The root cause of the CWE-190  vulnerability in the given code slice is:

The code uses the atoi() function to convert the command-line argument (optarg) to an integer value for horizSubSampling without proper validation. If the user provides a value of "0" as the argument, atoi() will return 0, which could later be used as a divisor in calculations, potentially leading to a divide-by-zero error.

The main issues contributing to this vulnerability are:

1. Lack of input validation: The code doesn't check if the input string is a valid positive integer before conversion.

2. Unchecked use of atoi(): The return value of atoi() is not validated to ensure it's not zero or negative.

3. Implicit trust in user input: The code assumes the user will provide a valid, non-zero value for horizSubSampling.

To fix this vulnerability, the code should include proper input validation and error handling to ensure that horizSubSampling is always set to a valid, positive, non-zero value before it's used in any calculations.