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James Gregory
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27-03.html
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27-03.html
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@ -36,12 +36,12 @@
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</TABLE>
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</CENTER>
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<P><BR></P>
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<P>“That’s an interesting application of write mode 2,” you may well say, “but is it really useful?” While the ability to convert chunky bitmaps into VGA bitmaps does have its uses, Listing 27.1 is primarily intended to illustrate the mechanics of write mode 2.
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<P>“That’s an interesting application of write mode 2,” you may well say, “but is it really useful?” While the ability to convert chunky bitmaps into VGA bitmaps does have its uses, Listing 27.1 is primarily intended to illustrate the mechanics of write mode 2.
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</P>
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<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP"><IMG SRC="images/i.jpg"><TD WIDTH="95%"><SMALL><I>For performance, it’s best to store 16-color bitmaps in pre-separated four-plane format in system memory, and copy one plane at a time to the screen. Ideally, such bitmaps should be copied one scan line at a time, with all four planes completed for one scan line before moving on to the next. I say this because when entire images are copied one plane at a time, nasty transient color effects can occur as one plane becomes visibly changed before other planes have been modified.</I></SMALL>
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<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP"><IMG SRC="images/i.jpg"><TD WIDTH="95%"><SMALL><I>For performance, it’s best to store 16-color bitmaps in pre-separated four-plane format in system memory, and copy one plane at a time to the screen. Ideally, such bitmaps should be copied one scan line at a time, with all four planes completed for one scan line before moving on to the next. I say this because when entire images are copied one plane at a time, nasty transient color effects can occur as one plane becomes visibly changed before other planes have been modified.</I></SMALL>
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</TABLE>
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<H4 ALIGN="LEFT"><A NAME="Heading6"></A><FONT COLOR="#000077">Drawing Color-Patterned Lines Using Write Mode 2</FONT></H4>
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<P>A more serviceable use of write mode 2 is shown in the program presented in Listing 27.2. The program draws multicolored horizontal, vertical, and diagonal lines, basing the color patterns on passed color tables. Write mode 2 is ideal because in this application color can vary from one pixel to the next, and in write mode 2 all that’s required to set pixel color is a change of the lower nibble of the byte written by the CPU. Set/reset could be used to achieve the same result, but an index/data pair of <B>OUT</B>s would be required to set the Set/Reset register to each new color. Similarly, the Map Mask register could be used in write mode 0 to set pixel color, but in this case not only would an index/data pair of <B>OUT</B>s be required but there would also be no guarantee that data already in display memory wouldn’t interfere with the color of the pixel being drawn, since the Map Mask register allows only selected planes to be drawn to.</P>
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<P>A more serviceable use of write mode 2 is shown in the program presented in Listing 27.2. The program draws multicolored horizontal, vertical, and diagonal lines, basing the color patterns on passed color tables. Write mode 2 is ideal because in this application color can vary from one pixel to the next, and in write mode 2 all that’s required to set pixel color is a change of the lower nibble of the byte written by the CPU. Set/reset could be used to achieve the same result, but an index/data pair of <B>OUT</B>s would be required to set the Set/Reset register to each new color. Similarly, the Map Mask register could be used in write mode 0 to set pixel color, but in this case not only would an index/data pair of <B>OUT</B>s be required but there would also be no guarantee that data already in display memory wouldn’t interfere with the color of the pixel being drawn, since the Map Mask register allows only selected planes to be drawn to.</P>
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<P>Listing 27.2 is hardly a comprehensive line drawing program. It draws only a few special line cases, and although it is reasonably fast, it is far from the fastest possible code to handle those cases, because it goes through a dot-plot routine and because it draws horizontal lines a pixel rather than a byte at a time. Write mode 2 would, however, serve just as well in a full-blown line drawing routine. For any type of patterned line drawing on the VGA, the basic approach remains the same: Use the bit mask to select the pixel (or pixels) to be altered and use the CPU byte in write mode 2 to select the color in which to draw.</P>
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<P><B>LISTING 27.2 L27-2.ASM</B></P>
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<!-- CODE //-->
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;
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; By Michael Abrash
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;
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Stack segment para stack ‘STACK’
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Stack segment para stack ‘STACK’
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db 512 dup(0)
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Stack ends
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@ -65,7 +65,7 @@ GC_INDEX equ 03ceh ;Graphics Controller Index reg
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GRAPHICS_MODE equ 5 ;index of Graphics Mode reg
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BIT_MASK equ 8 ;index of Bit Mask reg
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Data segment para common ‘DATA’
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Data segment para common ‘DATA’
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Pattern0 db 16
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db 0, 1, 2, 3, 4, 5, 6, 7, 8
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db 9, 10, 11, 12, 13, 14, 15
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@ -77,13 +77,13 @@ Pattern3 db 9
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db 1, 1, 1, 2, 2, 2, 4, 4, 4
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Data ends
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Code segment para public ‘CODE’
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Code segment para public ‘CODE’
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assume cs:Code, ds:Data
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Start proc near
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mov ax,Data
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mov ds,ax
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mov ax,10h
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int 10h ;select video mode 10h (640×350)
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int 10h ;select video mode 10h (640x350)
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;
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; Draw 8 radial lines in upper-left quadrant in pattern 0.
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;
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@ -268,7 +268,7 @@ LineUp proc near
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pop dx ;get back line length
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shl di,1
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jmp cs:[LineUpVectors+di]
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jmp cs:[LineUpVectors+di]
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;
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; Horizontal line to right.
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;
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