SobelFilter, different results from C and OpenCL Implementations

So a number of reasons might be the problem. The first is how you're sending and reading data to and from the kernel. Make sure the data that you read into the kernel is what you expect, the same with when you're reading from the kernel. Other problems maybe how you partition the work items and work groups. The order of operations if some operations depend on previous data can be out of place and thus give you the wrong results.