Sdcb.SimdCodecs.Pnm 1.2.0

Sdcb.SimdCodecs.Pnm

Pure-managed Netpbm decoding and encoding for .NET 10, written from the Netpbm format pages and the PFM description. It decodes straight into the caller's buffer in any 8-bit pixel layout without allocating, or to the file's native 8-bit, 16-bit or float samples. Its row kernels use 256-bit and 128-bit vectors, and each kernel has a scalar version that gives the same bytes.

using Sdcb.SimdCodecs.Abstractions;
using Sdcb.SimdCodecs.Pnm;

byte[] data = File.ReadAllBytes("image.ppm");
PnmInfo info = PnmDecoder.ReadInfo(data);       // size, format, channels, maximum value
PnmImage image = PnmDecoder.Decode(data);       // 8-bit, 16-bit or float samples, as the file holds them

var bgra = new byte[info.Width * info.Height * 4];
PnmDecoder.DecodeTo(data, bgra, ImagePixelFormat.Bgra32);

The decoder reads these formats:

Malformed data throws InvalidDataException. The Try methods and the facade adapters (PnmImageDecoder, PfmImageDecoder) return false instead. Images above 2^28 pixels are refused, which bounds what a decode allocates.

Encoding

byte[] ppm = PnmEncoder.Encode(rgb, width, height, ImagePixelFormat.Rgb24);           // P6
byte[] pam = PnmEncoder.Encode(rgba, width, height, ImagePixelFormat.Rgba32);         // P7, RGB_ALPHA
byte[] plain = PnmEncoder.Encode(rgb, width, height, ImagePixelFormat.Rgb24,
    new PnmEncodeOptions { Format = PnmFormat.PlainGraymap, MaxValue = 1023 });       // P2
byte[] deep = PnmEncoder.Encode(samples16, width, height, channels: 3);               // P6, maximum 65535
byte[] pfm = PnmEncoder.EncodePfm(floats, width, height, channels: 3);                // PF

Every format can be written, plain or raw. The input can be any 8-bit layout at any stride, 8-bit or 16-bit samples with 1 to 4 channels, or float samples for PFM.

How the encoder converts the input to the chosen format:

• A PBM or PGM takes the input's luma. A PBM pixel is black when the luma is below 128.
• A PPM drops alpha.
• A PAM keeps the channels it is given.

Plain output keeps every line within 70 characters and starts each image row on a new line. A PFM is written little-endian, which is scale −1, with its rows bottom-up as the format stores them. The layout, 16-bit and PFM overloads also have a Stream version that writes through one pooled buffer.

Decisions

The format pages leave these cases open. The table shows what this package does: