About the Envelope Tool
The Envelope tool lets you deform and warp part of a drawing using a grid envelope and Bezier handles. You can warp images to make them look like they are reflected in water or in a deformable mirror.
If you use the Envelope tool on a textured brush stroke, it is will deform its texture to fit the new shape of the brush stroke. Keep in mind that this will resample the texture, which is liable to make it lose quality.
You can also apply an animatable Envelope-like deformation on drawings or other elements in your scene using the Mesh Warp node—see Mesh Warp Node.