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EBM Format Specification

EBM files contain compressed bitmaps and are used exclusively to represent guild emblems.

Prerequisites

There really isn't much to read up on here:

  • Bitmap (BMP) is one of the standard image formats used in the RO client
  • DEFLATE is a standard compression algorithm and serves to reduce the file size on disk

Overview

EBM files are what the game creates when players upload regular image files as their guild emblem. They are always stored in the _tmpEmblem folder, which I'd call "emblem cache".

The emblems are decompressed and can then be displayed in only a few places:

  • Next to the character and guild name when hovering over a player character's sprite
  • In the Guild management UI that also allows uploading new emblems
  • On the guild flag model, when a guild has conquered the corresponding castle

Compression

The client simply applies the INFLATE (for decompressing) and DEFLATE(for compressing) standard algorithms, provided by zlib, to the raw bitmaps to convert from/to EBM.

This makes obtaining the original image file trivial as long as zlib can be used directly:

const fs = require('fs');
const zlib = require("zlib");

const SOME_EBM_PATH = "myEmblem.ebm";

const buffer = fs.readFileSync(SOME_EBM_PATH);
const bitmap = zlib.inflateSync(buffer);

fs.writeFileSync(SOME_EBM_PATH + ".bmp", bitmap);

Example: Decompressing EBM files using Node.js' builtin zlib module

See also