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4. Modeling grass

Next let's build a blade of grass using a parabolic arc as a spine.
Click here to review perpendicular lines.

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  • piceratops ultimate style avatar for user Freya Murphy
    How are the widths of the skeleton lines determined? Are they based on another parabola?
    (58 votes)
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  • male robot donald style avatar for user Servaich
    How the skeleton works mathematically?
    (9 votes)
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    • leafers ultimate style avatar for user Bryan Ray
      He kind of glossed over that bit, didn't he? Here's one approach:

      You can use the intersections of the individual line segments making up the curve to evenly distribute the skeleton lines along the curve. Say, every fourth intersection gets a displayed skeleton line.

      Then you need to determine the angle of each skeleton line. The skeleton lines are all perpendicular to the curve. Trouble is, we've located them on the intersections, where there is a very shallow corner. Well, you can't actually get a line that's perpendicular to a corner, but what you can do is bisect the angle of that corner. Create a line that intersects with the corner such that the angles formed with the line segments on either side of the new line are equal. Or such that the angles on opposite sides of the new line are equal to 180 degrees. Either way gets the same result.

      Now what about the length of the skeleton lines? If you look carefully at the endpoints of those lines, you might notice that they, like the main curve, describe the same kind of parabolic arc. The lower control points, where the base of the blade is, are offset from the center. There is probably a hidden parameter in the drawing program that determines the width of the base. The upper control point is in the same location as that of the main arc. Draw a line perpendicular to the curve that passes through the middle control point of the main arc, the middle control points of both skeleton curves will lie on that line. The width control determines the distance of those control points from the main arc.
      (10 votes)
  • aqualine sapling style avatar for user Sanya
    What kind of program is this?
    (6 votes)
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  • piceratops ultimate style avatar for user trek
    What about modeling something like ferns? Couldn't we simply adjust the skeleton algorithm? Looks like we'd need to have user inputs for tweaking the amplitude and period of the sine wave. I mean, sure, it is an increase in complexiy, but it would allow the artists to create other species of underbrush, right?
    (4 votes)
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  • aqualine sapling style avatar for user Jade Meinke
    what link was he talking about?
    (2 votes)
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  • leafers seed style avatar for user Matheus Luis da Silva
    Which Software I need to use for this ?
    (2 votes)
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  • aqualine seed style avatar for user emmanueld083
    what app or software can i install in my system for me to be able to do animations
    (2 votes)
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    • blobby green style avatar for user Mihai Walther Ianculescu
      You have a wide variety of tools to choose, from commercial ones to open source tools. I would recommend using Blender to start, because it's free and it contains pretty much everything you need to learn, from modeling tools to rigging, texturing, animating and rendering your work and move on to some commercial software if you intend to become a professional 3D artist. However it's not really about the tools, it's more about studying, a lot, understanding the way the computer works to calculate various simulations, studying the main techniques, etc. For example, when modeling, once you learn the proper way to distribute the polygons on a model (proper topology) in one software you will know how to do that in any other related software, you'll just have to accommodate with the key bindings and commands.
      (2 votes)
  • piceratops seedling style avatar for user Patricia
    What are the usual widths parameters used in Brave or other movies for creating grass? This is amazing!
    (2 votes)
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  • blobby green style avatar for user gatik goyal
    Do artists have to make each blade of grass or is there a program that makes them for the artist.
    (1 vote)
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  • primosaur ultimate style avatar for user Tyler Harris
    How do you create millions and millions of blades of grass?
    (1 vote)
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Video transcript

(clicking) Great work! Now we've got all the math we need for a computer program to draw as many lines as we need to create a good approximation to a parabola. Let's return to some of the artistic aspects of grass starting with the width. In this interactive, I can move the points around and I haven't drawn the legs of the control polygon just to make it look a little bit simpler and, let's see, that kind of shape for the spine looks pretty good. So let me move on to the next step which is to define the width. I'm going to do that using something we'll call a Skeleton. The Skeleton is a set of lines, like these, that cut across the blade of grass at 90 degree angles so here's a Skeleton line, here's a Skeleton line and so on. If I connect all the left sides of the Skeleton lines and the right side of the Skeleton lines, I create a region, let me fill that in, and that region looks a lot more like a blade of grass. (click) (tinkling) I can control the width using this slider here so that's just adjusting the length of those Skeleton lines and that width there looks pretty good. Finally, I can control the color. I can go from a deep green to a lighter green to a really light green. Our artists control parameters like this to create the kind of convincing grass that you see in "Brave." Use this interactive to see what kinds of blades of grass you can create and then, in the next video, we'll talk about how to animate it.