Exploring Altera product, would like to know what software skill is required.

An fpga can definitely do what you're asking. Using hdl or opencl are both good options. But be aware being a good programmer will not make you good at designing for fpgas in opencl and especially not hdl. You really need to be good with parellel processing like dsps or cuda as a start.

I have no experience with opencl myself, but the impression I get from colleagues who have tried it is that opencl it's great for rapid prototyping, but if you want to squeeze performance/area out of your fpga, you need to spend the time designing a full hdl design.