Texturing with Alpha channels in Softimage XSI 3.01

© 2003 Rob Wuijster a.k.a. Rork

- part 3 -


4. Blending images using the weight node

Now we know how to use the weight node to create an alpha channel with a B/W image, we can also use this technique to blend images together. First we're gonna move the excisting image connection over from the color1 "slot" to the Base_color "slot".
This image is now driving the color of the object, replacing the base color value of the "Mix2color" node.

Now add another image node with "Nodes>Texture>Image", browse for an image and assign the texture projection.
Connect this new image to the now free color1 "slot" of the "Mix2color" node.

This image is now used as the texture. The third image, still connected to the weight node wil "stamp" the new texture connected to the color1 "slot", to reveil the image now used as the base_color. Again you can use a invert node to switch the colors of the image.

Note: When disconnection nodes, be sure not to click on the refresh button untill you're finished, or your tree is gone. And nope, the Rendertree has NO undo!!!!

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



4a. Blending images using the weight node, but now WITH an alpha channel in the image

Now we know how to use the weight node to create an alpha channel with a B/W image, we can also use the same technique in a slightly different way. What if the image in the weight node DOES contain an alpha channel. How to use this instead of the RGB values from the B/W image??

We need to "extract" the alpha from the image with "Nodes>Color Channel>Pick". Disconnect the BW-image node and reconnect it to the input "slot" of the new "pick_channel" node, and connect the "pick_channel" node back into the weight "slot" of the "Mix2color" node. A "scalar2color" node is automatically inserted inbetween.

Now doubleclick the "pick_color node" to open the PPG and be sure to set the color space to RGBA and the channel RGBA to alpha. In the renderregion you will see the B/W created "alpha" result is replaced by the real alpha channel, changing from a text to a logo "stamp" through.

Note: When disconnection nodes, be sure not to click on the refresh button untill you're finished, or your tree is gone. And nope, the Rendertree has NO undo!!!!

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



4b. Oh no..., another rendertree but the same result:

To keep our options open we can do this again in a slightly different way. Instead of using the "pick_channel" and "scalar2color" nodes, we also can do this directly with "Nodes>Color Channel>Color2Alpha". Reconnect the image to the input "slot" of the new "color2alpha" node, and connect the "color2alpha" node back into the weight "slot" of the "Mix2color" node.
Now doubleclick the "color2alpha" node and leave the color input values alone. You can use the Factor value to blend your image with the underlying one..

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


So there you have it.

You can go the easy way and apply alpha to your images and apply these images directly from the interface, adding even more textures if needed. Change the blending of the images by adjusting the weight values and Mix options of the separate layers. The alpha channel(s) will automatically be added into the mix.

You can also set up the rendertree by hand and creating all the nodes manually, and creating alpha channels with an extra BW image by using the weight "slot". Adding to that, you can still use an image WITH alpha to plug into the weight "slot" and blend images this way by extracting the alpha channel.

I would say more than enough options to get your object textured the way you want.


That's all folks ;)
These tutorials are partly based on tutorials from Stefan Andersson and Patrick Vier, the links are found at http://www.edharriss.com/. Check this site out for a LOT more tutorials, plugins, add-ons etc.