FME unwanted conversion of RGB to palette in PNG writer

Question

I have a pile of TIFF images (aerial photography) that I want to convert to PNG. Band interpretation of my source TIFF files is allways RGB, cp. gdalinfo:

C:\OSGeo4W>gdalinfo -mm -stats s:\ER-GIS-Daten\Luftbild\2020\Lieferung\14_TrueOrthomosaic_RGB_TIFF_500x500m_5cm\330880.tif
Driver: GTiff/GeoTIFF
Files: s:\ER-GIS-Daten\Luftbild\2020\Lieferung\14_TrueOrthomosaic_RGB_TIFF_500x500m_5cm\330880.tif
       s:\ER-GIS-Daten\Luftbild\2020\Lieferung\14_TrueOrthomosaic_RGB_TIFF_500x500m_5cm\330880.tfw
Size is 10000, 10000
Origin = (633000.000000000000000,5488500.000000000000000)
Pixel Size = (0.050000000000000,-0.050000000000000)
Metadata:
  TIFFTAG_DATETIME=2020:05:19 13:57:41
  TIFFTAG_IMAGEDESCRIPTION=OrthoVistaProcessing ImageWriter
  TIFFTAG_RESOLUTIONUNIT=2 (pixels/inch)
  TIFFTAG_SOFTWARE=Adobe Photoshop 21.1 (Windows)
  TIFFTAG_XRESOLUTION=72
  TIFFTAG_YRESOLUTION=72
Image Structure Metadata:
  INTERLEAVE=PIXEL
Corner Coordinates:
Upper Left  (  633000.000, 5488500.000)
Lower Left  (  633000.000, 5488000.000)
Upper Right (  633500.000, 5488500.000)
Lower Right (  633500.000, 5488000.000)
Center      (  633250.000, 5488250.000)
Band 1 Block=10000x1 Type=Byte, ColorInterp=Red
    Computed Min/Max=0.000,255.000
  Minimum=0.000, Maximum=255.000, Mean=255.000, StdDev=0.208
  Metadata:
    STATISTICS_MAXIMUM=255
    STATISTICS_MEAN=254.99975402
    STATISTICS_MINIMUM=0
    STATISTICS_STDDEV=0.20842394174816
    STATISTICS_VALID_PERCENT=100
Band 2 Block=10000x1 Type=Byte, ColorInterp=Green
    Computed Min/Max=0.000,255.000
  Minimum=0.000, Maximum=255.000, Mean=255.000, StdDev=0.214
  Metadata:
    STATISTICS_MAXIMUM=255
    STATISTICS_MEAN=254.99974602
    STATISTICS_MINIMUM=0
    STATISTICS_STDDEV=0.21399129770661
    STATISTICS_VALID_PERCENT=100
Band 3 Block=10000x1 Type=Byte, ColorInterp=Blue
    Computed Min/Max=0.000,255.000
  Minimum=0.000, Maximum=255.000, Mean=255.000, StdDev=0.298
  Metadata:
    STATISTICS_MAXIMUM=255
    STATISTICS_MEAN=254.99963607
    STATISTICS_MINIMUM=0
    STATISTICS_STDDEV=0.29783649466604
    STATISTICS_VALID_PERCENT=100

In some (not all) of the resulting PNG's however, band interpretation turns from source rgb to palette, gdalinfo for the resulting PNG file associated with the above example:

Driver: PNG/Portable Network Graphics
Files: Q:/Luftbilder/2020/0.05/330880.png
       Q:/Luftbilder/2020/0.05/330880.wld
Size is 10000, 10000
Origin = (633000.000000000000000,5488500.000000000000000)
Pixel Size = (0.050000000000000,-0.050000000000000)
Corner Coordinates:
Upper Left  (  633000.000, 5488500.000) 
Lower Left  (  633000.000, 5488000.000) 
Upper Right (  633500.000, 5488500.000) 
Lower Right (  633500.000, 5488000.000) 
Center      (  633250.000, 5488250.000) 
Band 1 Block=10000x1 Type=Byte, ColorInterp=Palette
    Computed Min/Max=0.000,125.000
  Minimum=0.000, Maximum=125.000, Mean=125.000, StdDev=0.101
  Metadata:
    STATISTICS_MAXIMUM=125
    STATISTICS_MEAN=124.99989161
    STATISTICS_MINIMUM=0
    STATISTICS_STDDEV=0.10079413798236
    STATISTICS_VALID_PERCENT=100
  Color Table (RGB with 126 entries)
    0: 0,0,0,255
    1: 0,2,0,255
    2: 2,4,0,255
    3: 3,4,0,255
    4: 10,7,0,255
    5: 21,22,0,255
    6: 52,50,0,255
    7: 61,53,0,255
    8: 67,62,0,255
    9: 71,66,0,255
   10: 76,68,0,255
   11: 77,71,0,255
   12: 81,78,0,255
   13: 82,70,0,255
   14: 82,78,0,255
   15: 83,71,0,255
   16: 84,68,0,255
   17: 85,79,0,255
   18: 87,77,0,255
[...]

The workspace for this is simplest one can think of, GeoTIFF reader, PNG writer:

Why this (randomly) reinterpretation of the bands and adding a palette and how can I overcome this behavior and force PNG writer to allways use RGB interpretation when source data is like this?

Answer 1

The reason for this is that I stumbled upon the PNG Optimization Level-parameter which I set to "Level_1_(recommended)" ("recommended" sounds like a good choice at first sight):

Since some of the resulting images remain RGB and some a converted to palette, my working hypothesis is that if the parameter is set to a value >0 FME tries to estimate if a conversion to palette leads to a significant reduction in file size. So, leaving this to "No_optimization" always results in RGB interpretation provided the source is RGB.

Answer 2

Your answer is correct (and may I say that it was a very good observation). Setting optimization can result in some changes to the data structure.

In particular, it might result in:

• Adding a palette (as in your example)
• Altering a palette (if it has duplicate or unused entries)
• Reducing bit depth (reducing 16-bit to 8-bit if the data allows it)
• Reducing colours (an RGB image with only gray pixels becomes grayscale)
• Removing alpha channel (if all pixels are opaque)

Sometimes reducing colours or palette values can also enable a bit-depth reduction too, so they're not operating in isolation.

I'd guess that most of these could happen at a low level of optimization because they're considered as lossless. That's why level 1 is probably the recommended level. But the default is "no optimization" because we wouldn't want to change your data without your knowledge.

If you think the interface is a bit misleading then let me know and I can ask for changes. Certainly, I'm not sure if "recommended" means "we recommend this more than no optimization" or "we recommend this only if you want optimization".