Region-based image segmentation partitions an image into regions based on pixel properties like homogeneity and spatial proximity. The key region-based methods are thresholding, clustering, region growing, and split-and-merge. Region growing works by aggregating neighboring pixels with similar attributes into regions starting from seed pixels. Split-and-merge first over-segments an image and then refines the segmentation by splitting regions with high variance and merging similar adjacent regions. Region-based segmentation is used for tasks like object recognition, image compression, and medical imaging.