How can I use an image texture as a guide for weight painting?

Question

So I have an image of a map which I would like to use to paint vertex groups (for terrain feature particle systems) on certain areas of an object (a sphere).

I have mapped the map onto the sphere. But even if I am in material or textured view, when I go into weight paint mode the object turns blue so I can not see the map so I don't know where to paint.

My next idea was to have a second, slightly larger sphere (in the exact same place) with a semi-transparent material of the map on it such that I could see the map on the large sphere and the weights underneath on the sphere I am weight-paining. But it seems that material view does not show cycles transparent shaders. so the transparency on the large sphere shows up as opaque white and I can not see the weights on the sphere underneath that I am painting.

So is there a way to do this? Either by showing textures/materials in weight painting mode, or by showing the transparency of the material on the large sphere? Or some other way I haven't thought of yet to use an image as a reference to weight-paint on an object?

By the way, this is not a duplicate of this or this, both of those answers only work for BI materials it seems, I am using cycles.

Answer 1

Here is my solution: Create a plane and subdivide it (as many times as you like, depending on your weight paint resolution requirements). Unwrap the mesh. Now you want to create a sphere from a plane using shapekeys. See the Cegaton's answer on how to do it here: What is the best way to unwrap a sphere?

When your transformation is ready, check the Basis key and add your heightmap image as a background image. Scale the added image to fit the size of a plane and check the image display type to Front. Set the Opacity value as you like.

Now you can easily paint in Weight Paint Mode, seeing a heightmap texture.

Transform a plane to a sphere using shapekeys.

As you can see all work quite nice.

Answer 2

You can change the weight painting colors, which might help with this.

Go to the Edit menu and click on Preferences

Then go to Editing and find the Weight Paint section and click on Use Custom Colors. The default custom color ramp should have white at one end and black at the other. Adjust the colors in the ramp as you would any other color ramp in the node editor. If your ramp has white or a light grey on the left hand side, unpainted parts will show up more or less like a texture without an overlay. If you are in weight paint mode when you open preferences you should be able to see the overlay updating as you edit the ramp.