threshold doesnt seem to work quite right

linux system, latest svn clean checkout.

For colored images, I'd expect this to turn the image (all channels) into a 
monocrhome image based on the value being set (ie all values above this to to 
100%, all values below go to 0).

The behavior seems to be almost works, however, I end up with some color 
remaining.   Tested images are all large PNG's , ie imagemagick reports this:

Image: test7.png
  Format: PNG (Portable Network Graphics)
  Class: DirectClass
  Geometry: 2592x3872+0+0
  Resolution: 72x72
  Print size: 36x53.7778
  Units: Undefined
  Type: TrueColor
  Endianess: Undefined
  Colorspace: sRGB
  Depth: 8-bit
  Channel depth:
    red: 8-bit
    green: 8-bit
    blue: 8-bit
  Channel statistics:
    Red:
      min: 0 (0)
      max: 228 (0.894118)
      mean: 107.723 (0.422441)
      standard deviation: 65.6952 (0.257628)
      kurtosis: -1.52917
      skewness: -0.475536
    Green:
      min: 0 (0)
      max: 209 (0.819608)
      mean: 107.004 (0.419624)
      standard deviation: 65.9809 (0.258749)
      kurtosis: -1.57422
      skewness: -0.442728
    Blue:
      min: 0 (0)
      max: 232 (0.909804)
      mean: 108.015 (0.423589)
      standard deviation: 67.426 (0.264416)
      kurtosis: -1.59933
      skewness: -0.412485
  Image statistics:
    Overall:
      min: 0 (0)
      max: 232 (0.909804)
      mean: 107.581 (0.421885)
      standard deviation: 66.3717 (0.260281)
      kurtosis: -1.56778
      skewness: -0.442582
  Rendering intent: Perceptual
  Gamma: 0.454545
  Chromaticity:
    red primary: (0.64,0.33)
    green primary: (0.3,0.6)
    blue primary: (0.15,0.06)
    white point: (0.3127,0.329)
  Interlace: None
  Background color: white
  Border color: srgb(223,223,223)
  Matte color: grey74
  Transparent color: black
  Compose: Over
  Page geometry: 2592x3872+0+0
  Dispose: Undefined
  Iterations: 0
  Compression: Zip
  Orientation: Undefined
  Properties:
    date:create: 2013-07-10T15:30:39-07:00
    date:modify: 2013-07-10T15:30:39-07:00
    png:cHRM                 : chunk was found (see Chromaticity, above)
    png:gAMA                 : gamma=0.45454544 (See Gamma, above)
    png:IHDR.bit_depth       : 8
    png:IHDR.color_type      : 2 (Truecolor)
    png:IHDR.interlace_method: 0 (Not interlaced)
    png:IHDR.width,height    : 2592, 3872
    png:sRGB                 : intent=0 (See Rendering intent)
    signature: 1efb0001b79a9ef8702ad89ea682e98e76f3f7baa55d94dcd14e8f8f3df1a803
  Artifacts:
    filename: test7.png
    verbose: true
  Tainted: False
  Filesize: 11.24MB
  Number pixels: 10.04M
  Pixels per second: 24.48MB
  User time: 0.420u
  Elapsed time: 0:01.410
  Version: ImageMagick 6.7.7-10 2012-11-06 Q16 http://www.imagemagick.org
Original issue reported on code.google.com by brian.qu...@gmail.com on 11 Jul 2013 at 4:56
fate-extra / imagestack

threshold doesnt seem to work quite right #10
