mjhjmtu

Any suggestions on how to draw something like this picture using TikZ?

Thanks!

Here is a slight extension of Milo's nice answer. The main differences (improvements?) are:

1. I rewrote Alain Matthes' nice answer to allow for arbitrary functions determining the line width. Previously it was a linearly dependence, now it can be an arbitrary function such as a sine. All you need to do is to say declare function=varyinglw(x)=1+6*sin(1.8*x);. Apart from that, I made the code a bit more general, increased its speed (I think) and got rid of makeatletter since there was nothing that cannot be achieved with commands not containing @s.


2. This variation also does not nest tikzpictures. Rather, the arrows a re attached in the usual way, and can be even bent.

Here's how this works

• You need to find a function that determines how thick the line is at a given position of the path. This function has the name varyinglw. It's argument runs from 0 to 100. So if the function has a maximum at 50, the path will have reached its maximal width in the middle. An example for a function with this feature is declare function=varyinglw(x)=1+6*sin(1.8*x); below. If you want to use different functions, use scopes. Unfortunately I do not now how one can reset functions that are declared with declare functions. Therefore you should keep your functions local.


• If you have a very long path, you may want to increase the number of segments since otherwise it won't look smooth any more. This can be done by saying e.g. /pgf/decoration/varying line width steps=180 as in the examples below.


• Other than that you can use pretty much any path.

documentclass[tikz,border=3.14mm]standalone
usetikzlibrarydecorations,arrows.meta,bending
begindocument

pgfkeys/pgf/decoration/.cd,
 start color/.store in=startcolor,
 start color=black,
 end color/.store in=endcolor,
 end color=black,
 varying line width steps/.initial=100

pgfdeclaredecorationwidth and color changeinitial
 stateinitial[width=0pt, next state=line, persistent precomputation=%
 pgfmathparsepgfdecoratedpathlength/pgfkeysvalueof/pgf/decoration/varying line width steps%
 letincrement=pgfmathresult%
 defx0%
 ]
 stateline[width=increment pt, persistent postcomputation=%
 pgfmathsetmacroxx+increment
 ,next state=line]%
 pgfmathparseifthenelse(x<pgfdecoratedpathlength-5mm,varyinglw(100*(x/pgfdecoratedpathlength)),
 varyinglw(100*((pgfdecoratedpathlength-5mm)/pgfdecoratedpathlength))*(pgfdecoratedpathlength-x)/14) )
 pgfsetlinewidthpgfmathresult pt%
 pgfpathmovetopgfpointorigin%
 pgfmathsetmacrosteplength1.4*increment
 pgfpathlinetopgfqpointsteplength pt0pt%
 pgfmathsetmacroy100*(x/pgfdecoratedpathlength)
 pgfsetstrokecolorendcolor!y!startcolor%
 pgfusepathstroke%
 
 statefinal%
 pgfsetlinewidthpgflinewidth%
 pgfpathmovetopgfpointorigin%
 pgfmathsetmacroy100*(x/pgfdecoratedpathlength)
 colorendcolor!y!startcolor%
 pgfusepathstroke% 
 



begintikzpicture[varying arrow/.style=-Stealth[length=5mm,width=3.2mm,bend],color=endcolor,
postaction=/utils/exec=pgfsetarrows-,decorate,decoration=width and color change
]
beginscope[declare function=varyinglw(x)=1+6*sin(1.8*x);]
draw[varying arrow] (0,0) to[out=45,in=150] ++ (1.5,0) to[out=-30,in=-135] ++ (3,0);
draw[varying arrow,/pgf/decoration/varying line width steps=180]
(0,-3) to[out=45,in=150] ++ (1.5,0) to[out=-30,in=-135] ++ (3,0) 
 to[out=45,in=45] ++ (-2,1);
draw[varying arrow,/pgf/decoration/varying line width steps=180,
/pgf/decoration/start color=red,/pgf/decoration/end color=blue]
(0,-5) to[out=15,in=165] ++ (2,0) to[out=-15,in=90] ++ (1,-1) 
 to[out=-90,in=-20] ++ (-1.5,0);
endscope

beginscope[declare function=varyinglw(x)=4-3*cos(7.2*x);,xshift=6cm]
draw[varying arrow] (0,0) to[out=45,in=150] ++ (1.5,0) to[out=-30,in=-135] ++ (3,0);
draw[varying arrow,/pgf/decoration/varying line width steps=180]
(0,-3) to[out=45,in=150] ++ (1.5,0) to[out=-30,in=-135] ++ (3,0) 
 to[out=45,in=45] ++ (-2,1);
draw[varying arrow,/pgf/decoration/varying line width steps=180,
/pgf/decoration/start color=yellow,/pgf/decoration/end color=red]
(0,-5) to[out=15,in=165] ++ (2,0) to[out=-15,in=90] ++ (1,-1) 
 to[out=-90,in=-20] ++ (-1.5,0);
endscope

beginscope[declare function=varyinglw(x)=4-3*cos(5.6*x);,xshift=12cm]
draw[varying arrow] (0,0) to[out=45,in=150] ++ (1.5,0) to[out=-30,in=-135] ++ (3,0);
draw[varying arrow,/pgf/decoration/varying line width steps=180]
(0,-3) to[out=45,in=150] ++ (1.5,0) to[out=-30,in=-135] ++ (3,0) 
 to[out=45,in=45] ++ (-2,1);
draw[varying arrow,/pgf/decoration/varying line width steps=180,
/pgf/decoration/start color=yellow,/pgf/decoration/end color=blue]
(0,-5) to[out=15,in=165] ++ (2,0) to[out=-15,in=90] ++ (1,-1) 
 to[out=-90,in=-20] ++ (-1.5,0);
endscope
endtikzpicture
enddocument 

FUN: The mandatory animation.

documentclass[tikz,border=3.14mm]standalone
usetikzlibrarydecorations,arrows.meta,bending
begindocument

pgfkeys/pgf/decoration/.cd,
 start color/.store in=startcolor,
 start color=black,
 end color/.store in=endcolor,
 end color=black,
 varying line width steps/.initial=100

pgfdeclaredecorationwidth and color changeinitial
 stateinitial[width=0pt, next state=line, persistent precomputation=%
 pgfmathparsepgfdecoratedpathlength/pgfkeysvalueof/pgf/decoration/varying line width steps%
 letincrement=pgfmathresult%
 defx0%
 ]
 stateline[width=increment pt, persistent postcomputation=%
 pgfmathsetmacroxx+increment
 ,next state=line]%
 pgfmathparseifthenelse(x<pgfdecoratedpathlength-5mm,varyinglw(100*(x/pgfdecoratedpathlength)),
 varyinglw(100*((pgfdecoratedpathlength-5mm)/pgfdecoratedpathlength))*(pgfdecoratedpathlength-x)/14) )
 pgfsetlinewidthpgfmathresult pt%
 pgfpathmovetopgfpointorigin%
 pgfmathsetmacrosteplength1.4*increment
 pgfpathlinetopgfqpointsteplength pt0pt%
 pgfmathsetmacroy100*(x/pgfdecoratedpathlength)
 pgfsetstrokecolorendcolor!y!startcolor%
 pgfusepathstroke%
 
 statefinal%
 pgfsetlinewidthpgflinewidth%
 pgfpathmovetopgfpointorigin%
 pgfmathsetmacroy100*(x/pgfdecoratedpathlength)
 colorendcolor!y!startcolor%
 pgfusepathstroke% 
 


foreach Z in 0,10,...,720
begintikzpicture[varying arrow/.style=-Stealth[length=5mm,width=3.2mm,bend],color=endcolor,
postaction=/utils/exec=pgfsetarrows-,decorate,decoration=width and color change
]
path[use as bounding box] (-4,-4) rectangle (4,4);
beginscope[declare function=varyinglw(x)=1+6*sin(1.8*x);]
draw[varying arrow] plot[variable=z,domain=Z+90:Z+180]
(z:1+sqrt(z/90));
endscope

endtikzpicture
enddocument 

ALTERNATIVE: Some additional possibilities arise with the calligraphy library. (If I understand correctly, currently you need to download the latest version from this repository.)

documentclass[tikz,border=3.14mm]standalone
usepackagespath3
usetikzlibrarycalligraphy % need to download the repository from https://github.com/loopspace/spath3
% run "tex spath3.dtx" and then copy the spath3.sty file in the same directory
% as the tex file
usetikzlibraryarrows.meta,bending,decorations.pathreplacing
tikzsetcalligraph/.style=postaction=decorate,decoration=show path construction,
 moveto code=,
 lineto code=,
 curveto code=
 calligraphy (tikzinputsegmentfirst) .. controls
 (tikzinputsegmentsupporta) and (tikzinputsegmentsupportb)
 ..(tikzinputsegmentlast);
 ,
 closepath code=

begindocument
begintikzpicture[line width=1pt]
pen (-135:.125) -- (45:.125) ;
draw[calligraph,Stealth[length=5mm,width=3.2mm,bend]-] (0,0) to[out=45,in=150] ++ (1.5,0) to[out=-30,in=-135] ++ (3,0);
endtikzpicture

begintikzpicture[line width=1pt]
pen (-135:.125) -- (45:.125) ;
draw[calligraph,Stealth[length=5mm,width=3.2mm,bend]-] (0,0) to[out=45,in=150]
++ (1.5,0) to[out=-30,in=45] ++ (1,-2);
endtikzpicture
enddocument

This isn't a complete answer. But as a proof of concept, I tried adapting this answer from Stroke with variable thickness to make something like the arrows you want.

documentclass[tikz,margin=0.5cm]standalone
usetikzlibrarydecorations,arrows.meta

makeatletter
pgfkeys/pgf/decoration/.cd,
 start color/.store in =startcolor,
 end color/.store in =endcolor

pgfdeclaredecorationwidth and color changeinitial
 stateinitial[width=2pt, next state=line, persistent precomputation=%
 pgfmathdivide50pgfdecoratedpathlength%
 letincrement=pgfmathresult%
 defx0%
 ]
 stateline[width=.5pt, persistent postcomputation=%
 pgfmathadd@xincrement%
 letx=pgfmathresult%
 ]%
 pgfsetlinewidthx/20*0.005pt+pgflinewidth%
 pgfsetarrows-%
 pgfpathmovetopgfpointorigin%
 pgfpathlinetopgfqpoint.75pt0pt%
 pgfsetstrokecolorendcolor!x!startcolor%
 pgfusepathstroke%
 
 statefinal%
 pgfsetlinewidthpgflinewidth%
 pgfpathmovetopgfpointorigin%
 colorendcolor!x!startcolor%
 pgfusepathstroke% 
 

makeatother

begindocument

begintikzpicture

draw [line width=2pt, decoration=width and color change,start color=black, end color=black, decorate,] plot [smooth, tension=1] coordinates (0,0) (2,0.5) (5,0.7) (3,1.5) (1,1.5) node [rotate=100] tikz draw [-Stealth[length=7mm, width=10mm]](0,0); ;

endtikzpicture

enddocument