Reads the codestream's header without decoding the samples.
public bool TryReadHeader(ReadOnlySpan<byte> encoded, ImageFilterContext context, scoped out ImageHeader header)
The data the filter would decode.
The image's dictionary values, limits and the filter context.
The image's size and components as the codestream declares them.
false when the header cannot be read; DecodeImage(ReadOnlyMemory<Byte>, ImageFilterContext) is then not called.ITU-T T.81 §B.2.2 (frame header: X, Y, Nf, P); ISO 32000-2 §8.9.5.1 Table 87 (always 8 bits per component). The header says the image decodes at 1/2, 1/4 and 1/8 too (MaxReduction 3).