Как найти площадь яблока

One way to do this is to use DominantColors as follows.

im = Import["http://pngimg.com/upload/small/apple_PNG12438.png"];
res = DominantColors[im, Automatic, {"Count", "Color"}]

To be sure if the reddish color is indeed the eatable part of the apple check this.

eatable=DeleteSmallComponents@First@DominantColors[i, Automatic, "CoverageImage"]

Now the exact eatable part can be recovered as the first entry of the output.

DominantColors[eatable, Automatic, {"Count", "Color"}]

{152709, 109435}

Now what the OP wanted!

positions = ImageValuePositions[ColorNegate@EdgeDetect@eatable, 0];
apple = Graphics[
   FilledCurve[Line[positions[[Last@FindShortestTour[positions]]]]]] //
   DiscretizeGraphics[#, MaxCellMeasure -> 0.1] &

And we can get the area..

{Area[apple], NIntegrate[1, {x, y} [Element] apple]}

{152615.,152615.}

But who eats a 2D apple..

But we can also make a 3D apple and calculate how much surface we need to munch.

pts = positions[[Last@FindShortestTour[positions]]];
par = BSplineFunction[ExponentialMovingAverage,TranslationTransform[-Mean@pts] /@ pts, .25],
SplineClosed -> True, SplineDegree -> 2];
ap = RevolutionPlot3D[{First@par[t], Last@par[t], t}, {t, 0, 1},RevolutionAxis -> {0, 1, 0},
PlotPoints -> 60, MaxRecursion -> 3,Mesh -> None, Boxed -> False, Axes->False];
appleColor = res[[1, 2]];
apple3D = DiscretizeGraphics[
  Cases[Normal@ap, _GraphicsGroup, -1][[1]],
  MeshCellStyle -> {{2, All} -> 
     Directive[res[[1, 2]], Specularity[White, 20], 
      Glow[Darker[appleColor, .5]], Lighting -> "Neutral"],
    {1, All} -> Directive[Thin, Darker@res[[1, 2]]]}
  ]

And here goes the surface/munching area with some dimensional info..

{RegionDimension@apple3D,RegionEmbeddingDimension@apple3D,IntegerPart[RegionMeasure@apple3D]}

{2, 3, 1450102}

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