public static SerializeWorldResult Serialize(SerializeWorldRequest request)
{
// figure out how much data we need and allocate it
var(pngSize, zlibSize) = AllocationSize(request);
var array = new byte[pngSize + zlibSize];
var png = new Span <byte>(array, 0, pngSize);
var zlib = new Span <byte>(array, pngSize, zlibSize);
// CHUNK ORDER: https://www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html#C.Summary-of-standard-chunks
// using constants to prevent lots of slicing on the span, might save half a nanosecond :)
WritePngHeader(png);
WriteIHDRChunk(png.Slice(PngHeaderSize, IHDRSize), request.Width * request.Scale, request.Height * request.Scale);
WriteTEXTChunk(png.Slice(PngHeaderSize + IHDRSize, TEXTSize));
var written = WriteIDATChunk(png.Slice(PngHeaderSize + IHDRSize + TEXTSize), zlib, request.Width, request.Blocks.Span, request.Palette.Span[0], request.Palette.Span, request.Scale);
WriteIENDChunk(png.Slice(PngHeaderSize + IHDRSize + TEXTSize + written, IENDSize));
var totalSize = PngHeaderSize + IHDRSize + TEXTSize + written + IENDSize;
return(new SerializeWorldResult
{
Array = array,
Png = new Memory <byte>(array, 0, totalSize)
});
}
private static (int PngTarget, int RawPngData) AllocationSize(SerializeWorldRequest request)
{
var artificialWidth = request.Width * request.Scale;
var artificialHeight = request.Height * request.Scale;
// `n` byte(s) per block, plus 1 byte per row (`+ height`).
//
// This addition 1 byte per row comes from the PNG standard,
// the filter type per scanline (https://www.w3.org/TR/2003/REC-PNG-20031110/#4Concepts.EncodingFiltering)
int rawPngStreamLength =
// scale the width & height by what they need to get
// the correct amount of blocks. that will then be multiplied by 4,
// because of each RGBA pixel per block.
(artificialWidth * artificialHeight * 4)
// 1 byte per scanline, and there well be `scale` more scanlines if the
// world is scaled.
+ artificialHeight;
// Quote from https://zlib.net/zlib_tech.html:
// "In the worst possible case, [...] the only expansion is an overhead of five bytes per 16 KB block (about 0.03%),
// plus a one-time overhead of six bytes for the entire stream"
//
// This will allocate enough bytes to satisfy the worst case scenario.
// '16KB' does indeed refer to 16KiB.
const int ZlibBlockSize = 16384;
int zlibWorstCase =
// 6 bytes overhead for the stream
// (2 bytes header, 4 bytes alder32)
6 +
// 5 bytes per 16KiB block
// Integer rounding up: https://stackoverflow.com/a/2422722
// calc blocks
(((rawPngStreamLength + (ZlibBlockSize - 1)) / ZlibBlockSize)
// 5 bytes per block
* 5)
// after the zlib overhead comes the actual data
+ rawPngStreamLength;
// `zlib_worst_case` is the amount of bytes necessary to be allocated, just for compressing the PNG data.
// There must also be bytes allocated for the PNG itself.
//
// These header sizes are directly from the C# implementation:
// https://github.com/SirJosh3917/EEUMinimapApi/blob/c9d25d660ccbab0d8e09fc840e9207ccd5ab4883/EEUMinimapApi/EEUMinimapApi/WorldToImage.cs#L335-L340
const int CPI_PNG_HEADER_SIZE = (1 + 3 + 2 + 2);
const int CPI_PNG_tEXt_SIZE = (4 + 4 + /* strlen('Software') */ 8 + 1 + /* strlen('EEUMinimapApi') */ 13 + 4);
const int CPI_PNG_IHDR_SIZE = (4 + 4 + 4 + 4 + 1 + 1 + 1 + 1 + 1 + 4);
// PLTE unused now.
int CPI_PNG_PLTE_SIZE(int uniqueBlocks) => 0; // (4 + 4 + (3 * (uniqueBlocks /* + 1: implicitly implied Id0 color */ + 1)) + 4);
int CPI_PNG_IDAT_SIZE(int idatSize) => (4 + 4 + idatSize + 4);
const int CPI_PNG_IEND_SIZE = (4 + 4 + 4);
int png_size_with_zlib_worst_case =
CPI_PNG_HEADER_SIZE
+ CPI_PNG_tEXt_SIZE
+ CPI_PNG_IHDR_SIZE
+ CPI_PNG_PLTE_SIZE(request.Palette.Length)
+ CPI_PNG_IDAT_SIZE(zlibWorstCase)
+ CPI_PNG_IEND_SIZE;
return(png_size_with_zlib_worst_case, rawPngStreamLength);
}