Daggerfall Mod:Image formats/IMG

IMG files each contain a single ImgFile structure. An ImgFile structure (defined below) consists of an ImgHeader structure immediately followed by a PixelData structure (an array of UInt8 values). The length of the PixelData is declared by the ImgHeader.PixelDataLength value. Since IMG files make use of an Rle to compress the actual pixel data, decoding the described image can require additional work for some IMG files.

ImgHeader

ImgHeader Structure

Offset	Type	Name	Description
0-1	Int16	XOffset	This is used by the Daggerfall rendering engine to display the image at the appropriate position on the screen.
2-3	Int16	YOffset	This is used by the Daggerfall rendering engine to display the image at the appropriate position on the screen.
4-5	Int16	Width	The width of the image, in pixels.
6-7	Int16	Height	The height of the image, in pixels.
8-9	Compression (UInt16)	Compression	This field is an enumeration, described below.
10-11	Int16	PixelDataLength	This is the number of bytes contained in the PixelData structure immediately following this header. If the Compression flag indicates Uncompressed, then the PixelDataLength must be equal to Width × Height; otherwise the PixelDataLength will be less-than-or-equal-to Width × Height.

Compression Enumeration

Compression Enumeration

Value	Name	Definition
0x0000	Uncompressed	Image is not compressed
0x0002	RleCompressed	Image is Rle Compressed
0x0108	ImageRle	This is a special type of compression only valid in texture files.
0x1108	RecordRle	This is a special type of compression only valid in texture files.

Reading Image Data

In essence, one simply reads a byte array of DataLength long from the file. Decoding the retrieved data is a different matter, dependent on the Compression field's value.

Uncompressed

This is performed identically to the procedure used for RCI files.

RleCompressed

Decoding RleCompressed data is slightly more complicated, but quite manageable. The generic Run-Length Encoding algorithm is fully described here. The specific algorithm employed in Daggerfall uses single-byte counting, which places an upper-limit on "run" lengths. Compressed runs have their count encoded as ( count + 127 ), which places a maximum count of compressed runs at 128. Uncompressed runs have their count encoded as ( count - 1 ), and since they must be distinguishable from compressed runs, this places a maximum count of uncompressed runs also at 128 bytes because any value greater than 127 would be interpreted as a compressed run. These facts imply any count value of 255 (0xff) is an invalid count value.

The algorithm employed by Daggerfall then simply becomes a matter of reading a count value and then reading one ore more bytes of data. If the count value is greater than 127 (0x7f), then the next byte is to be repeated in the output buffer ( count - 127 ) times; otherwise the next ( count + 1 ) bytes are to be copied to the output buffer. This is to continue until ( ImgHeader.Width * ImgHeader.Height ) bytes have been copied to the output buffer. At that point the data is fully decompressed, and can be displayed as per RCI files.

Special IMG files

There are a number of exceptions to the IMG format.

Empty IMG files

FMAP0I00.IMG

FMAP0I01.IMG

FMAP0I16.IMG

These 12 byte files each contain nothing but an empty (0x00's) ImgHeader structure and no PixelData.

Invalid Compression Value files

FRAM00I0.IMG

TALK00I0.IMG

These 2 files each report an undefined (0x0800) Compression value. These files are decoded as if they were Uncompressed.

RCI files

AMAP00I0.IMG

BANK01I1.IMG

BANK01I2.IMG

BIOG00I0.IMG

BOOK00I0.IMG

BUTN01I0.IMG

CHAR00I0.IMG

CHAR01I0.IMG

CHAR02I0.IMG

CHAR03I0.IMG

CHAR04I0.IMG

CHAR05I0.IMG

CNFG00I0.IMG

COMPASS.IMG

CORT01I0.IMG

CUST00I0.IMG

DANK02I0.IMG

DESERT.IMG

GRAD01I0.IMG

ICON00I0.IMG

INFO00I0.IMG

INFO01I0.IMG

INVE00I0.IMG

INVE01I0.IMG

INVE02I0.IMG

INVE04I0.IMG

INVE05I0.IMG

INVE0FI0.IMG

ITEM00I0.IMG

ITEM01I0.IMG

LGBK00I0.IMG

LOAD00I0.IMG

MAP100I0.IMG

MASK00I0.IMG

MASK01I0.IMG

MASK02I0.IMG

MASK04I0.IMG

MASK05I0.IMG

MASK06I0.IMG

NITE00I0.IMG

NITE01I0.IMG

NITE02I0.IMG

NITE03I0.IMG

PICK01I0.IMG

SCRL04I0.IMG

SCRL05I0.IMG

SCRL07I0.IMG

SCRL08I0.IMG

SCRL09I0.IMG

SCRL10I0.IMG

SCRL11I0.IMG

SCRL12I0.IMG

SCRL13I0.IMG

SCRL14I0.IMG

TALK01I0.IMG

TALK02I0.IMG

TALK03I0.IMG

TAMRIEL.IMG

TAMRIEL2.IMG

TMAP00I0.IMG

TRAV01I0.IMG

TRAV01I1.IMG

TRAV0I00.IMG

TRAV0I01.IMG

TRAV0I03.IMG

These are actually RCI Files, containing one image each.

Palettized RCI files

CHGN00I0.IMG

DIE_00I0.IMG

PICK02I0.IMG

PICK03I0.IMG

PRIS00I0.IMG

TITL00I0.IMG

These are actually Palettized RCI Files, containing one image each.