System Information
Windows 8.1, Nvidia GTX 760 M

Blender Version
Broken: 2.79 5bd8ac9

Short description of error

Using Filter node as Image input to Scale node and Texture node as input to X and Y offsets of Scale node renders empty image.

Exact steps for others to reproduce the error

Switch to Compositing
Add Texture node with Clouds texture
Add Filter Node with Sobel on
Add Scale Node with Relative on
Add Viewer node
Enable Backdrop
Connect Images of Texture To Sobel To Scale To Viewer
Connect Texture Color output to X input of Scale node
After that in my case - the Backdrop is empty. Viewer Node in Image Editor is transparent or black if disabled Use Alpha.

File with the node setup: