Layer transparency slider in GIMP isn't working

Question

I've got two layers in a gimp file. I want to be able to slide the top layer in and out of transparency so I can use it as a guide for the image below. When I select the layer in the layers box and then scale the opacity bar at the top the image remains opaque until the slider hits 50% at which point the image underneath is fully visible with the top layer entirely transparent.

Is there a solution that will let me slide the layer in and out of transparency?

Answer 1

This is because your image is color-indexed (opened a GIF?). In this mode opacity is "binary". For progressive opacity you have to use Grayscale or RGB mode (Image>Mode>RGB).