OpenCV reads an image as a 3-channel image while PIL reads the same image as a 1-channel image

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I\'m trying to figure out the NYU-v2 for semantic segmentation dataset in this Repository. I\'m having a hard time trying to understand how the image labels have been stored

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天涯浪人

2021-01-06 10:16

It is stored as a palette (indexed) image. See here. OpenCV doesn't really like palette images and just converts to RGB.

If I dump it with ImageMagick in Terminal, you can see the palette (under Colormap) and the 7 colours under Histogram:

identify -verbose YourImage.png

Sample Output

Image: YourImage.png
  Format: PNG (Portable Network Graphics)
  Mime type: image/png
  Class: PseudoClass
  Geometry: 625x468+0+0
  Units: Undefined
  Colorspace: sRGB
  Type: Palette
  Base type: Undefined
  Endianess: Undefined
  Depth: 8-bit
  Channel depth:
    Red: 8-bit
    Green: 8-bit
    Blue: 8-bit
  Channel statistics:
    Pixels: 292500
    Red:
      min: 0  (0)
      max: 224 (0.878431)
      mean: 64.0019 (0.250988)
      standard deviation: 87.3558 (0.342572)
      kurtosis: -1.23408
      skewness: 0.749894
      entropy: 0.746999
    Green:
      min: 0  (0)
      max: 224 (0.878431)
      mean: 37.0024 (0.145107)
      standard deviation: 68.3573 (0.268068)
      kurtosis: 1.38689
      skewness: 1.64292
      entropy: 0.594363
    Blue:
      min: 0  (0)
      max: 192 (0.752941)
      mean: 61.0934 (0.239582)
      standard deviation: 80.7328 (0.316599)
      kurtosis: -1.34383
      skewness: 0.676587
      entropy: 0.842137
  Image statistics:
    Overall:
      min: 0  (0)
      max: 224 (0.878431)
      mean: 54.0326 (0.211892)
      standard deviation: 78.8153 (0.30908)
      kurtosis: -0.752325
      skewness: 0.974955
      entropy: 0.727833
  Colors: 7
  Histogram:
    163911: (  0,  0,  0) #000000 black
     16626: (  0,  0,128) #000080 navy
      5261: (  0,128,  0) #008000 green
     37681: (128,128,192) #8080C0 srgb(128,128,192)
     36210: (192,  0,128) #C00080 srgb(192,0,128)
     12644: (192, 64,  0) #C04000 srgb(192,64,0)
     20167: (224,224,192) #E0E0C0 srgb(224,224,192)
  Colormap entries: 256
  Colormap:
         0: (  0,  0,  0,255) #000000FF black
         1: (128,  0,  0,255) #800000FF maroon
         2: (  0,128,  0,255) #008000FF green
         3: (128,128,  0,255) #808000FF olive
         4: (  0,  0,128,255) #000080FF navy
         5: (128,  0,128,255) #800080FF purple
         6: (  0,128,128,255) #008080FF teal
         7: (128,128,128,255) #808080FF fractal
         8: ( 64,  0,  0,255) #400000FF srgba(64,0,0,1)
         9: (192,  0,  0,255) #C00000FF srgba(192,0,0,1)
        10: ( 64,128,  0,255) #408000FF srgba(64,128,0,1)
        11: (192,128,  0,255) #C08000FF srgba(192,128,0,1)
        12: ( 64,  0,128,255) #400080FF srgba(64,0,128,1)
        13: (192,  0,128,255) #C00080FF srgba(192,0,128,1)
        14: ( 64,128,128,255) #408080FF srgba(64,128,128,1)
        15: (192,128,128,255) #C08080FF srgba(192,128,128,1)
        16: (  0, 64,  0,255) #004000FF srgba(0,64,0,1)
        17: (128, 64,  0,255) #804000FF srgba(128,64,0,1)
        18: (  0,192,  0,255) #00C000FF srgba(0,192,0,1)
        19: (128,192,  0,255) #80C000FF srgba(128,192,0,1)
        20: (  0, 64,128,255) #004080FF srgba(0,64,128,1)
        21: (128, 64,128,255) #804080FF srgba(128,64,128,1)
        22: (  0,192,128,255) #00C080FF srgba(0,192,128,1)
        23: (128,192,128,255) #80C080FF srgba(128,192,128,1)
        24: ( 64, 64,  0,255) #404000FF srgba(64,64,0,1)
        25: (192, 64,  0,255) #C04000FF srgba(192,64,0,1)
        26: ( 64,192,  0,255) #40C000FF srgba(64,192,0,1)
        27: (192,192,  0,255) #C0C000FF srgba(192,192,0,1)
        28: ( 64, 64,128,255) #404080FF srgba(64,64,128,1)
        29: (192, 64,128,255) #C04080FF srgba(192,64,128,1)
        30: ( 64,192,128,255) #40C080FF srgba(64,192,128,1)
        31: (192,192,128,255) #C0C080FF srgba(192,192,128,1)
        32: (  0,  0, 64,255) #000040FF srgba(0,0,64,1)
        33: (128,  0, 64,255) #800040FF srgba(128,0,64,1)
        34: (  0,128, 64,255) #008040FF srgba(0,128,64,1)
        35: (128,128, 64,255) #808040FF srgba(128,128,64,1)
        36: (  0,  0,192,255) #0000C0FF srgba(0,0,192,1)
        37: (128,  0,192,255) #8000C0FF srgba(128,0,192,1)
        38: (  0,128,192,255) #0080C0FF srgba(0,128,192,1)
        39: (128,128,192,255) #8080C0FF srgba(128,128,192,1)
        40: ( 64,  0, 64,255) #400040FF srgba(64,0,64,1)
        41: (192,  0, 64,255) #C00040FF srgba(192,0,64,1)
        42: ( 64,128, 64,255) #408040FF srgba(64,128,64,1)
        43: (192,128, 64,255) #C08040FF srgba(192,128,64,1)
        44: ( 64,  0,192,255) #4000C0FF srgba(64,0,192,1)
        45: (192,  0,192,255) #C000C0FF srgba(192,0,192,1)
        46: ( 64,128,192,255) #4080C0FF srgba(64,128,192,1)
        47: (192,128,192,255) #C080C0FF srgba(192,128,192,1)
        48: (  0, 64, 64,255) #004040FF srgba(0,64,64,1)
        49: (128, 64, 64,255) #804040FF srgba(128,64,64,1)
        50: (  0,192, 64,255) #00C040FF srgba(0,192,64,1)
        51: (128,192, 64,255) #80C040FF srgba(128,192,64,1)
        52: (  0, 64,192,255) #0040C0FF srgba(0,64,192,1)
        53: (128, 64,192,255) #8040C0FF srgba(128,64,192,1)
        54: (  0,192,192,255) #00C0C0FF srgba(0,192,192,1)
        55: (128,192,192,255) #80C0C0FF srgba(128,192,192,1)
        56: ( 64, 64, 64,255) #404040FF grey25
        57: (192, 64, 64,255) #C04040FF srgba(192,64,64,1)
        58: ( 64,192, 64,255) #40C040FF srgba(64,192,64,1)
        59: (192,192, 64,255) #C0C040FF srgba(192,192,64,1)
        60: ( 64, 64,192,255) #4040C0FF srgba(64,64,192,1)
        61: (192, 64,192,255) #C040C0FF srgba(192,64,192,1)
        62: ( 64,192,192,255) #40C0C0FF srgba(64,192,192,1)
        63: (192,192,192,255) #C0C0C0FF silver
        64: ( 32,  0,  0,255) #200000FF srgba(32,0,0,1)
        65: (160,  0,  0,255) #A00000FF srgba(160,0,0,1)
        66: ( 32,128,  0,255) #208000FF srgba(32,128,0,1)
        67: (160,128,  0,255) #A08000FF srgba(160,128,0,1)
        68: ( 32,  0,128,255) #200080FF srgba(32,0,128,1)
        69: (160,  0,128,255) #A00080FF srgba(160,0,128,1)
        70: ( 32,128,128,255) #208080FF srgba(32,128,128,1)
        71: (160,128,128,255) #A08080FF srgba(160,128,128,1)
        72: ( 96,  0,  0,255) #600000FF srgba(96,0,0,1)
        73: (224,  0,  0,255) #E00000FF srgba(224,0,0,1)
        74: ( 96,128,  0,255) #608000FF srgba(96,128,0,1)
        75: (224,128,  0,255) #E08000FF srgba(224,128,0,1)
        76: ( 96,  0,128,255) #600080FF srgba(96,0,128,1)
        77: (224,  0,128,255) #E00080FF srgba(224,0,128,1)
        78: ( 96,128,128,255) #608080FF srgba(96,128,128,1)
        79: (224,128,128,255) #E08080FF srgba(224,128,128,1)
        80: ( 32, 64,  0,255) #204000FF srgba(32,64,0,1)
        81: (160, 64,  0,255) #A04000FF srgba(160,64,0,1)
        82: ( 32,192,  0,255) #20C000FF srgba(32,192,0,1)
        83: (160,192,  0,255) #A0C000FF srgba(160,192,0,1)
        84: ( 32, 64,128,255) #204080FF srgba(32,64,128,1)
        85: (160, 64,128,255) #A04080FF srgba(160,64,128,1)
        86: ( 32,192,128,255) #20C080FF srgba(32,192,128,1)
        87: (160,192,128,255) #A0C080FF srgba(160,192,128,1)
        88: ( 96, 64,  0,255) #604000FF srgba(96,64,0,1)
        89: (224, 64,  0,255) #E04000FF srgba(224,64,0,1)
        90: ( 96,192,  0,255) #60C000FF srgba(96,192,0,1)
        91: (224,192,  0,255) #E0C000FF srgba(224,192,0,1)
        92: ( 96, 64,128,255) #604080FF srgba(96,64,128,1)
        93: (224, 64,128,255) #E04080FF srgba(224,64,128,1)
        94: ( 96,192,128,255) #60C080FF srgba(96,192,128,1)
        95: (224,192,128,255) #E0C080FF srgba(224,192,128,1)
        96: ( 32,  0, 64,255) #200040FF srgba(32,0,64,1)
        97: (160,  0, 64,255) #A00040FF srgba(160,0,64,1)
        98: ( 32,128, 64,255) #208040FF srgba(32,128,64,1)
        99: (160,128, 64,255) #A08040FF srgba(160,128,64,1)
       100: ( 32,  0,192,255) #2000C0FF srgba(32,0,192,1)
       101: (160,  0,192,255) #A000C0FF srgba(160,0,192,1)
       102: ( 32,128,192,255) #2080C0FF srgba(32,128,192,1)
       103: (160,128,192,255) #A080C0FF srgba(160,128,192,1)
       104: ( 96,  0, 64,255) #600040FF srgba(96,0,64,1)
       105: (224,  0, 64,255) #E00040FF srgba(224,0,64,1)
       106: ( 96,128, 64,255) #608040FF srgba(96,128,64,1)
       107: (224,128, 64,255) #E08040FF srgba(224,128,64,1)
       108: ( 96,  0,192,255) #6000C0FF srgba(96,0,192,1)
       109: (224,  0,192,255) #E000C0FF srgba(224,0,192,1)
       110: ( 96,128,192,255) #6080C0FF srgba(96,128,192,1)
       111: (224,128,192,255) #E080C0FF srgba(224,128,192,1)
       112: ( 32, 64, 64,255) #204040FF srgba(32,64,64,1)
       113: (160, 64, 64,255) #A04040FF srgba(160,64,64,1)
       114: ( 32,192, 64,255) #20C040FF srgba(32,192,64,1)
       115: (160,192, 64,255) #A0C040FF srgba(160,192,64,1)
       116: ( 32, 64,192,255) #2040C0FF srgba(32,64,192,1)
       117: (160, 64,192,255) #A040C0FF srgba(160,64,192,1)
       118: ( 32,192,192,255) #20C0C0FF srgba(32,192,192,1)
       119: (160,192,192,255) #A0C0C0FF srgba(160,192,192,1)
       120: ( 96, 64, 64,255) #604040FF srgba(96,64,64,1)
       121: (224, 64, 64,255) #E04040FF srgba(224,64,64,1)
       122: ( 96,192, 64,255) #60C040FF srgba(96,192,64,1)
       123: (224,192, 64,255) #E0C040FF srgba(224,192,64,1)
       124: ( 96, 64,192,255) #6040C0FF srgba(96,64,192,1)
       125: (224, 64,192,255) #E040C0FF srgba(224,64,192,1)
       126: ( 96,192,192,255) #60C0C0FF srgba(96,192,192,1)
       127: (224,192,192,255) #E0C0C0FF srgba(224,192,192,1)
       128: (  0, 32,  0,255) #002000FF srgba(0,32,0,1)
       129: (128, 32,  0,255) #802000FF srgba(128,32,0,1)
       130: (  0,160,  0,255) #00A000FF srgba(0,160,0,1)
       131: (128,160,  0,255) #80A000FF srgba(128,160,0,1)
       132: (  0, 32,128,255) #002080FF srgba(0,32,128,1)
       133: (128, 32,128,255) #802080FF srgba(128,32,128,1)
       134: (  0,160,128,255) #00A080FF srgba(0,160,128,1)
       135: (128,160,128,255) #80A080FF srgba(128,160,128,1)
       136: ( 64, 32,  0,255) #402000FF srgba(64,32,0,1)
       137: (192, 32,  0,255) #C02000FF srgba(192,32,0,1)
       138: ( 64,160,  0,255) #40A000FF srgba(64,160,0,1)
       139: (192,160,  0,255) #C0A000FF srgba(192,160,0,1)
       140: ( 64, 32,128,255) #402080FF srgba(64,32,128,1)
       141: (192, 32,128,255) #C02080FF srgba(192,32,128,1)
       142: ( 64,160,128,255) #40A080FF srgba(64,160,128,1)
       143: (192,160,128,255) #C0A080FF srgba(192,160,128,1)
       144: (  0, 96,  0,255) #006000FF srgba(0,96,0,1)
       145: (128, 96,  0,255) #806000FF srgba(128,96,0,1)
       146: (  0,224,  0,255) #00E000FF srgba(0,224,0,1)
       147: (128,224,  0,255) #80E000FF srgba(128,224,0,1)
       148: (  0, 96,128,255) #006080FF srgba(0,96,128,1)
       149: (128, 96,128,255) #806080FF srgba(128,96,128,1)
       150: (  0,224,128,255) #00E080FF srgba(0,224,128,1)
       151: (128,224,128,255) #80E080FF srgba(128,224,128,1)
       152: ( 64, 96,  0,255) #406000FF srgba(64,96,0,1)
       153: (192, 96,  0,255) #C06000FF srgba(192,96,0,1)
       154: ( 64,224,  0,255) #40E000FF srgba(64,224,0,1)
       155: (192,224,  0,255) #C0E000FF srgba(192,224,0,1)
       156: ( 64, 96,128,255) #406080FF srgba(64,96,128,1)
       157: (192, 96,128,255) #C06080FF srgba(192,96,128,1)
       158: ( 64,224,128,255) #40E080FF srgba(64,224,128,1)
       159: (192,224,128,255) #C0E080FF srgba(192,224,128,1)
       160: (  0, 32, 64,255) #002040FF srgba(0,32,64,1)
       161: (128, 32, 64,255) #802040FF srgba(128,32,64,1)
       162: (  0,160, 64,255) #00A040FF srgba(0,160,64,1)
       163: (128,160, 64,255) #80A040FF srgba(128,160,64,1)
       164: (  0, 32,192,255) #0020C0FF srgba(0,32,192,1)
       165: (128, 32,192,255) #8020C0FF srgba(128,32,192,1)
       166: (  0,160,192,255) #00A0C0FF srgba(0,160,192,1)
       167: (128,160,192,255) #80A0C0FF srgba(128,160,192,1)
       168: ( 64, 32, 64,255) #402040FF srgba(64,32,64,1)
       169: (192, 32, 64,255) #C02040FF srgba(192,32,64,1)
       170: ( 64,160, 64,255) #40A040FF srgba(64,160,64,1)
       171: (192,160, 64,255) #C0A040FF srgba(192,160,64,1)
       172: ( 64, 32,192,255) #4020C0FF srgba(64,32,192,1)
       173: (192, 32,192,255) #C020C0FF srgba(192,32,192,1)
       174: ( 64,160,192,255) #40A0C0FF srgba(64,160,192,1)
       175: (192,160,192,255) #C0A0C0FF srgba(192,160,192,1)
       176: (  0, 96, 64,255) #006040FF srgba(0,96,64,1)
       177: (128, 96, 64,255) #806040FF srgba(128,96,64,1)
       178: (  0,224, 64,255) #00E040FF srgba(0,224,64,1)
       179: (128,224, 64,255) #80E040FF srgba(128,224,64,1)
       180: (  0, 96,192,255) #0060C0FF srgba(0,96,192,1)
       181: (128, 96,192,255) #8060C0FF srgba(128,96,192,1)
       182: (  0,224,192,255) #00E0C0FF srgba(0,224,192,1)
       183: (128,224,192,255) #80E0C0FF srgba(128,224,192,1)
       184: ( 64, 96, 64,255) #406040FF srgba(64,96,64,1)
       185: (192, 96, 64,255) #C06040FF srgba(192,96,64,1)
       186: ( 64,224, 64,255) #40E040FF srgba(64,224,64,1)
       187: (192,224, 64,255) #C0E040FF srgba(192,224,64,1)
       188: ( 64, 96,192,255) #4060C0FF srgba(64,96,192,1)
       189: (192, 96,192,255) #C060C0FF srgba(192,96,192,1)
       190: ( 64,224,192,255) #40E0C0FF srgba(64,224,192,1)
       191: (192,224,192,255) #C0E0C0FF srgba(192,224,192,1)
       192: ( 32, 32,  0,255) #202000FF srgba(32,32,0,1)
       193: (160, 32,  0,255) #A02000FF srgba(160,32,0,1)
       194: ( 32,160,  0,255) #20A000FF srgba(32,160,0,1)
       195: (160,160,  0,255) #A0A000FF srgba(160,160,0,1)
       196: ( 32, 32,128,255) #202080FF srgba(32,32,128,1)
       197: (160, 32,128,255) #A02080FF srgba(160,32,128,1)
       198: ( 32,160,128,255) #20A080FF srgba(32,160,128,1)
       199: (160,160,128,255) #A0A080FF srgba(160,160,128,1)
       200: ( 96, 32,  0,255) #602000FF srgba(96,32,0,1)
       201: (224, 32,  0,255) #E02000FF srgba(224,32,0,1)
       202: ( 96,160,  0,255) #60A000FF srgba(96,160,0,1)
       203: (224,160,  0,255) #E0A000FF srgba(224,160,0,1)
       204: ( 96, 32,128,255) #602080FF srgba(96,32,128,1)
       205: (224, 32,128,255) #E02080FF srgba(224,32,128,1)
       206: ( 96,160,128,255) #60A080FF srgba(96,160,128,1)
       207: (224,160,128,255) #E0A080FF srgba(224,160,128,1)
       208: ( 32, 96,  0,255) #206000FF srgba(32,96,0,1)
       209: (160, 96,  0,255) #A06000FF srgba(160,96,0,1)
       210: ( 32,224,  0,255) #20E000FF srgba(32,224,0,1)
       211: (160,224,  0,255) #A0E000FF srgba(160,224,0,1)
       212: ( 32, 96,128,255) #206080FF srgba(32,96,128,1)
       213: (160, 96,128,255) #A06080FF srgba(160,96,128,1)
       214: ( 32,224,128,255) #20E080FF srgba(32,224,128,1)
       215: (160,224,128,255) #A0E080FF srgba(160,224,128,1)
       216: ( 96, 96,  0,255) #606000FF srgba(96,96,0,1)
       217: (224, 96,  0,255) #E06000FF srgba(224,96,0,1)
       218: ( 96,224,  0,255) #60E000FF srgba(96,224,0,1)
       219: (224,224,  0,255) #E0E000FF srgba(224,224,0,1)
       220: ( 96, 96,128,255) #606080FF srgba(96,96,128,1)
       221: (224, 96,128,255) #E06080FF srgba(224,96,128,1)
       222: ( 96,224,128,255) #60E080FF srgba(96,224,128,1)
       223: (224,224,128,255) #E0E080FF srgba(224,224,128,1)
       224: ( 32, 32, 64,255) #202040FF srgba(32,32,64,1)
       225: (160, 32, 64,255) #A02040FF srgba(160,32,64,1)
       226: ( 32,160, 64,255) #20A040FF srgba(32,160,64,1)
       227: (160,160, 64,255) #A0A040FF srgba(160,160,64,1)
       228: ( 32, 32,192,255) #2020C0FF srgba(32,32,192,1)
       229: (160, 32,192,255) #A020C0FF srgba(160,32,192,1)
       230: ( 32,160,192,255) #20A0C0FF srgba(32,160,192,1)
       231: (160,160,192,255) #A0A0C0FF srgba(160,160,192,1)
       232: ( 96, 32, 64,255) #602040FF srgba(96,32,64,1)
       233: (224, 32, 64,255) #E02040FF srgba(224,32,64,1)
       234: ( 96,160, 64,255) #60A040FF srgba(96,160,64,1)
       235: (224,160, 64,255) #E0A040FF srgba(224,160,64,1)
       236: ( 96, 32,192,255) #6020C0FF srgba(96,32,192,1)
       237: (224, 32,192,255) #E020C0FF srgba(224,32,192,1)
       238: ( 96,160,192,255) #60A0C0FF srgba(96,160,192,1)
       239: (224,160,192,255) #E0A0C0FF srgba(224,160,192,1)
       240: ( 32, 96, 64,255) #206040FF srgba(32,96,64,1)
       241: (160, 96, 64,255) #A06040FF srgba(160,96,64,1)
       242: ( 32,224, 64,255) #20E040FF srgba(32,224,64,1)
       243: (160,224, 64,255) #A0E040FF srgba(160,224,64,1)
       244: ( 32, 96,192,255) #2060C0FF srgba(32,96,192,1)
       245: (160, 96,192,255) #A060C0FF srgba(160,96,192,1)
       246: ( 32,224,192,255) #20E0C0FF srgba(32,224,192,1)
       247: (160,224,192,255) #A0E0C0FF srgba(160,224,192,1)
       248: ( 96, 96, 64,255) #606040FF srgba(96,96,64,1)
       249: (224, 96, 64,255) #E06040FF srgba(224,96,64,1)
       250: ( 96,224, 64,255) #60E040FF srgba(96,224,64,1)
       251: (224,224, 64,255) #E0E040FF srgba(224,224,64,1)
       252: ( 96, 96,192,255) #6060C0FF srgba(96,96,192,1)
       253: (224, 96,192,255) #E060C0FF srgba(224,96,192,1)
       254: ( 96,224,192,255) #60E0C0FF srgba(96,224,192,1)
       255: (224,224,192,255) #E0E0C0FF srgba(224,224,192,1)
  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)
  Matte color: grey74
  Background color: white
  Border color: srgb(223,223,223)
  Transparent color: none
  Interlace: None
  Intensity: Undefined
  Compose: Over
  Page geometry: 625x468+0+0
  Dispose: Undefined
  Iterations: 0
  Compression: Zip
  Orientation: Undefined
  Properties:
    date:create: 2020-01-21T11:05:35+00:00
    date:modify: 2020-01-21T11:05:35+00:00
    png:IHDR.bit-depth-orig: 8
    png:IHDR.bit_depth: 8
    png:IHDR.color-type-orig: 3
    png:IHDR.color_type: 3 (Indexed)
    png:IHDR.interlace_method: 0 (Not interlaced)
    png:IHDR.width,height: 625, 468
    png:PLTE.number_colors: 256
    png:sRGB: intent=0 (Perceptual Intent)
    signature: 12c7d4ffffd28f23d431ea875c618757f305577bf8aec476ec418728c161d3de8e
  Artifacts:
    verbose: true
  Tainted: False
  Filesize: 6497B
  Number pixels: 292500
  Pixels per second: 54.4078MP
  User time: 0.000u
  Elapsed time: 0:01.005
  Version: ImageMagick 7.0.9-6 Q16 x86_64 2019-11-27 https://imagemagick.org

If ImageMagick is too big a dependency for you, you can also use the much smaller pngcheck like this:

pngcheck -vp YourImage.png

Sample Output

File: YourImage.png (6497 bytes)
  chunk IHDR at offset 0x0000c, length 13
    625 x 468 image, 8-bit palette, non-interlaced
  chunk PLTE at offset 0x00025, length 768: 256 palette entries
      0:  (  0,  0,  0) = (0x00,0x00,0x00)
      1:  (128,  0,  0) = (0x80,0x00,0x00)
      2:  (  0,128,  0) = (0x00,0x80,0x00)
      3:  (128,128,  0) = (0x80,0x80,0x00)
      4:  (  0,  0,128) = (0x00,0x00,0x80)
      5:  (128,  0,128) = (0x80,0x00,0x80)
      6:  (  0,128,128) = (0x00,0x80,0x80)
      7:  (128,128,128) = (0x80,0x80,0x80)
      8:  ( 64,  0,  0) = (0x40,0x00,0x00)
      9:  (192,  0,  0) = (0xc0,0x00,0x00)
     10:  ( 64,128,  0) = (0x40,0x80,0x00)
     11:  (192,128,  0) = (0xc0,0x80,0x00)
     12:  ( 64,  0,128) = (0x40,0x00,0x80)
     13:  (192,  0,128) = (0xc0,0x00,0x80)
     14:  ( 64,128,128) = (0x40,0x80,0x80)
     15:  (192,128,128) = (0xc0,0x80,0x80)
     16:  (  0, 64,  0) = (0x00,0x40,0x00)
     17:  (128, 64,  0) = (0x80,0x40,0x00)
     18:  (  0,192,  0) = (0x00,0xc0,0x00)
     19:  (128,192,  0) = (0x80,0xc0,0x00)
     20:  (  0, 64,128) = (0x00,0x40,0x80)
     21:  (128, 64,128) = (0x80,0x40,0x80)
     22:  (  0,192,128) = (0x00,0xc0,0x80)
     23:  (128,192,128) = (0x80,0xc0,0x80)
     24:  ( 64, 64,  0) = (0x40,0x40,0x00)
     25:  (192, 64,  0) = (0xc0,0x40,0x00)
    ...
    ...
    251:  (224,224, 64) = (0xe0,0xe0,0x40)
    252:  ( 96, 96,192) = (0x60,0x60,0xc0)
    253:  (224, 96,192) = (0xe0,0x60,0xc0)
    254:  ( 96,224,192) = (0x60,0xe0,0xc0)
    255:  (224,224,192) = (0xe0,0xe0,0xc0)
  chunk IDAT at offset 0x00331, length 5660
    zlib: deflated, 32K window, default compression
  chunk IEND at offset 0x01959, length 0
No errors detected in p.png (4 chunks, 97.8% compression).

I see you want to write your results out with the same palette as you started with. I don't believe OpenCV can do that, but you could write your results as a normal, unpalettised PNG with OpenCV and then remap that to your original palette with ImageMagick in Terminal.

So, we can make a 7x1 pixel image of the colours in your palette like this:

magick -size 1x1 xc:black xc:navy xc:green xc:'#8080C0' xc:'#C00080' xc:'#C04000' xc:'#E0E0C0' +append palette.png

That looks like this when enlarged:

You can now make any results images you like with OpenCV, but convert (i.e. "remap") them to that original palette like this:

magick OpenCVresult.png +dither -remap palette.png result.png

If your ImageMagick is v6, use convert in place of magick.

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