PnmEncoder Class

With PnmPixelLayout.Auto the variant follows the pixel format: gray pixel formats are written as a portable graymap (P5, or P2 with PnmEncoding.Plain), pixel formats without alpha as a portable pixmap (P6, or P3), and pixel formats with alpha as a portable arbitrary map (P7, tuple type RGB_ALPHA). MAXVAL is 255 for 8-bit pixel formats and 65,535 for 16-bit ones, so the output is always lossless.

A portable bitmap (P4, or P1 with PnmEncoding.Plain) is only written on request, with PnmPixelLayout.Bitmap. It is lossless too: every pixel must be exactly black or exactly white, and any other pixel is rejected with an UnsupportedImageFeatureException before anything is written. A portable bitmap is read back as PixelFormat.Gray8 with one bit per component.

An encoder never looks at the file extension. Only the save and writer overloads that take a path and no encoder do: they map .pbm to PnmPixelLayout.Bitmap and the other Netpbm extensions to the default settings.

Neither the plain forms nor a portable bitmap store alpha (PAM has no plain form): PnmEncoding.Plain or PnmPixelLayout.Bitmap with a pixel format that has alpha discards alpha and therefore requires PnmEncoder.BackgroundColor.

PNM stores exactly one frame: animated images and images with a poster frame are rejected with an UnsupportedImageFeatureException; use ImageFrame.Save to save one frame explicitly. Several images may be concatenated in one Netpbm file; this version never writes (nor reads) more than one.

Netpbm files store no metadata: an ICC profile, EXIF, XMP, an orientation other than ExifOrientation.TopLeft, a resolution and text entries follow ImageEncoder.MetadataHandling (rejected by default).