Victor Image Processing Library Application Notes

LZW Compression

What is LZW Compression?

LZW (short for Lempel-Ziv-Welch) is a lossless compression technique that is an integral part of the GIF image file format and an option in TIFF image files.

The Lempel-Ziv algorithm looks for repeated pixel combinations and builds a lookup table (on-the-fly) of compression codes to represent the combinations. The codes are Huffman-encoded for further compression efficiency.

LZW compression is commonly used for 1- through 8-bit palette color images and less often for 24-bit RGB images. Typically an LZW-compressed palette color image is 60 to 80% of the original size. But for some images LZW can create a larger file than the original uncompressed image requires. Compression efficiency varies based on image complexity.

Why Use LZW Compression?

Two reasons . . .

A Comparison of Compression Efficiencies

This picture of yellow dendrobium flowers began as a 24-bit RGB image (314 x 237 pixels). Table 1 shows the sizes of the resulting files when the image is saved using various compression techniques. 

Table 1. Comparison of Compression Efficiencies for a 24-bit Image

Compression     Image bits/pixel  File Format   Size (kb) (% of original)
none                  24             TIFF       223,646       100
LZW                   24             TIFF       243,829       109
run length            24             TGA        224,055       101
JPEG quality=75       24             JPEG        14,572         7
JPEG quality=50       24             JPEG         9,413         4
Notice that for the 24-bit image LZW (or run length encoding) is not really compressing but actually increasing the space requirements. This demonstrates why LZW compression is seldom used for 24-bit images. (Be sure to see the JPEG Application Note.)

Using color reduction (with optimized palette and diffusion scatter) the original image is converted to 8 bits per pixel. Color reduction cuts the image size by 2/3. Now examine the resulting file sizes when the image is saved in 8-bit formats. 


Table 2. Comparison of Compression Efficiencies for an 8-bit Image

Compression     Image bits/pixel  File Format   Size (kb) (% of original)
none                   8             TIFF        77,810       100
run length             8             PCX         79,028       102 
LZW                    8             GIF         59,582        77
Table 2 shows typical LZW compression efficiency for palette color images. While it is not always true, at least in this case LZW would not be the compression of choice. (To compare the image quality and speed of transmission of LZW vs. JPEG visit our website, www.catenary.com/victor and jump to this same application note. The relevent files are dendrob.gif and qual50.jpg.)

License Considerations

The Victor Library includes complete support for LZW compression and decompression within the GIF and TIFF-LZW image file formats. Unfortunately, due to the license agreement imposed by Unisys this LZW functionality is locked until you provide proof of a valid LZW software license agreement with Unisys.

Unisys holds the patent rights to the LZW compression technology used in GIF and TIFF-LZW image files. The LZW license agreement is necessary if you wish to use this technology. This is true regardless of how you implement LZW support -- whatever library you use or even if you write all the code yourself. Visit the Unisys website (www.unisys.com) for further information. The license generally requires modest royalty payments which will not cause financial hardship to any developer.

LZW Compression with Victor

The Victor Library includes complete support for LZW compression and decompression within the GIF and TIFF-LZW image file formats. Table 3 lists the relevant functions. 


Table 3. Victor Functions Supporting LZW Compression and Decompression

Victor Function   File Format    Comments
loadtif           TIFF           8- and 24-bit images may be LZW-compressed
loadtifpage       TIFF           8- and 24-bit images may be LZW-compressed
savetif           TIFF           LZW is an option for 8- and 24-bit images 
savetifpage       TIFF           LZW is an option for 8- and 24-bit images 
loadgif           GIF            all GIF images are LZW compressed
savegif           GIF            1- and 8-bit images are LZW compressed
savegifex         GIF            1-, 4-, and 8-bit images are LZW compressed
                                 with interlace and transparency options
The TIFF functions all perform normally for non-LZW compressed images, that is, no unlocking is necessary.

Sample Code - How to Load an Image from a GIF File

The following small C function shows how to load an image from a GIF file. The image is stored in memory as a device independent bitmap, DIB. 
int load_a_gif(char *fname, imgdes *image)
{
   GifData gdat;
   int rcode;

   // Get info on the GIF file we're to load 
   gifinfo(fname, &gdat);
   // Allocate space for the image based on the data in the GIF file
   allocimage(image, gdat.width, gdat.length, gdat.vbitcount);
    
   rcode = loadgif(fname, image); // Assumes LZW functionality previously unlocked
   return(rcode);
}
As soon as the image is loaded it is eligible for use with any of the Victor functions for processing, printing, saving, or display. See the list of Victor functions.

Sample Code - How to Save a Transparent Interlaced Image to a GIF File

The following small C function shows how to save an image to a GIF file. The image will be saved in interlaced mode with color number 43 transparent. 
#define TRANSP_INTERLACED = 3

int save_a_gif(char *fname, imgdes *image)
{
   int rcode;
   int savemode = TRANSP_INTERLACED;
   int transcolor = 43;
    
   rcode = savegifex(fname, image, savemode, transcolor); // Assumes LZW functionality previously unlocked
   return(rcode);
}

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