Tool for annotation of images for semantic segmentation
Diffgram is really great for this! I used it for a construction monitoring project. It's Open Source. From their site:
Semantic Segmentation Tools:

Auto Bordering: Automatically detects edges to create 100% coverage
masks. Simple select the intersecting shape.
Combo Shapes: Create shapes that are partially curves and partially straight lines.
Points to Full Polygon: Draw a box (or a few points), get a polygon.
Correct Pre-Label Segmentation Map: Load an existing segmentation prediction (pixels.) Get an editable polygon for correction.

Semantic Segmentation Data Annotation