I know you can take an object and leave an imprint in another object, but what about with particles? Can I make a field of grass and imprint something in it so it makes a crop circle? Just wondering if it’s possible before I bother even setting up a field of grass. Thanks
Is this for animation or a still shot? If animated, does the crop circle appear over time (animated)? How to do such a thing will depend strongly on how it’s being used.
I would use 2 particle systems. One with the grass standing and second one with the grass folded. The systems would use a texture to influence the length/size. The texture for the folded grass would be the negative of the one for the standing grass so that you can simply draw your crop circles in black on white.
For a still image, you use 2 single images and, for an animation, 2 image sequences created with dynamic paint.
The only unanswered question for me so far is if it’s possible to comb the hair in the crop circles the right direction… I haven’t played much with hair. (Hair on GPU is still quite new.)
I made a quick test… but it looked more like a lawn mower going across a field of grass. I didn’t keep it.
Animation. The crop circle would already be imprinted.
If the imprint isn’t changing, does it need to be a sequence, or can it just be a still?
A sequence isn’t needed as long as the crop circle isn’t being produced as part of the animation. Kaluura’s suggestion is a good one but could be simplified depending on whether you want the grass to react to anything in the scene, such as wind, or a character walking through or over it. If no reaction is needed, the crushed grass could be primarily an image texture, with a lesser number of “crushed grass particles” to give it some dimensionality. The upright grass would be a densely-populated particle system using either a fat hair strand or a particle object. Using an image texture for the crushed portion would let you control the look of the crushed blades more completely than particles alone, could even be photo-sourced if you can find the right pic.
When using a hair system with a particle object you can comb the parent strands in whatever direction you want the particles to orient, but you can’t introduce any flex in the object particles, and the object particles don’t react naturally to forces like wind. For flex and reaction to forces you’ll need to use particle hair strands made to look like grass, and then comb the crop circle into place. This can be kind of tedious, but it can be done with a single particle system, and gives a lot of control over the result if you are willing to take the time. You can also use vertex groups and/or image textures to control various particle aspects like density and length.
Thanks for the reply, chipmasque. I already have a grass field set up by following a tutorial. But how do I integrate my crop circle image? And how should the image be? Should I take an image of real grass, only include the crop circle shape, and make all other space in the image transparent? And then, how do I tie that into my particle system?
A lot depends on the design & color scheme of your scene, but I would probably use a crushed-grass image as a diffuse texture for the entire field, with the circular portion positioned as needed for your layout. Then use either image texture masks or a vertex group to limit the upright particle grass to everywhere except the circle. The particle grass will hide a great deal of the underlying image texture, but it will help “sell” the idea of a field of grass, and maybe make a very dense grass system less of a necessity. Using an image mask that’s the inverse of the image mask for the upright grass particle system, you can make a second system with bent grass blades to help make the painted circle more dimensional. By “painted” I mean it’s part of the underlying image texture on the particle emitter – I’d be inclined to use photo-sources and construct the crushed grass circle in Photshop or GIMP.
I did something similar in this image from The Nezina Project. Under the particle system is a tiled image of straw-like crushed grass/hay, mostly hidden, but it also adds color to the particle stems. There are two particle systems, one for straight grass, fairly dense, the other for the bent grass area under the figures, much thinner, and with a lot of length and density modification using image masks. Your crop circle would require a lot more carefully-planned masks and particle system setup, but in principle, it’s very similar.
My suggestion is in a much simpler and novitiate vein than the previous ones but it might suffice for less demanding purposes…
Use vertex groups to define the two particle systems, and weight paint away the areas in the long grass system where you want the crushed grass to appear.
In fact with a little experimentation with assigning different VGs for length and roughness, you might easily be able to do this with a single particle system. I know I have had coiffures turn out looking like crop circle fields so why not the other way, eh?
@ DruBan – that’s basically what’s been suggested, but with a few more options to improve the result. One thing about vertex groups that may be a factor is that their shape is restricted to the mesh grid pattern, not an ideal way to get a smooth-edged circle, or finer detail. Image masks give you more flexibility for producing complex patterns. A hair system is a good choice but takes a lot of effort to groom accurately, and the single-strand grass blades my not be an ideal solution, particularly with Cycles hair, which has a definitive geometry of its own. Using Object particles with either a standard or hair particle system allows custom blade geometry but the particles don’t react naturalistically to forces, no flex reaction, unless converted to normal geometry, which I have also done but forgot to mention. Too many ways to skin this cat! . In the end, it’s a matter of making choices based on how the end result is envisioned, and sometimes making some compromises and doing a workaround or two. I’ve done basically all the methods I describe, and use each according to the scene needs. None is a ideal solution so it’s always good to have options.