📄 tga.htm
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<title>file:///d:/程序资料/format2/tga.txt</title>
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<p align="center"><font size="6" color="#0000ff">tga format</font></p>
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<td width="100%"><pre>this file has been created to satisfy numerous requests for information
on targa image file formats. the information has been taken from
appendix c of the truevision technical guide. requests for further
information could be directed to:</pre>
<pre> at&t
electronic photography and imaging center
2002 wellesley ave.
indianapolis, in 42619</pre>
<pre>this document does not pretend to be complete, but it does pretend to
be accurate. if you discover any finger checks or erroneous information
please let me know, ( david mcduffee, 75530,2626), and i will upload the
corrections. thanks.</pre>
<pre>the lack of completeness is due to the fact that the targa recognizes
over half a dozen image file formats, some of which are more widely
used than others. i have chosen to re-key the details on only those
formats which i actually use. again, if you want to know more about
formats not covered here, you could contact your truevision representative.</pre>
<pre>all targa formats are identified by a data type field, which is a one
byte binary integer located in byte three of the file. the various
file types specified by this field are as follows:</pre>
<pre> 0 - no image data included.
1 - uncompressed, color-mapped images.
2 - uncompressed, rgb images.
3 - uncompressed, black and white images.
9 - runlength encoded color-mapped images.
10 - runlength encoded rgb images.
11 - compressed, black and white images.
32 - compressed color-mapped data, using huffman, delta, and
runlength encoding.
33 - compressed color-mapped data, using huffman, delta, and
runlength encoding. 4-pass quadtree-type process.</pre>
<pre>this document will describe only four formats: 1, 2, 9, and 10.
</pre>
<pre>--------------------------------------------------------------------------------
data type 1: color-mapped images. |
_______________________________________________________________________________|
| offset | length | description |
|--------|--------|------------------------------------------------------------|
|--------|--------|------------------------------------------------------------|
| 0 | 1 | number of characters in identification field. |
| | | |
| | | this field is a one-byte unsigned integer, specifying |
| | | the length of the image identification field. its range |
| | | is 0 to 255. a value of 0 means that no image |
| | | identification field is included. |
| | | |
|--------|--------|------------------------------------------------------------|
| 1 | 1 | color map type. |
| | | |
| | | this field contains a binary 1 for data type 1 images. |
| | | |
|--------|--------|------------------------------------------------------------|
| 2 | 1 | image type code. |
| | | |
| | | this field will always contain a binary 1. |
| | | ( that's what makes it data type 1 ). |
| | | |
|--------|--------|------------------------------------------------------------|
| 3 | 5 | color map specification. |
| | | |
| 3 | 2 | color map origin. |
| | | integer ( lo-hi ) index of first color map entry. |
| | | |
| 5 | 2 | color map length. |
| | | integer ( lo-hi ) count of color map entries. |
| | | |
| 7 | 1 | color map entry size. |
| | | number of bits in each color map entry. 16 for |
| | | the targa 16, 24 for the targa 24, 32 for the targa 32. |
| | | |
|--------|--------|------------------------------------------------------------|
| 8 | 10 | image specification. |
| | | |
| 8 | 2 | x origin of image. |
| | | integer ( lo-hi ) x coordinate of the lower left corner |
| | | of the image. |
| | | |
| 10 | 2 | y origin of image. |
| | | integer ( lo-hi ) y coordinate of the lower left corner |
| | | of the image. |
| | | |
| 12 | 2 | width of image. |
| | | integer ( lo-hi ) width of the image in pixels. |
| | | |
| 14 | 2 | height of image. |
| | | integer ( lo-hi ) height of the image in pixels. |
| | | |
| 16 | 1 | image pixel size. |
| | | number of bits in a stored pixel index. |
| | | |
| 17 | 1 | image descriptor byte. |
| | | bits 3-0 - number of attribute bits associated with each |
| | | pixel. |
| | | bit 4 - reserved. must be set to 0. |
| | | bit 5 - screen origin bit. |
| | | 0 = origin in lower left-hand corner. |
| | | 1 = origin in upper left-hand corner. |
| | | must be 0 for truevision images. |
| | | bits 7-6 - data storage interleaving flag. |
| | | 00 = non-interleaved. |
| | | 01 = two-way (even/odd) interleaving. |
| | | 10 = four way interleaving. |
| | | 11 = reserved. |
| | | this entire byte should be set to 0. don't ask me. |
| | | |
|--------|--------|------------------------------------------------------------|
| 18 | varies | image identification field. |
| | | contains a free-form identification field of the length |
| | | specified in byte 1 of the image record. it's usually |
| | | omitted ( length in byte 1 = 0 ), but can be up to 255 |
| | | characters. if more identification information is |
| | | required, it can be stored after the image data. |
| | | |
|--------|--------|------------------------------------------------------------|
| varies | varies | color map data. |
| | | |
| | | the offset is determined by the size of the image |
| | | identification field. the length is determined by |
| | | the color map specification, which describes the |
| | | size of each entry and the number of entries. |
| | | each color map entry is 2, 3, or 4 bytes. |
| | | unused bits are assumed to specify attribute bits. |
| | | |
| | | the 4 byte entry contains 1 byte for blue, 1 byte |
| | | for green, 1 byte for red, and 1 byte of attribute |
| | | information, in that order.
| | | |
| | | the 3 byte entry contains 1 byte each of blue, green, |
| | | and red. |
| | | |
| | | the 2 byte entry is broken down as follows: |
| | | arrrrrgg gggbbbbb, where each letter represents a bit. |
| | | but, because of the lo-hi storage order, the first byte |
| | | coming from the file will actually be gggbbbbb, and the |
| | | second will be arrrrrgg. "a" represents an attribute bit. |
| | | |
|--------|--------|------------------------------------------------------------|
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