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rres is a simple and easy-to-use file-format to package resources

rres has been designed to package game assets (images, fonts, text, audio, models...) into a simple self-contained comprehensive format, easy to read and write, designed to load data in a fast and efficient way.

rres is inspired by the following file-formats: XNB (used by XNA/MonoGame), RIFF, PNG and ZIP.


  1. Design Goals
  2. Usage Benefits
  3. Design History
  4. File Structure
  5. File Header: rresFileHeader
  6. Resource Chunk: rresResourceChunk
    1. Resource Chunk Info: rresResourceChunkInfo
    2. Resource Chunk Data: rresResourceChunkData
    3. Resource Data Type: rresResourceDataType
    4. Resource Chunk: Central Directory: rresCentralDir
  7. Custom Engine Implementation
  8. License

Design Goals

  • Simplicity: rres structure is simple, one file header and multiple resources one after the other. Still, every resource has a small (32 bytes) resource info header with multiple options available.

  • Ease-of-use: rres library to read .rres files is small with only the minimum required functions to read resource data from it's id. It only uses a small set of functions from the standard C library. An rrespacker tool with GUI/CLI is provided to easily create and view .rres files.

  • Flexibility: rres format supports any kind of input file, if data is not classified as a basic data type it can be just packed as raw file data.

  • Portability: rres file format is not tied to a specific engine, the base library just reads the resource data as packed and that data can be mapped to any engine structures. A usage example has been provided with the auxiliary library rres-raylib.h that maps rres data to raylib structures.

  • Security: rres support per-resource compression and encryption if required. Still, despite the support provided by the file format it is up to the user to implement it in the rres packer tool and the engine mapping library. rres does not force any specific compression/encryption algorithm by design

  • Extensibility: rres file format is extensible, supporting new data types, new features and custom data if required.

  • Free and open source: rres is an open spec and a free and open source library. It can be used with the provided tool (rrespacker) or a custom packer/loader can be implemented for any engine. The format description can also be used as a learning material for anyone willing to create its own data packaging file format.

Usage Benefits

There are some important reasons to package game assets data into a format like rres, here are some good reasons to do it.

  • Organization: Avoid thousands of files distributed in hundreds of directories in the final game build. All the game files data could be organized in a single or a few .rres files. It keeps the game directory clean and well-structured.

  • Performance: Keeping all required files for a level/map into a single .rres file reduces the need of file handles to access multiple files; file handles are an OS resource and the cost of opening a file is not insignificant, it may cause delays in the game. Data access times inside the .rres file are also important and depending on how the files are organized it could also improve the loading times.

  • Security: Avoid exposing all the game assets directly to the user for easy copy or modification. Data packed into a .rres file will be more difficult to extract and modify for most users; protect your game copyrighted assets. rres also supports per-resource data compression and encryption, adding an extra level of security when required.

  • Downloading times: If data needs to be accessed from a server and downloaded, the assets packed into a single or a few .rres files improves download times, in comparison to downloading each file individually.

Design History

rres has been in development since 2014. I started this project with the aim to create a packaging file-format similar to XNB for raylib. In the last 8 years the project has suffered multiple redesigns and improvements along a file-formats learning process. In that time I implemented loaders/writers for +20 different file formats and also created +12 custom file formats for multiple raylibtech custom tools.

rres file-format has gone through at least 4 big redesigns:

  • First design of the format was limited to packaging one resource after another, every resource consisted of one resource info header followed by a fixed set of four possible parameters and the resource data. Along the .rres file, a .h header file was generated mapping with defines the rresId with a resource filename (usually the original filename of the un-processed data). This model was pretty simple and intuitive, but it has some important downsides: it did not consider complex pieces of data that could require multiple chunks and there was no way to extract/retrieve original source files (or similar ones).

  • Second design was way more complex and tried to address first design shortcomings. In the second design every resource could include multiple chunks of data in a kind of tree structure. Actually, that design was similar to RIFF file-format: every chunk could contain additional chunks, each one with its header and data. Some additional resource options were also added but the format became quite complex to understand and manage. Finally, the testing implementation was discarded and a simpler alternative was investigated.

  • Third design was a return to first design: simplicity but keeping some of the options for the individual resources. The problem of multiple resources generated from a single input file was solved using a simple offset field in the resource info header pointing to the next linked resource when required. Resources were loaded as an array of resource chunks. An optional Central Directory resource chunk was added to keep references for the input files. Format was good but it still required implementation and further investigation, it needed to be engine-agnostic and it was still dependent on raylib structures and functionality.

  • Fourth design has been done along the implementation, almost all structures and fields have been reviewed and renamed for consistency and simplicity. A separate library has been created (rres-raylib.h) to map the loaded resources data into a custom library/engine data types, any dependency to raylib has been removed, making it a completely engine-agnostic file-format. Several usage examples for raylib have been implemented to illustrate the library-engine connection of rres and multiple types of resource loading have been implemented. rrespacker tool has been created from scratch to generate rres files, it supports a nice GUI interface with drag and drop support but also a powerful command-line for batch processing. Compression and encryption implementation has been moved to the user library implementation to be aligned with the packaging tool and keep the rres file-format cleaner and simpler.

It's been an 8 years project, working on it on-and-off, with many redesigns and revisions but I'm personally very happy with the final result. rres is a resource packaging file-format at the level of any professional engine package format BUT free and open-source, available to any game developer that wants to use it, implement it or create a custom version.

File Structure

rres file format consists of a file header (rresFileHeader) followed by a number of resource chunks (rresResourceChunk). Every resource chunk has a resource info header (rresResourceChunkInfo) that includes a FOURCC data type code and resource data information. The resource data (rresResourceChunkData) contains a small set of properties to identify data, depending on the type and could contain some additional data at the end.

rres v1.0

Fig 01. rres v1.0 file structure.

NOTE: rresResourceChunk(s) are generated from input files. It's important to note that resources could not be mapped to files 1:1, one input file could generate multiple resource chunks. For example, a .ttf input file could generate an image resource chunk (RRES_DATA_IMAGE type) plus a font glyph info resource chunk (RRES_DATA_FONT_GLYPHS type).

rresFileHeader               (16 bytes)
    Signature Id              (4 bytes)     // File signature id: 'rres'
    Version                   (2 bytes)     // Format version
    Resource Count            (2 bytes)     // Number of resource chunks contained
    CD Offset                 (4 bytes)     // Central Directory offset (if available)
    Reserved                  (4 bytes)     // <reserved>

    rresResourceChunkInfo    (32 bytes)
        Type                  (4 bytes)     // Resource type (FourCC)
        Id                    (4 bytes)     // Resource identifier (CRC32 filename hash or custom)
        Compressor            (1 byte)      // Data compression algorithm
        Cipher                (1 byte)      // Data encryption algorithm
        Flags                 (2 bytes)     // Data flags (if required)
        Packed data Size      (4 bytes)     // Packed data size (compressed/encrypted + custom data appended)
        Base data Size        (4 bytes)     // Base data size (uncompressed/unencrypted)
        Next Offset           (4 bytes)     // Next resource chunk offset (if required)
        Reserved              (4 bytes)     // <reserved>
        CRC32                 (4 bytes)     // Resource Chunk Data CRC32
    rresResourceChunkData     (n bytes)     // Packed data
        Property Count        (4 bytes)     // Number of properties contained
        Properties[]          (4*i bytes)   // Resource data required properties, depend on Type
        Data                  (m bytes)     // Resource data

File Header: rresFileHeader

The following C struct defines the rresFileHeader:

// rres file header (16 bytes)
typedef struct rresFileHeader {
    unsigned char id[4];            // File identifier: rres
    unsigned short version;         // File version: 100 for version 1.0
    unsigned short chunkCount;      // Number of resource chunks in the file (MAX: 65535)
    unsigned int cdOffset;          // Central Directory offset in file (0 if not available)
    unsigned int reserved;          // <reserved>
} rresFileHeader;
Field Description
id File signature identifier, it must be the four characters: r,r,e,s.
version Defines the version and subversion of the format.
chunkCount Number of resource chunks present in the file. Note that it could be greater than the number of input files processed.
cdOffset Central Directory absolute offset within the file, note that CDIR is just another resource chunk type, Central Directory can be present in the file or not. It's recommended to be placed as the last chunk in the file if a custom rres packer is implemented. Check rresCentralDir section for more details.
reserved This field is reserved for future additions if required.

Table 01. rresFileHeader fields description and details


  • rres files are limited by design to a maximum of 65535 resource chunks, in case more resources need to be packed is recommended to create multiple rres files.
  • rres files use 32 bit offsets to address the different resource chunks, consequently, no more than ~4GB of data can be addressed, please keep the rres files smaller than 4GB. In case of more space required to package resources, create multiple rres files.

Resource Chunk: rresResourceChunk

rres file contains a number of resource chunks. Every resource chunk represents a self-contained pack of data. Resource chunks are generated from input files on rres file creation by the rres packer tool; depending on the file extension, the rres packer tool extracts the required data from the file and generates one or more resource chunks. For example, for an image file, a resource chunk of type RRES_DATA_IMAGE is generated containing only the pixel data of the image and the required properties to read that data back from the resource file.

It's important to note that one input file could generate several resource chunks when the rres file is created. For example, a .ttf input file could generate an RRES_DATA_IMAGE resource chunk plus a RRES_DATA_FONT_GLYPHS resource chunk; it's also possible to just pack the file as a plain RRES_DATA_RAW resource chunk type, in that case the input file is not processed, just packed as raw data.

On rres creation, rres packer could create an additional resource chunk of type RRES_DATA_DIRECTORY containing data about the processed input files. It could be useful in some cases, for example to relate the input filename directly to the generated resource(s) id and also to extract the data in a similar file structure to the original input one.

Every resource chunk is divided in two parts: rresResourceChunkInfo + rresResourceData.

Resource Chunk Info: rresResourceChunkInfo

The following C struct defines the rresResourceChunkInfo:

// rres resource chunk info header (32 bytes)
typedef struct rresResourceChunkInfo {
    unsigned char type[4];          // Resource chunk type (FourCC)
    unsigned int id;                // Resource chunk identifier (generated from filename CRC32 hash)
    unsigned char compType;         // Data compression algorithm
    unsigned char cipherType;       // Data encription algorithm
    unsigned short flags;           // Data flags (if required)
    unsigned int packedSize;        // Data chunk size (compressed/encrypted + custom data appended)
    unsigned int baseSize;          // Data base size (uncompressed/unencrypted)
    unsigned int nextOffset;        // Next resource chunk global offset (if resource has multiple chunks)
    unsigned int reserved;          // <reserved>
    unsigned int crc32;             // Data chunk CRC32 (propCount + props[] + data)
} rresResourceChunkInfo;
Field Description
type A FourCC code and identifies the type of resource data contained in the rresResourceChunkData. Enum rresResourceDataType defines several data types, new ones could be added if required.
id A global resource identifier, it's generated from input filename using a CRC32 hash and it's not unique. One input file can generate multiple resource chunks, all the generated chunks share the same identifier and they are loaded together when the resource is loaded. For example, an input .ttf could generate two resource chunks (RRES_DATA_IMAGE + RRES_DATA_FONT_GLYPHS) with the same identifier that will be loaded together when their identifier is requested. It's up to the user to decide what to do with loaded data.
compType Defines the compression algorithm used for the resource chunk data. Compression depends on the middle library between rres and the engine, rres.h just defines some useful algorithm values to be used in case of implementing compression. Compression should always be applied before encryption and it compresses the full rresResourceData (Property Count + Properties[] + Data). If no data encryption is applied, packedSize defines the size of compressed data.
cipherType Defines the encryption algorithm used for the resource chunk data. Like compression, encryption depends on the middle library between rres and the engine, rres.h just defines some useful algorithm values to be used in case of implementing encryption. Encryption should be applied after compression. Depending on the encryption algorithm and encryption mode it could require some extra piece of data to be attached to the resource data (i.e encryption MAC), this is implementation dependent and the rres packer tool / rres middle library for the engines are responsible to manage that extra data. It's recommended to be just appended to resource data and considered on packedSize.
flags Reserved for additional flags, in case they are required by the implementation.
packedSize Defines the packed size (compressed/encrypted + additional user data) of rresResourceChunkData. Packaged data could contain appended user data at the end, after compressed/encrypted data, for example the nonce/MAC for the encrypted data, but it is implementation dependent, managed by the rres packer tool and the user library to load the chunks data into target engine structures.
baseSize Defines the base size (uncompressed/unencrypted) of rresResourceChunkData.
nextOffset Defines the global file position address for the next related resource chunk, it's useful for input files that generate multiple resources, like fonts or meshes.
reserved This field is reserved for future additions if required.
crc32 Calculated over the full rresResourceData chunk (packedSize) and it's intended to detect data corruption errors.

Table 02. rresResourceChunkInfo fields description and details

Resource Chunk Data: rresResourceChunkData

rresResourceChunkData contains the following data:

  • Property Count: Number of properties contained, depends on resource type
  • Properties[]: Resource data required properties, depend on resource type
  • Data: Resource raw data, depend on resource type

NOTE: rresResourceChunkData could contain additional user data, in those cases additional data size must be considered in packedSize.

Resource Data Type: rresResourceDataType

The resource type specified in the rresResourceChunkInfo defines the type of data and the number of properties contained in the resource chunk.

Here it is the currently defined data types. Please note that some input files could generate multiple resource chunks of multiple types. Additional resource types could be added if required.

// rres resource chunk data type
// NOTE 1: Data type determines the properties and the data included in every chunk
// NOTE 2: This enum defines the basic resource data types, some input files could generate multiple resource chunks
typedef enum rresResourceDataType {

    RRES_DATA_NULL         = 0,     // FourCC: NULL - Reserved for empty chunks, no props/data
    RRES_DATA_RAW          = 1,     // FourCC: RAWD - Raw file data, input file is not processed, just packed as is
    RRES_DATA_TEXT         = 2,     // FourCC: TEXT - Text file data, byte data extracted from text file
    RRES_DATA_IMAGE        = 3,     // FourCC: IMGE - Image file data, pixel data extracted from image file
    RRES_DATA_WAVE         = 4,     // FourCC: WAVE - Audio file data, samples data extracted from audio file
    RRES_DATA_VERTEX       = 5,     // FourCC: VRTX - Vertex file data, extracted from a mesh file
    RRES_DATA_FONT_GLYPHS  = 6,     // FourCC: FNTG - Font glyphs info, generated from an input font file
    RRES_DATA_LINK         = 99,    // FourCC: LINK - External linked file, filepath as provided on file input
    RRES_DATA_DIRECTORY    = 100,   // FourCC: CDIR - Central directory for input files relation to resource chunks
    // TODO: Add additional data types if required
} rresResourceDataType;

The currently defined data types consist of the following properties and data:

resource type FourCC propsCount props data
RRES_DATA_RAW RAWD 4 props[0]:size
raw file bytes
RRES_DATA_TEXT TEXT 4 props[0]:size
text data
RRES_DATA_IMAGE IMGE 4 props[0]:width
pixel data
RRES_DATA_WAVE WAVE 4 props[0]:frameCount
audio samples data
RRES_DATA_VERTEX VRTX 4 props[0]:vertexCount
vertex data
RRES_DATA_FONT_GLYPHS FNTG 4 props[0]:baseSize
RRES_DATA_LINK LINK 1 props[0]:size filepath data
RRES_DATA_DIRECTORY CDIR 1 props[0]:entryCount rresDirEntry[0..entryCount]

Table 03. rresResourceDataType defined values and details

NOTE: RRES_DATA_RAW contains the packaged file extension as part of its properties, char extension is converted to a 4-byte unsigned int big-endian value, starting with a dot. i.e ".png" => 0x2e706e67. In case the extension was not relevant, the user implementation could decide to set those properties to 0.

rres.h defines the following enums for convenience to assign some of the properties:

  • rresTextEncoding: Defines several possible text encodings, default value is 0 (UTF-8)
  • rresCodeLang: Defines several programming languages, useful in case of embedding code files or scripts
  • rresPixelFormat: Defines multiple pixel format values for image pixel data
  • rresVertexAttribute: Defines several vertex attribute types for convenience
  • rresVertexFormat: Defines several data formats for vertex data
  • rresFontStyle: Defines several font styles (Regular, Bold, Italic...), default value is 0 (RRES_FONT_STYLE_DEFAULT)

Resource Chunk: Central Directory: rresCentralDir

The Central Directory resource chunk is a special chunk that could be present or not in the rres file, it stores information about the input files processed to generate the multiple resource chunks and could be useful to:

  • Reference the resources by its original filename. This is very useful in terms of implementation to minimize the required code changes if rres packaging is done over an existing project or at the end of a project development, if all files loading has been done using the filenames directly.

  • Extract some of the resources to a similar input file. It will be only possible if the input file has not been processed in a destructive way. For example, if a.ttf file has been processed to generate a glyphs image atlas + glyphs data info, it won't be possible to retrieve the original .ttf file.

rres provides some helpful structures to deal with Central Directory resource chunk data:

// rres central directory entry
typedef struct rresDirEntry {
    unsigned int id;                // Resource id
    unsigned int offset;            // Resource global offset in file
    unsigned int reserved;          // reserved
    unsigned int fileNameSize;      // Resource fileName size (NULL terminator and 4-byte alignment padding considered)
    char fileName[RRES_MAX_CDIR_FILENAME_LENGTH];  // Resource original fileName (NULL terminated and padded to 4-byte alignment)
} rresDirEntry;

// rres central directory
// NOTE: This data represents the rresResourceChunkData
typedef struct rresCentralDir {
    unsigned int count;             // Central directory entries count
    rresDirEntry *entries;          // Central directory entries
} rresCentralDir;

NOTE: Central Directory filename entries are aligned to 4-byte padding to improve file-access times.

rres.h provides a function to load Central Directory from the rres file when available: rresLoadCentralDirectory() and also a function to get a resource identifiers from its original filename: rresGetIdFromFileName().

In case a rres file is generated with no Central Directory, a secondary header file (.h) should be provided with the id references for all resources, to be used in user code.

Custom Engine Implementation

rres is designed as an engine-agnostic file format that can be used with any game engine. Developers adopting rres can implement a custom library and a custom abstraction to map rres generic data to their own engines data structures and also custom rres packaging tools.

The following diagram shows a sample implementation of rres for raylib library.

rres v1.0

Fig 02. rres sample implementation: custom engine libs and tool.

rres implementation consist of several parts:

Base library: rres.h

Base rres library is in charge of reading rres files resource chunks into a generic resource structure, returned to the user. User exposed resource structure rresResourceChunkfollows rres specs structure (rresResourceChunkInfo + rresResourceChunkData). The following structures are provided:

  • rresResourceChunk contains a single chunk with user-required info and data: rresResourceChunkInfo + rresResourceChunkData
  • rresresourceChunkInfo contains the information about the loaded resource chunk
  • rresResourceChunkData contains the actual data for the resource: the required properties and the raw data. It's important to note that in the case data was compressed/encrypted, it's up to the user-library (rres-raylib.h) to process that data; in those cases contains the compressed/encrypted data and = 0 and = NULL; it's up to the user library to fill properties after decompression/decryption.
  • rresResourceMulti contains multiple rresResourceChunks for a processed input file
// rres resource chunk
typedef struct rresResourceChunk {
    rresResourceChunkInfo info;     // Resource chunk info
    rresResourceChunkData data;     // Resource chunk packed data, contains propCount, props[] and raw data
} rresResourceChunk;

// rres resource chunk info header (32 bytes)
typedef struct rresResourceChunkInfo {
    unsigned char type[4];          // Resource chunk type (FourCC)
    unsigned int id;                // Resource chunk identifier (generated from filename CRC32 hash)
    unsigned char compType;         // Data compression algorithm
    unsigned char cipherType;       // Data encription algorithm
    unsigned short flags;           // Data flags (if required)
    unsigned int packedSize;        // Data chunk size (compressed/encrypted + custom data appended)
    unsigned int baseSize;          // Data base size (uncompressed/unencrypted)
    unsigned int nextOffset;        // Next resource chunk global offset (if resource has multiple chunks)
    unsigned int reserved;          // <reserved>
    unsigned int crc32;             // Data chunk CRC32 (propCount + props[] + data)
} rresResourceChunkInfo;

// rres resource chunk data
typedef struct rresResourceChunkData {
    unsigned int propCount;         // Resource chunk properties count
    unsigned int *props;            // Resource chunk properties
    void *raw;                      // Resource chunk raw data
} rresResourceChunkData;

// rres resource multi
// NOTE: It supports multiple resource chunks
typedef struct rresResourceMulti {
    unsigned int count;             // Resource chunks count
    rresResourceChunk *chunks;      // Resource chunks
} rresResourceMulti;

A single rresResourceChunk can be loaded from the .rres file with the provided function: rresLoadResourceChunk() and unloaded with rresUnloadResourceChunk().

A full rresResourceMulti can be loaded from the .rres file with the provided function: rresLoadResourceMulti() and unloaded with rresUnloadResourceMulti().

Engine mapping library: rres-raylib.h

The mapping library includes rres.h and provides functionality to map the resource chunks data loaded from the rres file into raylib structures. The API provided is simple and intuitive, following raylib conventions:

RLAPI void *LoadDataFromResource(rresResourceChunk chunk, int *size);   // Load raw data from rres resource chunk
RLAPI char *LoadTextFromResource(rresResourceChunk chunk);              // Load text data from rres resource chunk
RLAPI Image LoadImageFromResource(rresResourceChunk chunk);             // Load Image data from rres resource chunk
RLAPI Wave LoadWaveFromResource(rresResourceChunk chunk);               // Load Wave data from rres resource chunk
RLAPI Font LoadFontFromResource(rresResourceMulti multi);               // Load Font data from rres resource multiple chunks
RLAPI Mesh LoadMeshFromResource(rresResourceMulti multi);               // Load Mesh data from rres resource multiple chunks

RLAPI int UnpackResourceChunk(rresResourceChunk *chunk);                // Unpack resource chunk data (decompres/decrypt data)
RLAPI void SetBaseDirectory(const char *baseDir);                       // Set base directory for externally linked data

Note that data decompression/decryption is implemented in this custom library, UnpackResourceChunk() is provided for the users. rresResourceChunk contains compressor/cipher identifier values for convenience. Compressors and ciphers support depends on user implementation and it must be aligned with the packaging tool (rrespacker).

rres file-format is engine-agnostic, libraries and tools can be created for any engine/framework in any programming language.

Packaging tool: rrespacker

rrespacker is the rres packing tool in charge of processing all the input files and creating the rres file, following the specification. In case some compression/encryption algorithms are supported it must be implemented by this tool and the same algorithms should be supported by the mapping library, in our case rres-raylib.h.

rrespacker v1.0

Fig 03. rrespacker tool, GUI interface, it also supports CLI for batch processing.

rrespacker can be used online (or downloaded) from


  • rres v1.0 (12-May-2022) - First release of the specification.


rres file-format specs, rres.h library and rres-raylib.h library are licensed under MIT license. Check LICENSE for further details.

Copyright (c) 2014-2023 Ramon Santamaria (@raysan5)