Disparity map on opencv 2.4

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You can get good results for the aloe vera plant that is found in the OpenCV examples data directory with sbm and sgbm respectively ;

      cvtColor( viewL, viewL, COLOR_RGB2GRAY );
  cvtColor( viewR, viewR, COLOR_RGB2GRAY );
  sStereo->alg = StereoCameraRig::STEREO_BM;
  sStereo->SADWindowSize = 9;
  sStereo->numberOfDisparities = 112;
  sStereo->preFilterSize = 5;
  sStereo->preFilterCap = 61;
  sStereo->minDisparity = -39;
  sStereo->textureThreshold = 507;
  sStereo->uniquenessRatio = 0;
  sStereo->speckleWindowSize = 0;
  sStereo->speckleRange = 8;
  sStereo->disp12MaxDiff = 1;

or : ..

   sStereo->alg = StereoCameraRig::STEREO_SGBM;
  sStereo->SADWindowSize = 11;
  sStereo->numberOfDisparities = 192;
  sStereo->preFilterCap = 4;
  sStereo->minDisparity = -64;
  sStereo->uniquenessRatio = 1;
  sStereo->speckleWindowSize = 150;
  sStereo->speckleRange = 1;
  sStereo->disp12MaxDiff = 1;
  sStereo->P1 = 600;
  sStereo->P2 = 2400;
  sStereo->textureThreshold = 500;

You have to convert images to grey for sbm while sgbm can work with color images without any problems.

Note: The left and right images of the plant are rectified meaning y-axis aligned. In order to make such a rectification (from scratch) you must stereo calibrate the system to get intrinsic and extrinsic parameters of the system.