TGA header

OK, Straight from 'Graphics File Formats, 2nd Edition' by David C. Kay & John R. Levine, here is the header format for the Targa image file.

Offset Length (in bytes) Description

------ ----------------- -----------

0 1 ID Field Length

1 1 Color-map Type

2 1 Image Type

(Color-map-specific Info)

3 2 First Color-map Entry

5 2 Color-map Length

7 1 Color-map Entry Size

(Image-specific Info)

8 2 Image X Origin

10 2 Image Y Origin

12 2 Image Width

14 2 Image Height

16 1 Bits-Per-Pixel

17 1 Image-Descriptor Bits

For True-color images the value of Color-map Type should be 0, while color-mapped images should set this to 1. If a color map is present, then Color-map Entry Size should be set to 15, 16, 24 or 32. For 15 and 16 values each color map entry is stored in two bytes in the format of:

High byte Low byte

A RRRRR GG GGG BBBBB

with the 'A' bit set to 0 for 15-bit color values. 24-bit-sized entries are stored as three bytes in the order of (B)lue, (G)reen, and (R)ed. 32-bit-sized color map entries are stored in four bytes ordered as (B)lue, (G)reen, (R)ed and (A)ttribute values.

Further, the Image Type code should contain one of the following values:

Code Description

---- -----------

0 No Image Present

1 Color-mapped, uncompressed

2 True-color, uncompressed

3 Black-&-White, uncompressed

9 Color-mapped, RLE compressed

10 True-color, RLE compressed

11 Black-&-White, RLE compressed

The Image X & Y Origins and the Image Width & Height fields are self-explanatory. Bits-Per-Pixel holds the number of bits per image pixel and should hold the values 8, 16, 24, or 32.

The Image Descriptor byte contains several bit fields that need to be extracted:

Bits Description

---- -----------

0-3 Attribute Bits (Explained later)

4 Left-to-Right orientation 0=L/R 1=R/L

5 Top/Bottom orientation 0=B/T 1=T/B

6-7 Scan-Line Interleaving 00H=None, 40H=2way, 80H=4way

The Attribute bits are used to define the attributes of the colors in color-mapped or true-color pixels. 0 means no alpha data, 1 means undefined and ignorable, 2 means undefined but should be preserved, 3 means regular alpha data and 4 means the pixel information has already been multiplied by the alpha value.

Version 2.0 Targa files also have a file footer that may contain additional image and comment information. A version 2.0 Targa file always ends with the null-terminated string 'TRUEVISION-TARGA.'. So, if your Targa image ends with the values 'TRUEVISION-TARGA.' + 00H then you can extract the eight bytes prior to the string to find the start of the extension area and the developer directory positions within the file. Basically the Version 2.0 footer takes the format:

Byte Length Description

----- ------ -----------

0 4 32-bit offset to Extension Area

4 4 32-bit offset to Developer Directory

8 17 TRUEVISION-TARGA.

25 1 Binary zero ($0)

I'm not going to give complete descriptions to the Developer's Directory or the Extension Area. Instead, I'm going to point out the postage-stamp info that the V2.0 Targa file *MAY* contain. This postage stamp is a miniature of the image, no larger than 64 X 64 pixels in size, *IF PRESENT*!

Extension Area

Offset Length Description

------ ------ -----------

0 2 Extension Area Size (should be 495)

2 41 Author's Name

43 81 Author's Comments

124 81 Author's Comments

205 81 Author's Comments

286 81 Author's Comments

367 2 Creation Month

369 2 Creation Day

371 2 Creation Year

.... ... ...

482 4 Color-correction table file offset

486 4 Postage-Stamp Image File Offset ******

490 4 Scan-line table file offset

494 1 Attribute byte

This postage-stamp image, if present, may be directly usable by you. It is an uncompressed image in the same color format (Color-mapped or True-color) as the full image.