SharpAstro.Pnm 3.32.1561

SharpAstro.Pnm

Pure-managed Netpbm reading and writing for .NET 10, written from the Netpbm format documentation (pbm, pgm, ppm, pam). The facade (SharpAstro.Codecs) sniffs P1..P7 and decodes through it. Writing is PnmWriter.

Reads every member of the family:

• P1/P4 PBM (bitmap, 1 = black), P2/P5 PGM, P3/P6 PPM, in plain (ASCII) and raw form; plain P1 may pack its digits with no separators.
• P7 PAM with TUPLTYPE BLACKANDWHITE, GRAYSCALE, RGB and their _ALPHA forms (or by DEPTH when absent). CMYK and other tuple types are refused.
• maxval 1..65535; above 255 raw samples are two bytes, big-endian.
• # comments wherever the format allows them.

A sample above maxval, a truncated raster or an out-of-range header throws InvalidDataException. A declared size over 2^28 samples (a caller-chosen ceiling) is refused before anything is allocated, and a raw raster must fit in the input.

using SharpAstro.Pnm;

PnmImage image = PnmReader.Read(File.ReadAllBytes("image.pgm"));
Console.WriteLine($"{image.Width} x {image.Height}, {image.Channels} channel(s), maxval {image.MaxValue}");
ushort sample = image.Samples[0]; // native value, 0..MaxValue

Read / write matrix

maxval 1..65535 both ways; above 255 a raw sample is two bytes, big-endian.

Writing

using SharpAstro.Pnm;

byte[] file = PnmWriter.Encode(rgb8, width, height, PnmTupleType.Rgb);               // P6, maxval 255
byte[] wide = PnmWriter.Encode(gray16, width, height, PnmTupleType.Grayscale);       // P5, maxval 65535
byte[] pam  = PnmWriter.Encode(rgba8, width, height, PnmTupleType.RgbAlpha);         // P7 RGB_ALPHA
PnmWriter.Write(stream, gray16, width, height, PnmTupleType.Grayscale, maxValue: 1023, pam: true);

Grey and RGB go out as P5 / P6 unless pam: true asks for P7; anything with alpha is always P7, since the PGM/PPM headers cannot carry it. Output is the raw form with Netpbm's own header layout and no comment. A short buffer, a tuple type that cannot be written (BlackAndWhite*), a maxval outside 1..65535 or a sample above it throws ArgumentException.

The writer never scales. Samples are the file's native values: with maxValue: 1023 they are 0..1023 and are written as they are. Scaling is a decision for the caller, and the reader's facade rule is the one to mirror (below): a maxval below the type's full range is stretched to fill it, round(v * full / maxval), which is what ImageMagick does too. Only maxval 255 and 65535 are written without any such question, and those are the two the tests compare against Magick.NET pixel for pixel; every other maxval is verified by our own reader round trip.

ImageMagick agrees sample for sample, except where it departs from the Netpbm documents: a binary raster with maxval 128..254 or 32768..65534 is read as full-scale samples, without dividing by maxval, and a 1-bit grey+alpha PAM comes back with its alpha inverted. This reader follows the documents there.

PnmReader returns the file's native samples. Through the facade (PnmImageDecoder), they are scaled to fill UInt8 (maxval ⇐ 255) or UInt16, with the rounding ImageMagick uses, so a maxval 15 file decodes to the pixels ImageMagick gives.