Combiners

Combiners are called this way because they are used to combine two different attributes. They could also be called mixers or selectors.So combiners are used to determine how much of the two attributes are mixed together. To see how, first lets import the "Cube.mdl" that is included on the CD. Make sure your grid is set at 5cm and select all the model and right-click to select "Snap to grid". Click-ON the "Snap Manipulator to Grid" button. Now move the cube vertically down so that its center is at y=0. OK, we have a 50cm cube centered at 0,0,0 so we're all set to start experimenting with materials.

Now create a new material. Right-click on its attribute and select "Change Type to" -> "Combiner" -> "Gradient". Keep the default settings but click the first attribute and set its color to white and then click the second attribute and set its color to black.

Click the material and drag it onto the cube object. Switch to front view and do a quick render.

First the obvious: What you see is a gradient from black to white. The gradient goes from -20cm down up to 20cm up on the cube. Below -20cm, the color is steady black and above 20cm the color is steady white. Exactly like the settings of the Gradient attributes panel.

What is less obvious is this: The gradient combiner actually does a combination of the black attribute and the white attribute. At -20cm, it selects 100% of the black attribute and 0% of the white attribute and at 20cm, it selects 100% of the white attribute and 0% of the black attribute and at midpoint (at 0cm) it selects 50% of black and 50% of white which gives a 50% gray.

So when used in this way, with intermediate values between 0% and 100%, the act as mixers of attributes. The fact that those attributes are black and white are just circumstantial.

Combiners are functions which output values between 0% and 100% (or in absolute values between 0 and 1). The way they output those values follows a programmed pattern. In the case of the gradient combiner, it simply transitions from 0% to 100% outputings all the intermediate values inbetween.

Now Right-Click on the Gradient Node and select "Change Type to" -> "Combiner" -> "Checker" and scale the material to 300% on both X, Y and Z and render.

The default settings for a Checker combiner is to output only either 0% or 100% values and no intermediates values. This way, it will produce only one or the other attribute and no mix of attributes.

When a combiner output a value of 0%, only the properties of the lower node will pass through and at 100%, only the properties of the upper node will pass through.

Used this way, with no intermediate values, combiners are used as selectors. They select one or the other attribute according to their output values. The black pattern we see here is the result of the value of the combiner letting passing only the attribute of the lower node. Conversely, the white pattern we see is only the result of the value of the combiner letting passing only the attribute of the upper node.

But you may also specify a Blur value for the Checker combiner which will produce intermediate output values. In this case, the checker combiner becomes a mixer.

Any combiner values between 0% and 100% will produce a mix between the properties of the two nodes. For instance, a combiner output value of 10% would mix 10% of the properties of the upper node with 90% of the properties of the lower node.

So when combiners are set to output only 0% or 100% values like the default checker or spherical combiners, they act as attribute separators. When combiners are set to output the whole range of values between 0% and 100% like the gradient combiner or the checker and spherical with blur, the act as mixers.

When working with combiners, it is important to grab this mixing of attributes concept. The attributes that gets mixed could be quite complex. They could be complete material trees in themselves. So the first thing to do is stop thinking about the combiners as if they where specialized kind of renderers. They are really used as mixers or selectors and using simple colors with them is just the superficial surface of their possible usage.