.tga File Extension (BIMAGE)

The ".tga" files used in DoomDOOM Eternal are not targa images. They are actually "BIM" (or .bimage) files.

These files are stored in two parts. The headerfirst part consists of the HEADER, MIPMAP, and NON_STREAMED_IMAGE sections. This is embedded in DoomDOOM Eternal's .resources files. The bodysecond part consists of the fileSTREAMED_IMAGE section, which is embedded in Doom Eternal's .streamdb files.

File Signature

DoomDOOM Eternal ".tga" files can be identified by the first 3 bytes of the file header,header stored in a .resources file, which is always 0x42 0x49 0x4D or "BIM", short for "bimage" or "binary image".

HeaderHowever, Structure

Thefiles .tgaare headerusually isstored extractedcompressed fromvia DoomOodle Eternal'sKraken, .resourceswhich files. may Itobscure consiststhe of afile 63-byteHEADER struct, followed by a variable number of 36-byte MIPMAP structs. Everything is little-endian.signature.

TheTo technicalview detailsthese offiles, .resources file extraction are beyond the scope of this article. Youyou can extract the .tga headers yourselffrom a .resources file using the EternalResourceExtractor tool. 

File Structure

A DOOM Eternal ".tga" file consists of 4 parts:

1. The HEADER part, which contains important details about the image size, format and encoding.
2. The MIPMAP section, which contains metadata for each individual image "mip"
3. The NON_STREAMED_IMAGE section, which contains non-streamed versions of the image. Usually, these are any "mips" that are smaller than about 32x32 pixels. Sometimes, though, it can be a full-sized image.
4. The STREAMED_IMAGE section, which is the portion of the image stored in .streamdb. The largest version of the image is normally stored in the .streamdb, along with several smaller mips.

The HEADER, MIPMAP, and NON_STREAMED_IMAGE sections are stored as a single compressed file, embedded within a DOOM Eternal .resources file. The STREAMED_IMAGE is stored separately in a .streamdb file. Even though they are stored separately, they are considered one "file" because exporting them to a usable format such as .DDS, .PNG, etc, requires that we connect all the disparate pieces.

struct ETERNAL_TGA_HEADERETERNAL_TGA_FILE
{
    HEADER header;
    MIPMAP mipmap[];							// Flexible array member,Array length varies with header.MipCount;
    NON_STREAMED_IMAGE non_streamed_image[];	// Smaller mips of the image.
    STREAMED_IMAGE streamed_image[];			// Full-size + larger mips of the image.
};

Header Section

The HEADER struct is a 63-byte sequence:

struct HEADER 
{		
	char Signature[3];				// "BIM"
    byte Version;					// 0x15
	int TextureType; 				// enum textureType_t
	int TextureMaterialKind;		// enum textureMaterialKind_t
	int PixelWidth;					// image width in pixels
	int PixelHeight;				// image height in pixels
	int Depth;
    int MipCount; 					// determines # of MIPMAP structs to follow
	int64_t MipLevel;				
	float unkFloat1; 				// usually 1.0, purpose unknown
	byte boolIsEnvironmentMap;		// 1 if image is an environment map
	int TextureFormat;				// enum textureFormat_t
	int Always7;					// literally always 7, purpose unknown
	int nullPadding;
	short AtlasPadding;
	byte boolIsStreamed;			// 1 if file is located in streamDB
	byte unkBool;
	byte boolNoMips;				// 1 if image has no mips
	byte boolFFTBloom;				// 1 if using FFT Bloom
	int StreamDBMipCount; 			// usually same number of mips stored in streamdb
};

• TextureType is a member of the enum textureType_t - it marks the image as 2-dimensional, 3-dimensional, or cubic.
• TextureMaterialKind is a member of the enum textureMaterialKind_t - it tells the engine what type of material this texture is (albedo, normal, specular, etc).
• TextureFormat is a member of the enum textureFormat_t - it tells the engine how the image is encoded (image format, block compression type, etc).

Mipmap Struct

The MIPMAP struct is a 36-byte sequence that comes immediately after the HEADER. This struct will be repeated n times, where n is equal to HEADER.MipCount above.

struct MIPMAP
{
	int64_t MipLevel;				// Starts at 0, increment by 1 each time it repeats
	int MipPixelWidth;				// Original PixelWidth reduced by 50% for each MipLevel
	int MipPixelHeight;				// Original PixelHeight reduced by 50% for each MipLevel
	int UnknownFlagA;
	int DecompressedSize;			// Decompressed size in bytes
	int FlagIsCompressed;			// 1 if the texture is compressed
	int CompressedSize;				// Compressed size in bytes
	int CumulativeSizeStreamDB;		
};

• CumulativeSizeStreamDB is always zero for the first mipmap. For additional mipmaps, it is the sum of previous mipmaps compressed sizes.

Non-Streamed Images

The totalNON_STREAMED_IMAGE sizesection (inbegins bytes)immediately ofafter the last TGA_FILE_HEADERMIPMAP. The starting offset of this section can be calculated as:as offset = 63 + (36 * HEADER.MipCount). 

struct However,NON_STREAMED_IMAGE
some{
	BYTE rawImageData[];					// Image format and encoding given in HEADER
};

struct STREAMED_IMAGE
{
	BYTE rawImageData[];					// Image format & encoding given in HEADER
};