Using VEX to Art Direct your Copy Stamp (Part 2)

In the last post I had spoken about how we can use VEX to Art Direct our Copy Stamps and Wedges but it was more of a manual approach to setting uthe system. We can now begin to build a system that would allow users to deal less with the VEX code itself and more with the Houdini Interface to not only setup their Art Direction parameters and systems but also create setups quickly! The goal is to slowly build an HDA that encompasses the entire tool and allows users the opportunity to simply add attributes and parameters on demand without having to deal with the pain of setting things up manually.

Before we begin with this post, lets look back at some of the issues with the from the previous system:

1. Manual Process: If an Artist wants to setup the Parameters he has to make sure the names of the parameters and the attribute name on the VEX Code match each other. This has a huge scope for mistakes and can cause things to break even if one or the other changes.

2. Tedious: The user has hard code the attribute names into the code along with its type, this can be tedious and reduce the scope of working fast or customizing the system according to your shot. What if you suddenly decide you want the attribute to be a global attribute (same value on all points)? What if you suddenly want to switch the type from a vector to float etc? What if you want to suddenly add attributes specific to a shot on demand? These issues mean that user have to pay attention to every step carefully and reduces the scope for a fast workflow or can lead to the system breaking easily.

3. Tough to debug immediately: Although it is not at a complex system as it involves several aspects of the basics of Houdini Parameter Interface, debugging things can be complicate as you have to run through all the steps every time to find the issue. As mentioned earlier, even one hiccup can cause the flow to break.

How do we Solve these Issues?

If we break the systems into three simple sections we can categories it as "The VEX or Expressions Section", "The Parameter Interface Section" and "The Copy Stamp or Wedge Workflow Section". In this Article I concentrated on getting rid of the "The VEX Code Section" by making a generic code that works entirely on the Parameter Interface allowing Artist to quickly set things up without dealing with Hard Coding things. However at this moment every time a user add a new attribute, he or she still has to manually add a parameter with the same name to match it, this is a later step to tackle things and automate it using python etc.

In the previous code we were concentrating on creating the value for the attribute on each point or wedge:

string pt = itoa(@ptnum);

@val = ch("value" + pt);

However in this case we be looking at the other side, giving user controls to decide the name of the attribute itself. This is important as the points can be used for various things also besides copy stamping, the other advantage is that we can give common names to attributes that Houdini Copy Stamp or Instance already recognizes like "orient (vector4)" etc. This would mean the user doesnt even have to use the stamp function for simple things and can take full advantage Houdini's powerful Instancing. To do that, just like last time I created Folder (Multiparm Block List) abut this time I specifically renamed it to Attrib_List (This will be very important in the VEX Code as I will be referencing this integer parameter) and added three parameters to it:

globalVariable# - Toggle

type# - Ordered Menu (Contains a List of Options to select Int, Float, Vector or Strings)

Attrib_Name# - String

Till now these parameter serve no purpose as they are just a bunch of information, we need to store them in variables and use them to declare our attribute names. In this case ill press the "+" and create 5 attributes and its types.

Lets take a look at the VEX Code that will use this information:

//Lets store the number of attributes created in an integer

int cnt = chi("Attrib_List");

//Now we need to store the names of the Attributes, to do this I am using an array to create a list.

string attrib[];

//Finally lets declare some variables to use later int i, type, global;

//The next step would be to create a Loop that will iterate on the "cnt" and append each name in every parameter to the array as well check to see if it is a global variable (One value for all points) and its type

//declare attribs for(i = 0; i <= cnt; i++) { type = chi("type" + itoa(i)); global = chi("globalVariable" + itoa(i)); append(attrib, chs("Attrib_Name" + itoa(i)));


Now that we got our information stored kept we can use to set attributes, its type and its value on each point.

Lets run a quick check to see our array string attribute has the names stored correctly from the parameters.

s@temp = ATTRIB_ARRAY[index_number]

s@temp = attrib[0]

In the geometry spread sheet you should be able to see the value as you keep changing the index of the array.

Why was this useful and easier than hard coding the names and type?

This will now allow us to simply use all this information provided by users and declare values for custom named attributes immediately allowing users to add and subtract attributes, change its type and use one value for all anytime without changing too much in the code. It will allow an artistic freedom without the tedious work of taking care of not breaking things.

How do we use this Information?

We can create a dictionary or library to reference depending on the situation or as in this case I have used If Else and Else If Conditions to keep things simple. Inside the If Else condition I used a setpointattrib to declare the name and type of the attribute, I did this because it was easy to control and the setpointattrib changes the attribute type depending on the variable it has been fed. That means that if I declare a variable called "val" and set it to "string" the setpointattrib will set my attribute also to string but if I change my type to "int" it will create my attribute as an integer.

int val = chi(attrib[i] + slidercnt);

setpointattrib(geoself(), attrib[i], @ptnum, val, "set");

Lets take a look at the second half of the VEX Code:

//declare variables

int cnt = chi("Attrib_List"); string slidercnt = itoa(@ptnum); string attrib[]; int i, type, global;

//declare attribs

for(i = 0; i <= cnt; i++) { type = chi("type" + itoa(i)); global = chi("globalVariable" + itoa(i)); append(attrib, chs("Attrib_Name" + itoa(i)));

//check if global variable toggle on if(global == 0) { //check type

if(type == 0) { int val = chi(attrib[i] + slidercnt); setpointattrib(geoself(), attrib[i], @ptnum, val, "set"); } else if(type == 1) { float val = ch(attrib[i] + slidercnt); setpointattrib(geoself(), attrib[i], @ptnum, val, "set"); } else if(type == 2) { string val = chs(attrib[i] + slidercnt); setpointattrib(geoself(), attrib[i], @ptnum, val, "set"); } else if(type == 3) { vector val = chv(attrib[i] + slidercnt); setpointattrib(geoself(), attrib[i], @ptnum, val, "set"); } } else { if(type == 0) { int val = chi(attrib[i]); setpointattrib(geoself(), attrib[i], @ptnum, val, "set"); } else if(type == 1) { float val = chf(attrib[i]); setpointattrib(geoself(), attrib[i], @ptnum, val, "set"); } else if(type == 2) { string val = chs(attrib[i]); setpointattrib(geoself(), attrib[i], @ptnum, val, "set"); } else if(type == 3) { vector val = chv(attrib[i]); setpointattrib(geoself(), attrib[i], @ptnum, val, "set"); } } }

Similar to Last time we setup a Parameter Interface that have the equivalent names to the Attrib_List names and now we have a fully working system that Artist can control without worrying about the VEX Code.

I have a simple example where I used this system to create raining Squabs falling. In the hip example I first simulated the squabs falling using wedges but I gave Artist the freedom to decide if the squab hits the Light Post or not, rotations, position offsets and scale.

Using this cache data I setup a Copy Stamp network in which Artist can Art Direct the crowd according to their needs. I even used the same system to randomize the BG Squabs.

It's Raining Squabs:

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