PNGTemplate.bt

//--------------------------------------
//--- 010 Editor v2.0.1 Binary Template
//
//    File: PNGTemplate.bt
//  Author: Kevin O. Grover <kevin@kevingrover.net>
// Purpose: A template for parsing PNG (Portable Network Graphics) files.
// Version: 1.1   2009-02-25
// History:
// 2005-05-11  KOG  Initial version
// 2005-06-29  KOG  Fixed typos in comments
// 2009-02-23  KOG  Decode IHDR and tEXt chunks
//
// This template was written to the PNG 1.2 Specification.
// Note however, that it does not check nor parse chunk subdata, so it
// should work with all future PNG specifications.
//
// Also, a possible caveat: PNG encourages the type of chunks to be
// mapped to strings of the form "[a-zA-Z]{4}".  However, it's not a requirement.
//
// Summary of PNG Format:
// A PNG file consists of an 8 byte ID followed by a series of chunks.
// Each Chunk is
//   length (4 bytes), chunk_type (4 bytes), data (length bytes), crc (4 bytes)
//   CRC Does NOT include the length bytes.
//--------------------------------------

BigEndian();                  // PNG files are in Network Byte order

const uint64 PNGMAGIC = 0x89504E470D0A1A0AL;

// Chunk Type
typedef union {
    uint32  ctype <format=hex>; // Chunk Type
    char    cname[4];           // character representation
} CTYPE <read=readCTYPE>;

string readCTYPE(local CTYPE &t) {
    return t.cname;
}

// -- Specific Chunks

// IHDR - Image Header

typedef struct {
    uint32   width;
    uint32   height;
    ubyte    bits;
    ubyte    color_type;
    ubyte    compression;
    ubyte    filter;
    ubyte    interlace;
} IHDR <read=readIHDR>;

string readIHDR(local IHDR &ihdr) {
    local string s;
    SPrintf(s, "%i x %i (x%i)", ihdr.width, ihdr.height, ihdr.bits);
    return s;
}

// tEXt - Text Data

typedef struct {
    string label;                                // to the first NULL (including)
    char   data[length - Strlen(label) - 1];     // rest of the data
} tEXt <read=readtEXt>;

string readtEXt(local tEXt &text) {
    local string s;
    SPrintf(s, "%s = %s", text.label, text.data);
    return s;
}

// -- End: Specific Chunks

local uint32 CHUNK_CNT = 0;

// Generic Chunks
typedef struct {
    uint32  length;             // Number of data bytes (not including length,type, or crc)
    local quad pos_start = FTell();
    CTYPE   type;               // Type of chunk
    if (type.cname == "IHDR") {
        IHDR    ihdr;
    } else if (type.cname == "tEXt") {
        tEXt    text;
    } else {
        ubyte   data[length];       // Data (or not present)
    }
    local quad data_size = FTell() - pos_start;
    uint32  crc <format=hex>;   // CRC type and data (not including length or crc)
    local uint32 crc_calc = Checksum(CHECKSUM_CRC32, pos_start, data_size);
    if (crc != crc_calc) {
        local string msg;
        SPrintf(msg, "*WARNING CRC Mismatch @ chunk[%d] (%08x != %08x)\n", CHUNK_CNT, crc, crc_calc);
        Warning(msg);
        Printf(msg);
    }
    CHUNK_CNT++;
} CHUNK <read=readCHUNK>;


// Chunks can be in any order: HOWEVER, IHDR must be first, IEND must be last
// Bit 5s in chunk type bytes are used to flag some things:
//    Ancillary bit: bit 5 of 1st byte: 0=Critical, 1=Ancillary
//      Private bit: bit 5 of 2nd byte: 0=Public, 1=Private
//     Reserved bit: bit 5 of 3rd byte: MUST be 0
// Safe to Copy bit: bit 5 of 4th byte: 0=Unsafe to Copy, 1=Safe to Copy
string readCHUNK(local CHUNK &c) {
    local string s;
    s=readCTYPE(c.type)+"  (";
    s += (c.type.cname[0] & 0x20) ? "Ancillary, "      : "Critical, ";
    s += (c.type.cname[1] & 0x20) ? "Private, "        : "Public, ";
    s += (c.type.cname[2] & 0x20) ? "ERROR_RESERVED, " : "";
    s += (c.type.cname[3] & 0x20) ? "Safe to Copy)"    : "Unsafe to Copy)";
    return s;
}

// ---------------------------------------------------------------------------
// MAIN  --  Here's where we really allocate the data
// ---------------------------------------------------------------------------

uint64 pngid <format=hex>;
if (pngid != PNGMAGIC) {
    Warning("Invalid PNG File: Bad Magic Number");
    return -1;
}
while(!FEof()) {
    CHUNK chunk;
}
if (CHUNK_CNT > 1) {
    if ((chunk[0].type.cname != "IHDR") || (chunk[CHUNK_CNT-1].type.cname != "IEND")) {
        local string msg;
        SPrintf(msg, "*WARNING: Chunk IHDR must be first and chunk IEND must be last!\n");
        Warning(msg);
        Printf(msg);
    }
}