COST_COLOR_GRAD vs COST_COLOR

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What are the differences between COST_COLOR_GRAD and COST_COLOR in the GraphCutSeamFinder class? And what criteria should be used to choose either or the other? The documentation doesn't explain their meaning. I've tried to measure the performance, and though I expected COST_COLOR_GRAD to be slower, the results are actually the same, neither I could see any difference in the overall final results.