Decoding the width and height of a GIF file

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Overview

The width and height of a GIF image might seem irrelevant to the usual programmer, but when it is required many programmers might not realize how easy it is to decode.Unlike the height and width of a JPEG file, the height of the gif is extremely easy to decode from its header. There is a manual conversion from 2 bytes to a short in this code to make it compatible for any processor and platform - a problem I acutally run into with some code while doing programming for both 68K and x86 CPUs.

Source Code

Decoding Function C++ source code in 9 lines

//Gets the GIF size from the array of data passed to the function

static char get_gif_size(unsigned char* data, unsigned int data_size, unsigned short *width, unsigned short *height) {

   //Check for valid GIF file (min 10 bytes for header and size, and the GIF signature)

   if(data_size >= 10 && (data[0] == 'G' && data[1] == 'I' && data[2] == 'F')) {

               *height = data[7]*256 + data[6];

               *width = data[9]*256 + data[8];

               return true;

   } return false;                     //Not a valid GIF file

}

Change Log

June 10, 2006

small fix for a bug which retrieved reversed heights

References

Gif File Format Specification - a very detailed specification of the gif file format, can be used for a full decoder

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All article content is copyright of Michael Petrov, 2012©.
The source code presented in the articles is distributed as freeware, please feel free to use it in your own projects - both commercial and non commercial.