Mixing bumpmaps with the bumpmap generator node

		Simple bumpmap mixing in Softimage XSI 3.0
		© 2004 Rob Wuijster Introduction: Simple bumpmapping is pretty straight forward in XSI, just by turning on bumpmapping in the texture PPG and fiddle with the settings. Or you can get a Bump map generator, get an image and work from there the same way. But what if you want to mix your bumps? You could slapping on a second texture and turning on the bump here too, adding it to the mix. But this method doesn't produces very predictable results, so we have to look for another method.
		Rendertree Setups Most people use the "vector2color" and "color2vector" nodes in combination with the "mixer2color" node and create elaborate Render trees from this. But what if you want to just simply mix 2 simple bumps and go on to the next thing. Well, you can do this by 'daisychaining' the "Bumpmap generator nodes" and setting its 'flow'. Note: Unfortuanetly for mixing displacements you cannot use this setup, you have to build yourself a more complex tree. For the example we create a simple polygon grid, assign a lambert material, add a XZ Projection and a simple grayscale cloudtexture build in PS. For the bumps we're using two simple B/W images with horizontal or vertical stripes. No need for a tutorial here, right??
		How to go on: Open the Rendertree, get your first "Bumpmap generator node" and connect it to the "Bump Map" slot of the Material node, now open the PPG. Select one of the two B/W images. (You can do this also within the PPG's, it's just how you prefer to work.) If you look at the "Normal" setting, you see that there are two options for the "Base normal". Change the "Relative to current surface normal" setting to "Relative to input normal". Get your second "Bumpmap generator node" and connect it to the "normal" slot of the first generator node. Again open the PPG and select the other B/W image. For this second node the "Normal" setting should state "Relative to current surface normal" (default value).
		Rendertree setup:
		Mixed like this by 'chaining' the bumpmap generators will give us a gridlike bump. You can even turn on the bump for the image itself to add to the mix. You can just add another generator to this set, just be sure the settings match. The last node (furthest away from the material node) must always be set to "Relative to current surface normal", the others should be set to "Relative to input normal". Note: Be aware of the following limitations, mixing negative and positive bumpvalues don't seem to work very well, and the node that connects to the Material's bump "slot" defines the bump output. So if you turn off bump for this node, nothing behind it in the other nodes will "flow through" to the material's bump slot. You can however turn off bumpnodes in between, or even add the bump from the texture (cloud in this case) to the mix. Well, that's all there is to it. Happy bumpin'
		Endresult You can get the same result by turning on the bump of the cloudtexture, instead of adding another "generator" node to the rendertree. A vector mixer node for mixing more complex setups are to be found here and here. You can also check out the forum at xsibase for more information about bumpmapping in XSI.