Dfyjkt

Hi I am trying to fill the area between two curves. One curve is two joined arcs, and the other is one freehand curve. The part I want to fill is the white area. I use the fillbetween library but I just can't get the upper curve right.

Your help is much appreciated. A MWE is as follows

documentclass[margin=10pt]standalone
usepackagetikz
usepackagepgfplots
pgfplotssetcompat=newest
usepgfplotslibraryfillbetween
usetikzlibraryintersections


begindocument
begintikzpicture%[x=1cm, y=1cm]
defrsmall2
defrlarge4
defalpha60
defbeta40
defgamma20
defAoutalpha+gamma-90
defBin270-beta-gamma

pgfmathsetmacroax-rsmall+cos(alpha)*rsmall
pgfmathsetmacroaysin(alpha)*rsmall
pgfmathsetmacrobxrlarge-cos(beta)*rlarge
pgfmathsetmacrobysin(beta)*rlarge

filldraw[name path=arcs, fill=gray!20, thick]
 (180:2*rsmall) arc (180:0:rsmall) arc (180:0:rlarge);

coordinate (A) at (ax, ay);
coordinate (B) at (bx, by);

draw[name path=unduloid, thick] (A) to[out=Aout, in=Bin] (B);


fill[blue!20,intersection segments=of=unduloid and arcs,sequence=L2--R2];


endtikzpicture
enddocument

You only need to add a [reverse] to one of the segments.

documentclass[margin=10pt]standalone
usepackagetikz
usepackagepgfplots
pgfplotssetcompat=newest
usepgfplotslibraryfillbetween
usetikzlibraryintersections


begindocument
begintikzpicture%[x=1cm, y=1cm]
defrsmall2
defrlarge4
defalpha60
defbeta40
defgamma20
defAoutalpha+gamma-90
defBin270-beta-gamma

pgfmathsetmacroax-rsmall+cos(alpha)*rsmall
pgfmathsetmacroaysin(alpha)*rsmall
pgfmathsetmacrobxrlarge-cos(beta)*rlarge
pgfmathsetmacrobysin(beta)*rlarge

filldraw[name path=arcs, fill=gray!20, thick]
 (180:2*rsmall) arc (180:0:rsmall) arc (180:0:rlarge);

coordinate (A) at (ax, ay);
coordinate (B) at (bx, by);

draw[name path=unduloid, thick] (A) to[out=Aout, in=Bin] (B);


path[%draw=red,thick,
fill=blue!20,intersection segments=of=unduloid and arcs,sequence=L2--R2[reverse]];


endtikzpicture
enddocument

How can one debug this? Just draw curves with arrows on the segments in question. Here one would add

draw[red,thick,-latex,
intersection segments=of=unduloid and arcs,sequence=L2];

draw[blue,thick,-latex,
intersection segments=of=unduloid and arcs,sequence=R2];

and then get

This means that the paths start and end at the same points. To fix this, i.e. to produce a cycle, you have to run through one of the paths in the reverse direction. If you don't, TikZ will close the path on its own by a straight line. That's why you got the straight line for one of the bounding curves.

ADDENDUM: In this situation you do not really need all the segments nor the fillbetween library. Personally I would also refrain from redefining alpha, beta etc. since you may need these Greek symbols.

documentclass[margin=10pt]standalone
usepackagetikz
usetikzlibrarycalc


begindocument
begintikzpicture%[x=1cm, y=1cm]
defrsmall2
defrlarge4
defmyalpha60
defmybeta40
defgamma20
defAoutmyalpha+gamma-90
defBin270-mybeta-gamma


filldraw[fill=gray!20, thick]
 (180:2*rsmall) arc (180:0:rsmall) arc (180:0:rlarge);

coordinate (A) at ($(-rsmall,0)+(myalpha:rsmall)$);
coordinate (B) at ($(rlarge,0)+(180-mybeta:rlarge)$);

draw[red,fill=blue!20, thick] (A) to[out=Aout, in=Bin] (B)
arc(180-mybeta:180:rlarge) arc(0:myalpha:rsmall) -- cycle;

endtikzpicture
enddocument