Texturing with Alpha channels in Softimage XSI 3.01

2003 Rob Wuijster a.k.a. Rork

- part 2 -


2. Manually linking nodes, but what goes where?


With the examples from part 1, we understand what's happening "under the hood" of XSI when texturing a model. Also the creation and linking of nodes is more understandable from the previous examples.
If you started with part 1, just re-select the model and assign a new Phong material to it. This will get rid of the excisting Rendertree, but will keep the Texture Projection.

With the Rendertree still open in viewport C, we will create some nodes ourselfs. Click on the "Nodes button"in the top bar, and get an image node with "Nodes>Texture>Image", and a mixer node with "Nodes>Mixers>Mix 2 color".

Note:
A mix 2 color node is a smaller version of the Mix 8 color that XSI will use when assiging textures, but will work exactly the same.

Double click on the Image node to open the Image Property Page, from here we can again browse for a image and assign the texture projection. Now link the Image node by dragging the red "dot" to the color 1 "slot" of the mix2color node.

Note:
Be aware that for reassigning the texture support, the imagelink must have a complete "path" to the material node. E.g. the image node must be connected to the mixer 2 color node, that must be connected to the material node. Otherwise the dropdown menu for the texture support will be left blank. If there's only one texture support XSI will automatically assign this to the texture.

Now connect the Mix2color node to the ambient and diffuse "slots" of the Phong node.

Now your rendertree will look like the next image. If you don't see anything in the rendertree, be sure the object is still selected and click the update button. Draw a render region.


Basic Mix2color node values

But the result will look different than the previous method. The alpha channel is not showing up, and the texture blending seems way off. What happened??

Double click on the Mix 2 color node to open the Mixer Property Page, here we have the same options we saw in the Mixer 8 color node.
The only difference here is that the default weight values are wrong, changing them to 1,1,1 will correct the texture color. Also the "Multiply weight by alpha" is turned off by default. Turn this on to see the alpha channel in the renderregion.
Much better, in the render region it looks exactly like the previous method XSI automatically presented us.

When we open the Phong node Property Page we see that we also still have control over the specular color value and are able to blend/link even more textures here.


Still the texture will apply to the rules of the Phong node according to transparancy, reflection etc. If we want to change that, we need to add more nodes in the tree.


Adjusted weight values and "Multiply weight by alpha" option turned on


3. Creating a alpha channel with an extra B/W image


You can also use a black and white image to create a alpha channel. The second image will do exactly the same as the alpha channel, creating a "stamp through" hole in your texture.
But where/how to connect this extra texture?

First d
ouble click on the image node and browse for an image without the alpha channel. Open the Mix2color node by clicking on the triangular widget. Click on the "Nodes button"in the top bar, and get an extra image node with "Nodes>Texture>Image", and browse for the B/W image. Now link the Image node by dragging the red "dot" to the weight "slot" of the mix2color node. Assign the texture projection for the second image.

It doesn't really matter is your image is B/W or W/B, you can always add a invert node between the image and the mix2color node to switch the colors of the image and get the desired result. Get the invert node with the "Nodes>Image processing>Invert" command.

Now your rendertree(s) will look like the next image(s). If you don't see anything, be sure the object is still selected and click the update button.