;offset of first source rect pixel in image
mov ax,si
add si,[bx+MaskPtr] ;point to mask offset of first mask pixel in DS
mov bx,[bx+ImagePtr] ;offset of first source rect pixel
add bx,ax ; in display memory
mov ax,[bp+SourceStartX] ;calculate # of addresses across
add ax,cx ; rect, shifting if necessary to
add cx,[bp+SourceEndX] ; account for alignment
cmp cx,ax
jle CopyDone ;skip if 0 or negative width
add cx,3
and ax,not 011b
sub cx,ax
shr cx,1
shr cx,1 ;# of addresses across rectangle to copy
mov ax,[bp+SourceEndY]
sub ax,[bp+SourceStartY] ;AX = height of rectangle
jle CopyDone ;skip if 0 or negative height
mov [bp+RectHeight],ax
mov ax,[bp+DestBitmapWidth]
shr ax,1 ;convert to width in addresses
shr ax,1
sub ax,cx ;distance from end of one dest scan line to start of next
mov [bp+DestNextScanOffset],ax
mov ax,[bp+SourceBitmapWidth] ;width in addresses
sub ax,cx ;distance from end of source scan line to start of next
mov [bp+SourceNextScanOffset],ax
mov [bp+RectAddrWidth],cx ;remember width in addresses
mov dx,SC_INDEX
mov al,MAP_MASK
out dx,al ;point SC Index register to Map Mask
inc dx ;point to SC Data register
CopyRowsLoop:
mov cx,[bp+RectAddrWidth] ;width across
CopyScanLineLoop:
lodsb ;get the mask for this four-pixel set
; and advance the mask pointer
out dx,al ;set the mask
mov al,es:[bx] ;load the latches with four-pixel set from source
mov es:[di],al ;copy the four-pixel set to the dest
inc bx ;advance the source pointer
inc di ;advance the destination pointer
dec cx ;count off four-pixel sets
jnz CopyScanLineLoop
mov ax,[bp+SourceNextScanOffset]
add si,ax ;point to the start of
add bx,ax ; the next source, mask,
add di,[bp+DestNextScanOffset] ; and dest lines
dec word ptr [bp+RectHeight] ;count down scan lines
jnz CopyRowsLoop
CopyDone:
mov dx,GC_INDEX+1 ;restore the bit mask to its default,
mov al,0ffh ; which selects all bits from the CPU
out dx,al ; and none from the latches (the GC
; Index still points to Bit Mask)
pop di ;restore caller's register variables
pop si
mov sp,bp ;discard storage for local variables
pop bp ;restore caller's stack frame
ret
_CopyScreenToScreenMaskedX endp
end
It would be handy to have a function that, given a base image and mask,
generates the four image and mask alignments and fills in the
MaskedImage structure. Listing 49.3, together with the include file
in Listing 49.4 and the system memory-to-display memory block-copy
routine in Listing 48.4 (in the previous chapter) does just that. It
would be faster if Listing 49.3 were in assembly language, but there's
no reason to think that generating aligned images needs to be
particularly fast; in such cases, I prefer to use C, for reasons of
coding speed, fewer bugs, and maintainability.
LISTING 49.3 L49-3.C
/* Generates all four possible mode X image/mask alignments, stores image
alignments in display memory, allocates memory for and generates mask
alignments, and fills out an AlignedMaskedImage structure. Image and mask must
both be in byte-per-pixel form, and must both be of width ImageWidth. Mask
maps isomorphically (one to one) onto image, with each 0-byte in mask masking
off corresponding image pixel (causing it not to be drawn), and each non-0-byte
allowing corresponding image pixel to be drawn. Returns 0 if failure, or # of
display memory addresses (4-pixel sets) used if success. For simplicity,
allocated memory is not deallocated in case of failure. Compiled with
Borland C++ in C compilation mode. */
#include <stdio.h>
#include <stdlib.h>
#include "maskim.h"
extern void CopySystemToScreenX(int, int, int, int, int, int, char *,
unsigned int, int, int);
unsigned int CreateAlignedMaskedImage(MaskedImage * ImageToSet,
unsigned int DispMemStart, char * Image, int ImageWidth,
int ImageHeight, char * Mask)
{
int Align, ScanLine, BitNum, Size, TempImageWidth;
unsigned char MaskTemp;
unsigned int DispMemOffset = DispMemStart;
AlignedMaskedImage *WorkingAMImage;
char *NewMaskPtr, *OldMaskPtr;
/* Generate each of the four alignments in turn. */
for (Align = 0; Align < 4; Align++) {
/* Allocate space for the AlignedMaskedImage struct for this alignment. */
if ((WorkingAMImage = ImageToSet->Alignments[Align] =
malloc(sizeof(AlignedMaskedImage))) == NULL)
return 0;
WorkingAMImage->ImageWidth =
(ImageWidth + Align + 3) / 4; /* width in 4-pixel sets */
WorkingAMImage->ImagePtr = DispMemOffset; /* image dest */
/* Download this alignment of the image. */
CopySystemToScreenX(0, 0, ImageWidth, ImageHeight, Align, 0,
Image, DispMemOffset, ImageWidth, WorkingAMImage->ImageWidth * 4);
/* Calculate the number of bytes needed to store the mask in
nibble (Map Mask-ready) form, then allocate that space. */
Size = WorkingAMImage->ImageWidth * ImageHeight;
if ((WorkingAMImage->MaskPtr = malloc(Size)) == NULL)
return 0;
/* Generate this nibble oriented (Map Mask-ready) alignment of
the mask, one scan line at a time. */
OldMaskPtr = Mask;
NewMaskPtr = WorkingAMImage->MaskPtr;
for (ScanLine = 0; ScanLine < ImageHeight; ScanLine++) {
BitNum = Align;
MaskTemp = 0;
TempImageWidth = ImageWidth;
do {
/* Set the mask bit for next pixel according to its alignment. */
MaskTemp |= (*OldMaskPtr++ != 0) << BitNum;
if (++BitNum > 3) {
*NewMaskPtr++ = MaskTemp;
MaskTemp = BitNum = 0;
}
} while (--TempImageWidth);
/* Set any partial final mask on this scan line. */
if (BitNum != 0) *NewMaskPtr++ = MaskTemp;
}
DispMemOffset += Size; /* mark off the space we just used */
}
return DispMemOffset - DispMemStart;
}
**LISTING 49.4 MASKIM.H**
/* MASKIM.H: structures used for storing and manipulating masked
images */
/* Describes one alignment of a mask-image pair. */
typedef struct {
int ImageWidth; /* image width in addresses in display memory (also
mask width in bytes) */
unsigned int ImagePtr; /* offset of image bitmap in display mem */
char *MaskPtr; /* pointer to mask bitmap */
} AlignedMaskedImage;
/* Describes all four alignments of a mask-image pair. */
typedef struct {
AlignedMaskedImage *Alignments[4]; /* ptrs to AlignedMaskedImage
structs for four possible destination
image alignments */
} MaskedImage;
Notes on Masked Copying
Listings 49.1 and 49.2, like all Mode X code I've presented, perform no clipping, because clipping code would complicate the listings too much. While clipping can be implemented directly in the low-level Mode X routines (at the beginning of Listing 49.1, for instance), another, potentially simpler approach would be to perform clipping at a higher level, modifying the coordinates and dimensions passed to low-level routines such as Listings 49.1 and 49.2 as necessary to accomplish the desired clipping. It is for precisely this reason that the low-level Mode X routines support programmable start coordinates in the source images, rather than assuming (0,0); likewise for the distinction between the width of the image and the width of the area of the image to draw.
Also, it would be more efficient to make up structures that describe the source and destination bitmaps, with dimensions and coordinates built in, and simply pass pointers to these structures to the low level, rather than passing many separate parameters, as is now the case. I've used separate parameters for simplicity and flexibility.
Be aware that as nifty as Mode X hardware-assisted masked copying is, whether or not it's actually faster than software-only masked or transparent copying depends upon the processor and the video adapter. The advantage of Mode X masked copying is the 32-bit parallelism; the disadvantages are the need to read display memory and the need to perform an
OUTfor every four pixels. (OUTis a slow 486/Pentium instruction, and most VGAs respond toOUTs much more slowly than to display memory writes.)
Animation
Gosh. There's just no way I can discuss high-level animation fundamentals in any detail here; I could spend an entire (and entirely separate) book on animation techniques alone. You might want to have a look at Chapters 43 through 46 before attacking the code in this chapter; that will have to do us for the present volume. (I will return to 3-D animation in the next chapter.)
Basically, I'm going to perform page flipped animation, in which one page (that is, a bitmap large enough to hold a full screen) of display memory is displayed while another page is drawn to. When the drawing is finished, the newly modified page is displayed, and the other—now invisible—page is drawn to. The process repeats ad infinitum. For further information, some good places to start are Computer Graphics, by Foley and van Dam (Addison-Wesley); Principles of Interactive Computer Graphics, by Newman and Sproull (McGraw Hill); and "Real-Time Animation" by Rahner James (January 1990, Dr. Dobb's Journal).
Some of the code in this chapter was adapted for Mode X from the code in Chapter 44—yet another reason to read that chapter before finishing this one.
Mode X Animation in Action
Listing 49.5 ties together everything I've discussed about Mode X so far in a compact but surprisingly powerful animation package. Listing 49.5 first uses solid and patterned fills and system-memory-to-screen-memory masked copying to draw a static background containing a mountain, a sun, a plain, water, and a house with puffs of smoke coming out of the chimney, and sets up the four alignments of a masked kite image. The background is transferred to both display pages, and drawing of 20 kite images in the nondisplayed page using fast masked copying begins. After all images have been drawn, the page is flipped to show the newly updated screen, and the kites are moved and drawn in the other page, which is no longer displayed. Kites are erased at their old positions in the nondisplayed page by block copying from the background page. (See the discussion in the previous chapter for the display memory organization used by Listing 49.5.) So far as the displayed image is concerned, there is never any hint of flicker or disturbance of the background. This continues at a rate of up to 60 times a second until Esc is pressed to exit the program. See Figure 49.1 for a screen shot of the resulting image—add the animation in your imagination.
LISTING 49.5 L49-5.C
/* Sample mode X VGA animation program. Portions of this code first appeared
in PC Techniques. Compiled with Borland C++ 2.0 in C compilation mode. */
#include <stdio.h>
#include <conio.h>
#include <dos.h>
#include <math.h>
#include "maskim.h"
#define SCREEN_SEG 0xA000
#define SCREEN_WIDTH 320
#define SCREEN_HEIGHT 240
#define PAGE0_START_OFFSET 0
#define PAGE1_START_OFFSET (((long)SCREEN_HEIGHT*SCREEN_WIDTH)/4)
#define BG_START_OFFSET (((long)SCREEN_HEIGHT*SCREEN_WIDTH*2)/4)
#define DOWNLOAD_START_OFFSET (((long)SCREEN_HEIGHT*SCREEN_WIDTH*3)/4)
static unsigned int PageStartOffsets[2] = {PAGE0_START_OFFSET,PAGE1_START_OFFSET};
static char GreenAndBrownPattern[] = {2,6,2,6, 6,2,6,2, 2,6,2,6, 6,2,6,2};
static char PineTreePattern[] = {2,2,2,2, 2,6,2,6, 2,2,6,2, 2,2,2,2};
static char BrickPattern[] = {6,6,7,6, 7,7,7,7, 7,6,6,6, 7,7,7,7,};
static char RoofPattern[] = {8,8,8,7, 7,7,7,7, 8,8,8,7, 8,8,8,7};
#define SMOKE_WIDTH 7
#define SMOKE_HEIGHT 7
static char SmokePixels[] = {
0, 0,15,15,15, 0, 0,
0, 7, 7,15,15,15, 0,
8, 7, 7, 7,15,15,15,
8, 7, 7, 7, 7,15,15,
0, 8, 7, 7, 7, 7,15,
0, 0, 8, 7, 7, 7, 0,
0, 0, 0, 8, 8, 0, 0};
static char SmokeMask[] = {
0, 0, 1, 1, 1, 0, 0,
0, 1, 1, 1, 1, 1, 0,
1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1,
0, 1, 1, 1, 1, 1, 0,
0, 0, 1, 1, 1, 0, 0};
#define KITE_WIDTH 10
#define KITE_HEIGHT 16
static char KitePixels[] = {
0, 0, 0, 0,45, 0, 0, 0, 0, 0,
0, 0, 0,46,46,46, 0, 0, 0, 0,
0, 0,47,47,47,47,47, 0, 0, 0,
0,48,48,48,48,48,48,48, 0, 0,
49,49,49,49,49,49,49,49,49, 0,
0,50,50,50,50,50,50,50, 0, 0,
0,51,51,51,51,51,51,51, 0, 0,
0, 0,52,52,52,52,52, 0, 0, 0,
0, 0,53,53,53,53,53, 0, 0, 0,
0, 0, 0,54,54,54, 0, 0, 0, 0,
0, 0, 0,55,55,55, 0, 0, 0, 0,
0, 0, 0, 0,58, 0, 0, 0, 0, 0,
0, 0, 0, 0,59, 0, 0, 0, 0,66,
0, 0, 0, 0,60, 0, 0,64, 0,65,
0, 0, 0, 0, 0,61, 0, 0,64, 0,
0, 0, 0, 0, 0, 0,62,63, 0,64};
static char KiteMask[] = {
0, 0, 0, 0, 1, 0, 0, 0, 0, 0,
0, 0, 0, 1, 1, 1, 0, 0, 0, 0,
0, 0, 1, 1, 1, 1, 1, 0, 0, 0,
0, 1, 1, 1, 1, 1, 1, 1, 0, 0,
1, 1, 1, 1, 1, 1, 1, 1, 1, 0,
0, 1, 1, 1, 1, 1, 1, 1, 0, 0,
0, 1, 1, 1, 1, 1, 1, 1, 0, 0,
0, 0, 1, 1, 1, 1, 1, 0, 0, 0,
0, 0, 1, 1, 1, 1, 1, 0, 0, 0,
0, 0, 0, 1, 1, 1, 0, 0, 0, 0,
0, 0, 0, 1, 1, 1, 0, 0, 0, 0,
0, 0, 0, 0, 1, 0, 0, 0, 0, 0,
0, 0, 0, 0, 1, 0, 0, 0, 0, 1,
0, 0, 0, 0, 1, 0, 0, 1, 0, 1,
0, 0, 0, 0, 0, 1, 0, 0, 1, 0,
0, 0, 0, 0, 0, 0, 1, 1, 0, 1};
static MaskedImage KiteImage;
#define NUM_OBJECTS 20
typedef struct {
int X,Y,Width,Height,XDir,YDir,XOtherPage,YOtherPage;
MaskedImage *Image;
} AnimatedObject;
AnimatedObject AnimatedObjects[] = {
{ 0, 0,KITE_WIDTH,KITE_HEIGHT, 1, 1, 0, 0,&KiteImage},
{ 10, 10,KITE_WIDTH,KITE_HEIGHT, 0, 1, 10, 10,&KiteImage},
{ 20, 20,KITE_WIDTH,KITE_HEIGHT,-1, 1, 20, 20,&KiteImage},
{ 30, 30,KITE_WIDTH,KITE_HEIGHT,-1,-1, 30, 30,&KiteImage},
{ 40, 40,KITE_WIDTH,KITE_HEIGHT, 1,-1, 40, 40,&KiteImage},
{ 50, 50,KITE_WIDTH,KITE_HEIGHT, 0,-1, 50, 50,&KiteImage},
{ 60, 60,KITE_WIDTH,KITE_HEIGHT, 1, 0, 60, 60,&KiteImage},
{ 70, 70,KITE_WIDTH,KITE_HEIGHT,-1, 0, 70, 70,&KiteImage},
{ 80, 80,KITE_WIDTH,KITE_HEIGHT, 1, 2, 80, 80,&KiteImage},
{ 90, 90,KITE_WIDTH,KITE_HEIGHT, 0, 2, 90, 90,&KiteImage},
{100,100,KITE_WIDTH,KITE_HEIGHT,-1, 2,100,100,&KiteImage},
{110,110,KITE_WIDTH,KITE_HEIGHT,-1,-2,110,110,&KiteImage},
{120,120,KITE_WIDTH,KITE_HEIGHT, 1,-2,120,120,&KiteImage},
{130,130,KITE_WIDTH,KITE_HEIGHT, 0,-2,130,130,&KiteImage},
{140,140,KITE_WIDTH,KITE_HEIGHT, 2, 0,140,140,&KiteImage},
{150,150,KITE_WIDTH,KITE_HEIGHT,-2, 0,150,150,&KiteImage},
{160,160,KITE_WIDTH,KITE_HEIGHT, 2, 2,160,160,&KiteImage},
{170,170,KITE_WIDTH,KITE_HEIGHT,-2, 2,170,170,&KiteImage},
{180,180,KITE_WIDTH,KITE_HEIGHT,-2,-2,180,180,&KiteImage},
{190,190,KITE_WIDTH,KITE_HEIGHT, 2,-2,190,190,&KiteImage},
};
void main(void);
void DrawBackground(unsigned int);
void MoveObject(AnimatedObject *);
extern void Set320x240Mode(void);
extern void FillRectangleX(int, int, int, int, unsigned int, int);
extern void FillPatternX(int, int, int, int, unsigned int, char*);
extern void CopySystemToScreenMaskedX(int, int, int, int, int, int,
char *, unsigned int, int, int, char *);
extern void CopyScreenToScreenX(int, int, int, int, int, int,
unsigned int, unsigned int, int, int);
extern unsigned int CreateAlignedMaskedImage(MaskedImage *,
unsigned int, char *, int, int, char *);
extern void CopyScreenToScreenMaskedX(int, int, int, int, int, int,
MaskedImage *, unsigned int, int);
extern void ShowPage(unsigned int);
void main()
{
int DisplayedPage, NonDisplayedPage, Done, i;
union REGS regset;
Set320x240Mode();
/* Download the kite image for fast copying later. */
if (CreateAlignedMaskedImage(&KiteImage, DOWNLOAD_START_OFFSET,
KitePixels, KITE_WIDTH, KITE_HEIGHT, KiteMask) == 0) {
regset.x.ax = 0x0003; int86(0x10, ®set, ®set);
printf("Couldn't get memory\n"); exit();
}
/* Draw the background to the background page. */
DrawBackground(BG_START_OFFSET);
/* Copy the background to both displayable pages. */
CopyScreenToScreenX(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT, 0, 0,
BG_START_OFFSET, PAGE0_START_OFFSET, SCREEN_WIDTH, SCREEN_WIDTH);
CopyScreenToScreenX(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT, 0, 0,
BG_START_OFFSET, PAGE1_START_OFFSET, SCREEN_WIDTH, SCREEN_WIDTH);
/* Move the objects and update their images in the nondisplayed
page, then flip the page, until Esc is pressed. */
Done = DisplayedPage = 0;
do {
NonDisplayedPage = DisplayedPage ^ 1;
/* Erase each object in nondisplayed page by copying block from
background page at last location in that page. */
for (i=0; i<NUM_OBJECTS; i++) {
CopyScreenToScreenX(AnimatedObjects[i].XOtherPage,
AnimatedObjects[i].YOtherPage,
AnimatedObjects[i].XOtherPage +
AnimatedObjects[i].Width,
AnimatedObjects[i].YOtherPage +
AnimatedObjects[i].Height,
AnimatedObjects[i].XOtherPage,
AnimatedObjects[i].YOtherPage, BG_START_OFFSET,
PageStartOffsets[NonDisplayedPage], SCREEN_WIDTH, SCREEN_WIDTH);
}
/* Move and draw each object in the nondisplayed page. */
for (i=0; i<NUM_OBJECTS; i++) {
MoveObject(&AnimatedObjects[i]);
/* Draw object into nondisplayed page at new location */
CopyScreenToScreenMaskedX(0, 0, AnimatedObjects[i].Width,
AnimatedObjects[i].Height, AnimatedObjects[i].X,
AnimatedObjects[i].Y, AnimatedObjects[i].Image,
PageStartOffsets[NonDisplayedPage], SCREEN_WIDTH);
}
/* Flip to the page into which we just drew. */
ShowPage(PageStartOffsets[DisplayedPage = NonDisplayedPage]);
/* See if it's time to end. */
if (kbhit()) {
if (getch() == 0x1B) Done = 1; /* Esc to end */
}
} while (!Done);
/* Restore text mode and done. */
regset.x.ax = 0x0003; int86(0x10, ®set, ®set);
}
void DrawBackground(unsigned int PageStart)
{
int i,j,Temp;
/* Fill the screen with cyan. */
FillRectangleX(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT, PageStart, 11);
/* Draw a green and brown rectangle to create a flat plain. */
FillPatternX(0, 160, SCREEN_WIDTH, SCREEN_HEIGHT, PageStart,
GreenAndBrownPattern);
/* Draw blue water at the bottom of the screen. */
FillRectangleX(0, SCREEN_HEIGHT-30, SCREEN_WIDTH, SCREEN_HEIGHT,
PageStart, 1);
/* Draw a brown mountain rising out of the plain. */
for (i=0; i<120; i++)
FillRectangleX(SCREEN_WIDTH/2-30-i, 51+i, SCREEN_WIDTH/2-30+i+1,
51+i+1, PageStart, 6);
/* Draw a yellow sun by overlapping rects of various shapes. */
for (i=0; i<=20; i++) {
Temp = (int)(sqrt(20.0*20.0 - (float)i*(float)i) + 0.5);
FillRectangleX(SCREEN_WIDTH-25-i, 30-Temp, SCREEN_WIDTH-25+i+1,
30+Temp+1, PageStart, 14);
}
/* Draw green trees down the side of the mountain. */
for (i=10; i<90; i += 15)
for (j=0; j<20; j++)
FillPatternX(SCREEN_WIDTH/2+i-j/3-15, i+j+51,SCREEN_WIDTH/2+i+j/3-15+1,
i+j+51+1, PageStart, PineTreePattern);
/* Draw a house on the plain. */
FillPatternX(265, 150, 295, 170, PageStart, BrickPattern);
FillPatternX(265, 130, 270, 150, PageStart, BrickPattern);
for (i=0; i<12; i++)
FillPatternX(280-i*2, 138+i, 280+i*2+1, 138+i+1, PageStart, RoofPattern);
/* Finally, draw puffs of smoke rising from the chimney. */
for (i=0; i<4; i++)
CopySystemToScreenMaskedX(0, 0, SMOKE_WIDTH, SMOKE_HEIGHT, 264,
110-i*20, SmokePixels, PageStart, SMOKE_WIDTH,SCREEN_WIDTH, SmokeMask);
}
/* Move the specified object, bouncing at the edges of the screen and
remembering where the object was before the move for erasing next time. */
void MoveObject(AnimatedObject * ObjectToMove) {
int X, Y;
X = ObjectToMove->X + ObjectToMove->XDir;
Y = ObjectToMove->Y + ObjectToMove->YDir;
if ((X < 0) || (X > (SCREEN_WIDTH - ObjectToMove->Width))) {
ObjectToMove->XDir = -ObjectToMove->XDir;
X = ObjectToMove->X + ObjectToMove->XDir;
}
if ((Y < 0) || (Y > (SCREEN_HEIGHT - ObjectToMove->Height))) {
ObjectToMove->YDir = -ObjectToMove->YDir;
Y = ObjectToMove->Y + ObjectToMove->YDir;
}
/* Remember previous location for erasing purposes. */
ObjectToMove->XOtherPage = ObjectToMove->X;
ObjectToMove->YOtherPage = ObjectToMove->Y;
ObjectToMove->X = X; /* set new location */
ObjectToMove->Y = Y;
}
Here's something worth noting: The animation is extremely smooth on a 20 MHz 386. It is somewhat more jerky on an 8 MHz 286, because only 30 frames a second can be processed. If animation looks jerky on your PC, try reducing the number of kites.
The kites draw perfectly into the background, with no interference or fringe, thanks to masked copying. In fact, the kites also cross with no interference (the last-drawn kite is always in front), although that's not readily apparent because they all look the same anyway and are moving fast. Listing 49.5 isn't inherently limited to kites; create your own images and initialize the object list to display a mix of those images and see the full power of Mode X animation.
The external functions called by Listing 49.5 can be found in Listings 49.1, 49.2, 49.3, and 49.6, and in the listings for the previous two chapters.
LISTING 49.6 L49-6.ASM
; Shows the page at the specified offset in the bitmap. Page is displayed when
; this routine returns.
; C near-callable as: void ShowPage(unsigned int StartOffset);
INPUT_STATUS_1 equ 03dah ;Input Status 1 register
CRTC_INDEX equ 03d4h ;CRT Controller Index reg
START_ADDRESS_HIGH equ 0ch ;bitmap start address high byte
START_ADDRESS_LOW equ 0dh ;bitmap start address low byte
ShowPageParms struc
dw 2 dup (?) ;pushed BP and return address
StartOffset dw ? ;offset in bitmap of page to display
ShowPageParms ends
.model small
.code
public _ShowPage
_ShowPage proc near
push bp ;preserve caller's stack frame
mov bp,sp ;point to local stack frame
; Wait for display enable to be active (status is active low), to be
; sure both halves of the start address will take in the same frame.
mov bl,START_ADDRESS_LOW ;preload for fastest
mov bh,byte ptr StartOffset[bp] ; flipping once display
mov cl,START_ADDRESS_HIGH ; enable is detected
mov ch,byte ptr StartOffset+1[bp]
mov dx,INPUT_STATUS_1
WaitDE:
in al,dx
test al,01h
jnz WaitDE ;display enable is active low (0 = active)
; Set the start offset in display memory of the page to display.
mov dx,CRTC_INDEX
mov ax,bx
out dx,ax ;start address low
mov ax,cx
out dx,ax ;start address high
; Now wait for vertical sync, so the other page will be invisible when
; we start drawing to it.
mov dx,INPUT_STATUS_1
WaitVS:
in al,dx
test al,08h
jz WaitVS ;vertical sync is active high (1 = active)
pop bp ;restore caller's stack frame
ret
_ShowPage endp
end
Works Fast, Looks Great
We now end our exploration of Mode X, although we'll use it again shortly for 3-D animation. Mode X admittedly has its complexities; that's why I've provided a broad and flexible primitive set. Still, so what if it is complex? Take a look at Listing 49.5 in action. That sort of colorful, high-performance animation is worth jumping through a few hoops for; drawing 20, or even 10, fair-sized objects at a rate of 60 Hz, with no flicker, interference, or fringe, is no mean accomplishment, even on a 386.
There's much more we could do with animation in general and with Mode X in particular, but it's time to move on to new challenges. In closing, I'd like to point out that all of the VGA's hardware features, including the built-in AND, OR, and XOR functions, are available in Mode X, just as they are in the standard VGA modes. If you understand the VGA's hardware in mode 12H, try applying that knowledge to Mode X; you might be surprised at what you find you can do.