TGA

Full Name	TGA (Truevision Targa)
Format ID	IG_FORMAT_TGA = 39
File Extension(s)	.tga, .tpic
Data Type	Raster Image
Data Encoding	Binary
Color Profile Support	No
ImageGear Multipage Support	No
ImageGear Alpha Channel Support	Single alpha channel for read/write
ImageGear Platforms Support	WIN32, WIN64, Unix, Unix64, .NET, .NET64, MAC

ImageGear Supported Versions:

Version 2.0, 1991
Version 1.0, 1984

ImageGear Supported Features:

IG_FLTR_DETECTSUPPORT - autodetection
IG_FLTR_PAGEREADSUPPORT - single page file reading
IG_FLTR_PAGEINSERTSUPPORT - single-page file writing

ImageGear Read Support:

IG_COMPRESSION_NONE:
- Grayscale: 1 bpp;
- Indexed RGB: 8 bpp;
- RGB: 15, 24 bpp;
- RGB 15 bpp + Alpha 1 bpp;
- RGB 24 bpp + Alpha 8 bpp;
- RGB 15 bpp + Premultiplied Alpha 1 bpp;
- RGB 24 bpp + Premultiplied Alpha 8 bpp;
- RGB 15 bpp + Extra 1 bpp;
- RGB 24 bpp + Extra 8 bpp;
IG_COMPRESSION_RLE:
- Grayscale: 1 bpp;
- Indexed RGB: 8 bpp;
- RGB: 15, 24 bpp;
- RGB 15 bpp + Alpha 1 bpp;
- RGB 24 bpp + Alpha 8 bpp;
- RGB 15 bpp + Premultiplied Alpha 1 bpp;
- RGB 24 bpp + Premultiplied Alpha 8 bpp;
- RGB 15 bpp + Extra 1 bpp;
- RGB 24 bpp + Extra 8 bpp;

ImageGear Write Support:

IG_COMPRESSION_NONE:
- Indexed RGB: 8 bpp;
- RGB: 15, 24 bpp;
- RGB 15 bpp + Alpha 1 bpp;
- RGB 24 bpp + Alpha 8 bpp;
- RGB 15 bpp + Extra 1 bpp;
- RGB 24 bpp + Extra 8 bpp;
IG_COMPRESSION_RLE:
- Indexed RGB: 8 bpp;
- RGB: 15, 24 bpp;
- RGB 15 bpp + Alpha 1 bpp;
- RGB 24 bpp + Alpha 8 bpp;
- RGB 15 bpp + Extra 1 bpp;
- RGB 24 bpp + Extra 8 bpp;

ImageGear Filter Control Parameters:

Filter Control Parameter	Type	Default Value	Available Values	Description
AUTHOR_COMMENT	STRING	" "		Author's comments.
AUTHOR_NAME	STRING	" "		Name of author.
IMAGE_ID	STRING	" "		Image ID.
SOFTWARE_ID	STRING	" "		Software ID.
STAMP_DAY	WORD	0		Day stamp.
STAMP_HOUR	WORD	0		Hour stamp.
STAMP_MINUTE	WORD	0		Minute stamp.
STAMP_MONTH	WORD	0		Month stamp.
STAMP_SECOND	WORD	0		Second stamp.
STAMP_YEAR	WORD	0		Year stamp.
KEEP_ALPHA	BOOL	FALSE	TRUE, FALSE	FALSE means to ignore Alpha bits. Non-zero (TRUE) means to pass alpha bits back through callback.
PROMOTE16	BOOL	FALSE	TRUE, FALSE	FALSE means to create 16 bit DIB without promoting to 24 bits. Non-zero (TRUE) means promoting.
HEADER_TYPE	INT	0	1 or 2	1 or 2 for revision level of header.
STORES_TAMP	BOOL	FALSE	TRUE, FALSE	FALSE for no stamp, TRUE to save a stamp with image. HEADER_TYPE must be 2.
STAMP_WIDTH	UINT	0	0-64	Stamps width must not be larger than 64 pixels.
STAMP_HEIGHT	UINT	0	0-64	Stamps height must not be larger than 64 pixels.
PALETTE	BOOL	FALSE	TRUE, FALSE	TRUE indicates that color map should be created.
CONV_TO_16	BOOL	FALSE	TRUE, FALSE	If TRUE, convert to 16 bpp when saving.
THUMB_FLAG	UINT	0	TRUE, FALSE	TRUE if save should include a thumbnail.
THUMB_WIDTH	UINT	0		Width of thumbnail.
THUMB_HEIGHT	UINT	0		Height of thumbnail - Thumbnails are 24 bit.

Comments:

This file format was originally developed by AT&T for use with its image capture boards. The format was taken over by Truevision when it acquired the product line from AT&T. It is now commonly used for digitized images and also for high-quality images produced by ray tracers and other graphics applications.

It became a popular file format mainly because it was the first 24-bit truecolor format to come to the PC market. There are several varieties of Targa files; the most commonly used are the Targa 16, Targa 24, and Targa 32. The names are derived from the type of hardware used to create them.

The fixed-sized header information of the Targa format includes: the existence (or not) and colormap, location, size, pixel depth, image location, colormap (if it exists), and finally the image data itself.

Version 2.0 introduced a file footer that identifies it as the newest version and contains pointers to additional fields in two main structures: the "extension area" and the "developer directory". The extension area contains the addresses of many optional fields, one of the most popular being the "postage stamp image" (miniature of the main image).

The developer directory can be used to store proprietary information. Developers can register their own private fields with Truevision. A null-terminated ASCII string containing "TRUEVISION-XFILE.", and positioned at the end of the file, indicates that the footer is valid.

Targa defines 3 color methods: pseudo color, direct color, and truecolor. Pseudo color uses an index to a color palette. Direct color is like pseudo color except that the RGB components are looked up separately. In truecolor files, the color information is stored directly in the image data. The palettes used by Targa files are variable in size; they do not necessarily correlate to the bit depth of the image. The presence of a palette does not always mean that it is used to display the image.

Alpha channel handling

KEEP_ALPHA control parameter must be set to TRUE to enable loading and saving of Alpha/Extra channels from TGA images.

If bits 0..3 of Image Descriptor field are set to non-zero (additional channel is present), ImageGear treats additional bits as follows, depending on the Attributes Type field:

Attributes Type value	Attributes type value meaning	Additional bits are loaded as:
0	No Alpha Data included	Non-premultiplied Alpha (for incompliant images where Image Descriptor tells there is an additional channel but Attributes Type field is not set.)
1	Undefined data, can be ignored	Extra channel
2	Undefined data, should be retained	Extra channel
3	Non-premultiplied Alpha	Non-premultiplied Alpha
4	Premultiplied Alpha	Premultiplied Alpha
Other	Reserved or unassigned	Non-premultiplied Alpha

If bits 0..3 of Image Descriptor field are set to zero (no additional channel is present), ImageGear treats additional bits as Extra channel. This allows reading additional channel from images that have incorrect Image Descriptor field, but does not spoil image display if there is no meaningful data in the additional channel.

References Used

Brown, C. Wayne, and Barry J. Shepherd. Graphics File Formats: Reference and Guide. Greenwich, CT.: Manning Publications, 1992.

Kay, David C. and John R. Levine. Graphics File Formats. Windcrest Books, 1992.

Murray, James D. and William vanRyper. Encyclopedia of Graphics File Formats. Sebastopol, CA: O'Reilly & Associates, Inc., 1994.