Basic Grasshopper Functionality
For the first real definition we cover, we are going to make a widget to put into practice what was covered in the previous interface section. Moreso, this widget will demonstrate in a toy manner, some of the major uses of Grasshopper. See this link for the file.
We will cover this and future definitions, step by step. Refer to the numbered component groups in the image below.
01 - How a Geometry Component Works
01A - Parallels with a Rhino Command
A single component in Grasshopper often roughly corresponds to a command in Rhino.
For example, run the Rhino command Sphere. You will be taken through a 2 step process via the Rhino command line.
- Rhino will first prompt you for the center of the sphere which you can provide either with your cursor or with coordinates through the command line.
- Rhino will prompt you for a radius which you can provide in one of the two ways above.
Grasshopper provides an analogous component, also named Sphere. To use it, open Grasshopper, click the Surface tab, then the Primitive dropdown menu, and select Sphere.
Take a look at the component on the canvas. Remembering that Grasshopper flows from left to right, we can intuit that we have 2 inputs: Base and Radius. Our single output, Sphere is the geometry itself. This Grasshopper component requires the same inputs to create the same output as the corresponding Rhino command.
This is fundamentally how Grasshopper works. We use components as flexible and non-destructive (parametric) stand-ins for commands we traditionally run through Rhino. In other words, once you've created that sphere in Rhino, you can't modify its properties without resorting to other additional commands such as scale3D, Ctrl+Z, etc. With Grasshopper, we can tweak the inputs to the sphere all day long!
01B - How Component Geometry is Represented Visually
Viewport Representation
Grasshopper geometry is represented much differently that Rhino geometry. To see this, zoom in towards the origin of your Rhino viewport. You'll see a translucent red sphere. Click the Sphere component, and the translucent sphere will turn green. This little ghostly sphere corresponds to the Sphere component.
Grasshopper geometry is a representation of geometry within a component
If a Grasshopper component creates, alters, or reorganizes geometry, there will be a corresponding representation in the Rhino viewport. This representation will be green if the component is selected in the GH canvas and red if the component is not selected in the canvas.
Now, if you try clicking on the Grasshopper sphere in Rhino, what happens? Nothing! You cannot interact with the Grasshopper geometry directly in Rhino in this way. Think back to that one undergrad philosophy course you took, and recall an allegory about a cave (but don't recall it too well!) Rhino representations of Grasshopper geometry are just that, representations of specific points in your parametric modeling process.
To illustrate the point that Rhino previews of Grasshopper geometry are representations, try placing a Move component onto the canvas, and hooking the output of Sphere into the Geometry input. You will see that both the original Sphere geometry and the new sphere, output from Move are present in the viewport.
Both spheres present in the viewport are representations of specific steps of the modeling process. This hilights another important point:
Grasshopper components are non-destructive
A grasshopper component does not erase the geometry or information contained in the previous step. Every step of the parametric modeling process is retained.
We will get into ways we can interact with Grasshopper through the Rhino viewport, when we talk about referencing geometry, but in general, the primary way that we interact with Grasshopper geometry is through the Grasshopper canvas.
Preview
Since the geometry we see in the Rhino viewport is a representation, we are able to turn on and off its visibility without effecting the geometry actually contained within the component. Try rmb Sphere and toggling preview.
When we toggle preview we are toggling whether or not the geometry representation is visible in Rhino. Turning off preview does not alter the underlying Grasshopper geometry or affect the following components.
Also note that when a component's preview is turned off, the component itself becomes a slightly darker shade of grey.
Enable
When we toggle enable we are toggling both the Rhino preview and the actual Grasshopper component. You can see that both spheres dissapear since Move doesn't have the original sphere flowing into it. Disabling a component disables the underlying geometry and makes the component's information unavailable to downstream components.
When a component is disabled, it takes on a more muted gray scheme. The orange wire also indicates that no data is flowing from the disabled component's output.
Delete
Finally, deleting a component from the canvas, will wipe its corresponding representation from the viewport, and make the now-deleted information unavailable to all downstream components. I'm deleting the Sphere in the .gif below, but try deleting the Move component to follow along with the next steps. You just lmb Move and hit Del.
01C - How Component Information is Represented Textually
While textual representations of geometry may seem overkill at this point, the textual representations of component data are often more useful than visual representations in the viewport. This will become more apparent when we start considering the organization of data within a definition, but even at this point, we can use these menus to learn the inputs and outputs of a specific component. Hover over the inputs, outputs, and icon of Sphere.
Hover over everything
In the input and output boxes shown above, we are shown three important pieces of data. Going from top to bottom:
- The name of the input or output.
- A description of what sort of data can be passed through the input or output.
- A description of the data currently flowing through an input or output.
01D - Baking
We've covered how the representations of Grasshopper geometry are not Rhino geometry. But how do we then bring our GH work into Rhino so we can finish our project? We bake it! Baking refers to converting Grasshopper geometry to Rhino geometry.
To bring geometry from Grasshopper into Rhino at any step of your definition, rmb the component you want to bake and select 🍳Bake.
This will bring up the generically titled Attributes window. Here, you can select which layer you want to bake your selected Grasshopper geometry to. You can also choose to group the geometry or set some display options.
You have now created a Rhino object from the component you baked that you can interact with in Rhino. Note, that this Rhino object that you have just baked exists totally independently of all your Grasshopper geometry. You can delete or transform it without any side effects in the Grasshopper canvas.
02 / 03 - Setting Parameters
Parameters in the context of Grasshopper refer to 2 things:
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Parameters are the variables of a Grasshopper definition. Think back to high school math - the function \(y=sin(x)\) has a parameter or variable \(x\). As \(x\) changes, the output \(y\) also changes. In this analogy, \(y\) is the geometry we will utlimately bake into Rhino, \(sin()\) is our Grasshopper definition, and \(x\) are our input parameters. However, in Grasshopper, instead of just numbers, we are able to pass geometry, text, user interaction and all sorts of things in as parameters.
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We also use the terms parameter to refer to the Grasshopper components which are used to pass in values. These special components are handily stored in the parameters tab.
Parameters & Components - What's the Difference
All the little 'nodes' that make up a Grasshopper definition (script) are called components. Parameters are a class of component that either reference data from Rhino, reference data from outside of Rhino / Grasshopper, or reference user input.
In this section of our definition, we are using 3 Number Slider parameters to define the X, Y, and Z values of a 3D point. To convert these values into the geometric point, we use Construct Point.
We also use a Number Slider to define the Radius of the Sphere.
The output of Construct Point is plugged into the the Base input of the Sphere component.
Grasshopper Type Conversions
Grasshopper often converts geometrical types that are close enough upon input. In this example, Construct Point outputs a 3D point which is plugged into Base in Sphere despite Base asking for a 3D plane. In cases like these Grasshopper will automatically convert a 3D point into a plane with an origin defined by that 3D point. Other conversions happen - try plugging the 'wrong' geometry into inputs to see what happens, but don't be surprised if you get an error sometimes!
04 / 05 - Drawing a Line
06 - The Input Expression Editor
Grasshopper offers a really useful way to quickly alter numerical values on component input. Let's say we want to ensure that our widget arm is always half a wide as the widget base. To do so, we can rmb the Radius of Pipe, select Expression and then write a short algebraic statement in the input box. In this case we can write x / 2. This will set our radius to always be half of the input value, and since our input Number Slider also controls the Sphere's radius, the radius of our pipe will always be the sphere radius / 2.
Always X!
The input Expression field can only ever contain one variable, X. If you need a more complex expression, you can always build it up outside of an input from components found in the Maths tab.
07 - Boolean Operations
08 - Geometry Analysis
Group 8 demonstrates how Grasshopper can be used for geometrical analysis, and how you can quickly build up algebraic functions.
In this instance, our goal is to determine how far off our widget's arm length is from a user set desired length. We begin by measuring the length of the Line component with Length. We use a Number Slider to define the desired length. We subtract the actual length from the desired length using Subtraction to give us the signed difference. Since we don't care about whether the arm length is less than or greater than the desired length, we get rid of the sign using Absolute. Finally we use a Panel to get a text output of how far away we are from our goal.
09 - Visualization
Often, visual feedback within the Rhino viewport is the most intuitive way to display conformance with design criteria. In this instance we will color our widget with Custom Preview. The color that feeds into the Material input is given by Gradient. If the widget is red, we are far from our goal, if it is green, we are close to or at our goal length.
Our output from Absolute is fed into Display's Parameter. Here we are telling Display to return a color based on the position of Absolute's output within the numerical range defined between Lower Limit and Upper Limit. By default, this range is 0 to 1.
We don't know how far from the goal we can get, but let's start with 100 for the Upper Limit.
Setting a Parameter Value Internally
If you rmb on a given parameter and select Set ..., you will be able to internally set a static value.
Now that we've set our upper limit to 100, we will see our widget's color update as we move further and closer from our design goal!
