Gnuplot:渐变彩色箭头

Question

我想绘制一个不是单一颜色的箭头，而是沿其长度方向的颜色渐变。有谁知道如何实现这一目标？一些用于创建以红色开始并以蓝色结束的箭头的伪代码:

set palette defined (0 "red", 1 "blue")
set cbr [0:1]
set arrow from 0,0 to 1,1 linecolor palette cb [0:1] # this doesn't work

Answer 1

除了@Friedrich 的解决方案，我想提出一个更通用的解决方案(尽管更复杂)。

我假设您想绘制箭头以外的其他内容。如果您的图表需要使用调色板，我猜您有麻烦了，因为我不确定 gnuplot 是否在 plot 命令中支持多个调色板(参见 Gnuplot 5.2 splot: Multiple pm3d palette in one plot call)。所以，你必须自己为你的箭头实现调色板(参见例如 Gnuplot: transparency of data points when using palette )。如果您想使用 Cubic Bézier 检查 ( https://stackoverflow.com/a/60389081/7295599 ) 来做弯曲的箭头。

代码:

### arrow with color gradient (besides other palette in plot)
reset session

array A[4] = [-4,-2,4,2]   # arrow coordinates x0,y0,x1,y1
Ax(t) = A[1] + t*(A[3]-A[1])
Ay(t) = A[2] + t*(A[4]-A[2])
AColorStart = 0xff0000   # red
AColorEnd =   0x0000ff   # blue
r(c) = (c & 0xff0000)>>16
g(c) = (c & 0x00ff00)>>8
b(c) = (c & 0x0000ff)
AColor(t) = ((int(r(AColorStart)*(1-t)+r(AColorEnd)*t))<<16) + \
            ((int(g(AColorStart)*(1-t)+g(AColorEnd)*t))<<8)  + \
              int(b(AColorStart)*(1-t)+b(AColorEnd)*t)

array AHead[1]   # dummy array for plotting a single point, here: arrow head
set angle degrees
set style arrow 1 lw 3 lc rgb var size 0.5,15 fixed

set palette grey

plot '++' u 1:2:($1*$2) w image notitle, \
     [0:0.99] '+' u (Ax($1)):(Ay($1)):(AColor($1)) w l lw 3 lc rgb var notitle,\
     AHead u (Ax(0.99)):(Ay(0.99)):(Ax(1)-Ax(0.99)):(Ay(1)-Ay(0.99)):(AColor($1)) w vec as 1 notitle
### end of code

结果:

添加:

值得一提的是，这里有一个变体，它允许绘制多个箭头，每个箭头都有不同的调色板。我猜它需要 gnuplot 5.2，因为索引数据 block $PALETTE[i]。

代码:

### multiple arrows each with different color gradients (besides other palette in plot)
reset session

# define palettes
set print $myPalettes
    test palette                # get default palette into datablock $PALETTE
    print $PALETTE              # add palette to $myPalettes
    set palette rgb 33,13,10    # define next palette
    test palette                # get palette into datablock $PALETTE
    print $PALETTE              # add palette to $myPalettes
    set palette defined (0 "blue", 1 "black", 2 "red")   # define next palette
    test palette                                         # get palette into datablock $PALETTE
    print $PALETTE                                       # add palette to $myPalettes
set print
    
ColorComp(p,t,c) = int(word($myPalettes[p*257+int(255*t)+1],c+1)*0xff)
AColor(p,t) = (ColorComp(p,t,1)<<16) + (ColorComp(p,t,2)<<8) + ColorComp(p,t,3)
           
set size ratio -1
set angle degrees
unset key
set style arrow 1 lw 3 lc rgb var size 0.5,15 fixed
array AHead[1]      # dummy array for plotting a single point, here: arrow head
set palette grey    # yet another palette for the background

# x0 y0 x1  y1  paletteNo
$Arrows <<EOD
-4  -4   4   0   0
-4  -2   4   2   1
-4   0   4   4   2
EOD

Ax(i,t) = word($Arrows[i],1) + t*(word($Arrows[i],3)-word($Arrows[i],1))
Ay(i,t) = word($Arrows[i],2) + t*(word($Arrows[i],4)-word($Arrows[i],2))
Palette(i) = int(word($Arrows[i],5))

plot '++' u 1:2:($1*$2) w image, \
     for [i=1:|$Arrows|] [0:0.99:0.01] '+' u (Ax(i,$1)):(Ay(i,$1)):(AColor(Palette(i),$1)) w l lw 4 lc rgb var, \
     for [i=1:|$Arrows|] AHead u (Ax(i,0.99)):(Ay(i,0.99)): \
         (Ax(i,1)-Ax(i,0.99)):(Ay(i,1)-Ay(i,0.99)):(AColor(Palette(i),$1)) w vec as 1
### end of code

结果: