Информационный сервер для программистов: Исходники со всего света. Паскальные исходники со всего света
  Powered by Поисковый сервер Яndex: Найдется ВСЁ!
На Главную Исходники Форум Информер Страны мира
   Графика    >>    lines
   
 
 LINES - Draw BitMap Lines [640x480x16]  Cary Ravitz 12.07.1990

This article will cover how to draw lines on a bit map, i.e. convert lines from vector format to raster format with width, shading, dash patterna, and color. Docs, ASM and PAS included



9k 
 

Cary Ravitz This article will cover how to draw lines on a bit map, i.e. convert lines from vector format to raster format. Drawing one pel wide lines efficiently has been covered in detail in many places, but for the real world this is not sufficient. Lines need emphasis so that they can be distinguished or highlighted. This can be accomplished by using wide lines, shading patterns, dashed lines, and color. The algorithms and code presented here are simple techniques to accomplish this. They are not meant to be the fastest available algorithms, but to provide usable, modifiable, straight forward code. WIDE LINES There are two (at least) ways to attack the problem of drawing wide lines. One way is to draw multiple one pel lines. Another way is to draw the line once with wide dots. I have chosen the second way here because it makes things simpler when dealing with dashed lines. I have not used Bresenham's method for calculating the points along a line. The routine that draws a dot will be a much bigger computational hit that calculating the dot locations, so simplicity is better here. The basic algorithm for drawing a line from x1,y1 to x2,y2 is shown here: if distance_from_x1_to_x2 > distance_from_y1_to_y2 then for x = x1..x2 draw a dot at x,y_value_at_x else for y = y1..y2 draw a dot at x_value_at_y,y For each step along a line, one floating point addition and one floating point to integer conversion is done, but there are no multiplies or divides. SHADING Shading is accomplished by masking the dots that are ored into the bit map. If you mask a shading pattern into the dots then, as a line is created with dots that overlap by one pel, the patterns will interfere and become solid. The trick that we need is to tie the shading mask location to the bit map, not the dot. Then when dots overlap, the shading will not interfere. So we will wait until after the dot is shifted to align with the bit map in the x direction (m:=dot[w,i] shr l; in DRAWDOT) before applying the shade mask. The dot is never shifted to align in the y direction and instead the starting scan line in the mask is chosen to match the dot. DASHED LINES Since each point on a line is drawn as a large dot, the line may be dashed by simply leaving out series of dots. If we keep track of how long the line is as we draw each point on it, and compare this to a true/false pattern, when the pattern is true we draw a dot, and when it is false we move on to the next point. COLOR Drawing in eight colors (including the background) is a matter of using mulitple color planes. Eight basic colors - red, green, blue, cyan, magenta, yellow, white, and black - can be represented in three color planes - either red/green/blue or yellow/magenta/cyan. MIXING COLORS WITH SHADING You can use shading to mix the eight basic colors. For example, draw a line solid in one color plane and checkerboard in another color plane. This mixes the two colors and greatly expands the available colors on eight color devices. SAMPLE CODE A sample program that demonstrates these techniques is available from COMPUTER LANGUAGE's CompuServe forum and BBS network. Included in the program listing is a sample program that invokes VGA 640x480 16 color mode and draws a number of lines with various attributes. The code is mostly in Turbo Pascal, with DRAWDOT in assembler (DRAWDOT's constants are in the Pascal code). Independent software developer Cary Ravitz is the author of the Ravitz Editor and other shareware products.