Hi,
I have been working on an example houdini file. There is a “Line” SOP going into a “CreateAttrib” SOP… in the “CreateAttrib” SOP there is a variable called “width”… and then there is a value.

So this creates a variable called width! But who uses it?!… Why this variable is able to actually changle the width of the line created by the “Line” SOP?

I am confused.

The ‘width’ attribute in regards to lines or curves is used to determine the width of the line when it is rendered.

Thanks!
So it is a hidden attribute that can be manipulated only through a CreateAttrib SOP? How can one see all hidden attributes that may be manipulated through a CreateAttrib SOP? Because there is nothing like WIDTH in the parameters of the Line SOP…

it's not a /hidden/ attribute..
the attributCreate SOP - /creates/ the attribute…
you can create any kind of attribute you want…even a string attribute called “fogaboogadingdong” with a value of “HelloEvilClown”
note that this attribute is almost useless…
in the case of the ‘width’ attribute it is useful for rendering curves as grahamt has pointed out.

Ok…

But it seems like the “width” attribute is hardcoded somewhere…
and the line SOP somehow uses it… how can I see a list of useful attributes that actually do not exist in the SOP itself… bur can be called with the CreatAttrib SOP…

Also… it is not working when I rename it “widthjklt”… and there is no other expression calling “widthjklt”… so it is a hardcoded attribute that changes something for the line SOP or the curve SOP…

It is not something like the STOMP function inside the COPY SOP where you can define a variable and call it somewhere UP in the network… right?

Thanks for your help… I am a real newbie… trying to find my way inside Houdini…

how can I see a list of useful attributes that actually do not exist in the SOP itself… bur can be called with the CreatAttrib SOP…

A valid question, without a simple answer. “widith” is special because mantra will use it at render time. Other attributes are used other places - for example, “klinear” is a special attribute that has meaning inside of DOPs. Generally, people create attributes because they know those attributes will be used elsewhere in Houdini. Then that use defines what the attribute is. If you are creating sprites, then special attributes are required for that, etc. So the problem you are trying to solve will determine the attributes that you want to use. And of course you can invent your own, for your own special purposes.

You can see what attributes are attached to your geometry by either:
1) Middle Mouse on a SOP tile. You'll see them listed there, along with the mapping to the local variable name that you can use to reference that attribute in an expression
2) Right Mouse on a node, and choose “Geometry Spreadsheet”. This will let you see all the attributes and their values.

Thanks. I kind of understand the attribute thing now.
I actually made some experiments. I had some lines to move with the “sin($F)” function. I created an attribute called “WIND” and keyframed it… then called it inside a “POINT” SOP and used it with the “sin($F)” like “sin($F)*$wind”… to manipulate the strenght of the motion.

Thanks.

Hi Symbolic,

I haven't worked much with attributes yet (still a noob too) but i found some reference in the houdini help system

hdox//houdini/content/base/model_attrs.html
hdox//houdini/content/base/mantra_attrs.html

Guess that there are specific attributes for the POP and DOP environments too…

The “Details View” Pane (hit ALT+4 in any pane) is particularly useful. It displays the attrbutes for the currently selected SOP/DOP etc.

Also note that attributes have a ‘class’: ie. you can have attributes assigned to points, verts, prims, and/or detail (which means one per object). When using the “Details View”, make sure the menu at the top of the Pane is set to the sort of attribute class you're examining. That is, it's quite possible to have 2 different attributes with the same name (eg. “blah”) where one is assigned to points, and the other to primitives. The two can have different values. Colour is a good example of an attribute.

cheers,
b.

ben simons
The “Details View” Pane (hit ALT+4 in any pane) is particularly useful. It displays the attrbutes for the currently selected SOP/DOP etc.

Also a useful tool for learning what gets passed from SOP to SOP (attribute wise, that is) is to write out a .geo file and *look* at it! Then stash it away, make a slight modification with the attribute SOP, then write that out, look at it, and best of all, do a ‘diff’ on them to see the difference between the two.
Once you've done that, you can recorrelate that difference in what you see with mouse-middle on the SOP itself.

– Antoine