A PPM file consists of two parts, a header and the image data. The header consists of at least three parts normally delinineated by carriage returns and/or linefeeds but the PPM specification only requires white space. The first “line” is a magic PPM identifier, it can be “P3″ or “P6″ (not including the double quotes!). The next line consists of the width and height of the image as ascii numbers. The last part of the header gives the maximum value of the colour components for the pixels, this allows the format to describe more than single byte (0..255) colour values. In addition to the above required lines, a comment can be placed anywhere with a “#” character, the comment extends to the end of the line.

The following are all valid PPM headers.

Header example 1
P6 1024 788 255

Header example 2
P6
1024 788
# A comment
255

Header example 3
P3
1024 # the image width
788 # the image height
# A comment
1023

The format of the image data itself depends on the magic PPM identifier. If it is “P3″ then the image is given as ascii text, the numerical value of each pixel ranges from 0 to the maximum value given in the header. The lines should not be longer than 70 characters.

PPM example 4
P3
# example from the man page
4 4
15
0 0 0 0 0 0 0 0 0 15 0 15
0 0 0 0 15 7 0 0 0 0 0 0
0 0 0 0 0 0 0 15 7 0 0 0
15 0 15 0 0 0 0 0 0 0 0 0

If the PPM magic identifier is “P6″ then the image data is stored in byte format, one byte per colour component (r,g,b). Comments can only occur before the last field of the header and only one byte may appear after the last header field, normally a carriage return or line feed. “P6″ image files are obviously smaller than “P3″ and much faster to read. Note that “P6″ PPM files can only be used for single byte colours.

While not required by the format specification it is a standard convention to store the image in top to bottom, left to right order. Each pixel is stored as a byte, value 0 == black, value 255 == white. The components are stored in the “usual” order, red – green – blue.

图形文件格式列表

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