Ooops
July 25, 2021, 17:12:19

### Author Topic: [bmx] Draw evenly spaced dots along a line by Warpy [ 1+ years ago ]  (Read 438 times)

#### BlitzBot

• Jr. Member
• Posts: 1
##### [bmx] Draw evenly spaced dots along a line by Warpy [ 1+ years ago ]
« on: June 29, 2017, 00:28:40 »
Title : Draw evenly spaced dots along a line
Author : Warpy
Posted : 1+ years ago

Description : Suppose you've got a set of points you want to join up with dotted lines, spaced a certain distance apart.

Bresenham isn't the right algorithm for this - the gaps between the dots are bound to be a lot bigger than single pixels, so 'stupid' linear interpolation should do the job satisfactorily.

However, if you want to make sure you definitely draw the start and end points of a line, as well as dots in between, or if you want to draw a line made up of several segments, it gets a bit more complicated.

Here are two methods:

The first tries to draw each line segment so that its start and end points get a dot, and then it changes the size of the gaps slightly so that all the dots are evenly spaced right up to the end.

The second goes along between the points, drawing dots at the given distance apart from each other. It might not draw dots exactly on the given points, but the dots are always exactly the right distance apart.

Code :
Code: BlitzMax
1. Strict
2.
3. 'this method will draw each line segment individually
4. 'it will change the gap slightly so that the dots are spread evenly along each segment
5. 'it always draws a dot on each corner and endpoint
6. Function drawdots1(points#[], gap#)
7.         If Len(points)<4 Return 'need at least two pairs of co-ordinates
8.
9.         Local sx#,sy#,ex#,ey#
10.         Local dx#,dy#,d#,steps,tgap#
11.         Local x#,y#
12.
13.         DrawDot points[0],points[1]     'draw a dot at the start of the line
14.
15.         For Local i=0 To Len(points)-3 Step 2           'draw each line segment
16.
17.                 sx=points[i]                    'get start and end of this segment
18.                 sy=points[i+1]
19.                 ex=points[i+2]
20.                 ey=points[i+3]
21.
22.                 dx# = ex-sx                     'calculate a vector in the direction of the line, so we can get the distance
23.                 dy# = ey-sy
24.                 d# = Sqr(dx*dx+dy*dy)
25.
26.                 steps = round(d/gap)    'work out how many dots to draw (distance between the points, divide by the desired gap size). Round to nearest whole number
27.                 tgap# = d/steps         'work out what the gap needs to be to space the calculated number of needed dots evenly
28.
29.                 dx:*tgap/d                      'work out the vector between each dot
30.                 dy:*tgap/d
31.
32.                 For Local j=1 To steps
33.                         x# = sx+j*dx    'calculate the dot's position by adding the gap vector to the position of the start of the line
34.                         y# = sy+j*dy
35.                         DrawDot x,y     'draw a dot
36.                 Next
37.         Next
38. End Function
39.
40. 'this method will draw dots evenly spaced along the whole set of lines
41. 'it might not draw dots on the corners.
42. Function drawdots2(points#[],gap#)
43.         If Len(points)<4 Return 'need at least two pairs of co-ordinates
44.
45.         Local sx#,sy#,ex#,ey#
46.         Local dx#,dy#,d#,tstep#
47.         Local x#,y#
48.
49.         Local t#=0      't will keep track of how far along each line segment we are - 0 for at the start, 1 for at the end
50.
51.         For Local i=0 To Len(points)-3 Step 2   'go through the line segments
52.
53.                 sx=points[i]                    'get start and end of this segment
54.                 sy=points[i+1]
55.                 ex=points[i+2]
56.                 ey=points[i+3]
57.
58.                 dx = ex-sx                      'work out a vector in the direction of the line
59.                 dy = ey-sy
60.                 d = Sqr(dx*dx+dy*dy)    'work out the length of the line
61.
62.                 tstep# = gap/d          'work out what fraction of the line each gap represents
63.
64.                 While t<1               'draw dots until we reach the end of the line
65.
66.                         x = sx+dx*t     'work out the position of the dot by multiplying the vector by t
67.                         y = sy+dy*t
68.                         DrawDot x,y     'draw the dot
69.                         t:+tstep                'increase t by the amount corresponding to a gap
70.                 Wend
71.
72.                 t:-1                            'when we reach the end of the line, t might be more than 1, meaning we didn't manage to get a whole gap in at the end
73.                                                 'subtract 1 from t, and carry over the remainder to the next line segment
74.         Next
75.
76.         DrawDot ex,ey                   'draw the end point of the line.
77. End Function
78.
79. Function DrawDot(x#,y#)
80.         DrawOval x-2,y-2,4,4
81. End Function
82.
83. Function Round(f#)      'round a floating point number to the nearest whole number
84.         Local i = Floor(f)
85.         If f-i>=.5
86.                 Return i+1
87.         Else
88.                 Return i
89.         EndIf
90. End Function
91.
92.
93. Graphics 800,600,0
94. Local points#[]
95. Local mode=0
96. Local gapsize#=20
97.
98. While Not (KeyHit(KEY_ESCAPE) Or AppTerminate())
99.         DrawText "Click the mouse!",0,0
100.         DrawText "Right-click to change methods",0,15
101.         DrawText "Press up/down to change gap size",0,30
102.
103.         If MouseHit(1)
104.                 points :+ [Float(MouseX()),Float(MouseY())]
105.                 If Len(points)>16
106.                         points=points[2..]
107.                 EndIf
108.         EndIf
109.
110.         If MouseHit(2)
111.                 mode=1-mode
112.         EndIf
113.
114.         gapsize :+ (KeyDown(KEY_UP)-KeyDown(KEY_DOWN))*.25
115.         If gapsize<1 gapsize=1
116.         DrawText "Gap Size: "+gapsize,400,15
117.
118.         If mode=0
119.                 drawdots1(points,gapsize)
120.                 DrawText "Using Method 1",400,0
121.         Else
122.                 drawdots2(points,gapsize)
123.                 DrawText "Using Method 2",400,0
124.         EndIf
125.         Flip
126.         Cls
127. Wend