Is std zlib usable for png

ericlang · July 23, 2025, 1:27pm

I know png images have their zlib compression scheme, which caused me some headaches in the past in C#. because there seems to be a tiny difference in definitions of zlib.
Is the std version (zlib.zig) the one which is usable for compressing png images?

ForeverZer0 · July 23, 2025, 6:02pm

I am unsure, though so long as the DEFLATE algorithm is compatible, it should be trivial to workaround. If memory serves me correct, Zlib is just a header, DEFLATE compressed data, and then a checksum. It has been a long time, but I seem to remember that there was something different with .NET’s implementation than what is commonly seen in the wild, and I had to implement a basic ZlibStream class when doing some Minecraft NBT project years ago.

zigimg uses a pure Zig reader/writer implementation, so it might be worth a peek to see how they did it. If they don’t use std.zlib, it may be an indicator that it suffers from the same issue.

ericlang · July 23, 2025, 6:13pm

github.com/zigimg/zigimg

src/formats/png/zlib_compressor.zig

master

const std = @import("std");
const io = std.io;
const deflate = std.compress.flate;

/// Zlib Compressor (Deflate) with a writer interface
pub fn ZlibCompressor(comptime WriterType: type) type {
    return struct {
        raw_writer: WriterType,
        compressor: deflate.Compressor(WriterType),
        adler: std.hash.Adler32,

        const Self = @This();

        // TODO: find why doing it an other way segfaults
        /// Inits a zlibcompressor
        /// This is made this way because not doing it in place segfaults for a reason
        pub fn init(self: *Self, stream: WriterType) !void {
            self.raw_writer = stream;
            self.compressor = try deflate.compressor(self.raw_writer, .{});
            self.adler = std.hash.Adler32.init();

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They use I think this. If I remember correctly the only difference was a checksum thingy.

ForeverZer0 · July 23, 2025, 6:34pm

I believe so, and TIL Zig has an adler32 implementation in std, which makes it even easier if you opt for a custom implementation. This does make me curious what the incompatibility issue is with Zlib implementations, as I assume zigimg didn’t implement this for fun.

ericlang · July 23, 2025, 7:39pm

Most programs open / show “unoffical” png’s without a problem. That is why I was so confused a few years ago. It is indeed a tiny difference. Yes it was this adler thing.
Telegram for example requires a fully qualified png.

Thanks

adria · July 24, 2025, 11:21am

Oh I didn’t know the std lib provided those functions.
Over the last few weeks I wrote my own PNG encoder/decoder over at

github.com/bfactory-ai/zignal

src/deflate.zig

master

//! Pure Zig implementation of DEFLATE compression and decompression (RFC 1951)
//! Used by PNG for IDAT chunk compression/decompression

const std = @import("std");
const assert = std.debug.assert;
const Allocator = std.mem.Allocator;
const ArrayList = std.ArrayList;

// Fixed Huffman code lengths for literal/length alphabet (RFC 1951)
const FIXED_LITERAL_LENGTHS = [_]u8{
    8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, // 0-15
    8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, // 16-31
    8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, // 32-47
    8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, // 48-63
    8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, // 64-79
    8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, // 80-95
    8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, // 96-111
    8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, // 112-127
    8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, // 128-143
    9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, // 144-159

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I will see if I can remove some stuff in favor of the std lib. But I’m worried it might break. Right now it’s pretty self contained.

theo · July 24, 2025, 11:43am

I would keep it, if you’re confident of its correctness. There’s nothing wrong with reimplementing certain functions that exist in the std lib for your own purposes.

ericlang · July 24, 2025, 12:40pm

I see in deflate.zig

// Compression algorithm explained in rfc-1951 (slightly edited for this case)

adria · January 28, 2026, 11:18am

I guess I can now confidently say that the answer to the original question is yes.