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I'd like to have the number of particles greater in the bottom , and they get lower towards the upper parts of the cube, it's because I don't want them to spread homogenously. How can I make it I have no idea? can you please help me? thanks

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Use weight painting to create a vertex group which will control density of the particles or create new texture in the particle system settings.

Weight painting

Prepare mesh topology

Weight paint affects on vertices, so in order painting to be smoother you'll need a lot of vertices. The more of them, the smoother will be painting, and so distribution of particles.

Use loopcuts or Subdivision Surface modifier to make topology of the mesh more dense.

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Use Weight painting to determine how to spread particles

Create new vertex group (or it will be added automatically), switch Object mode to Weight Paint and use brushes to paint areas where you'd like particles to be located more dense. Weight of 0 will make no particles in that area, weight of 1 will make to add maximum amount of particles (if the vertex group effect isn't inverted).
You can also use Weight tools, located in Tools panel (opened with T) to mirror, invert, normalize weights etc.

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Set created vertex group to control density of particles

In Vertex Groups scroll locate Density field and choose created vertex group.

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Note that you could also invert vertex group influence, to make particles being distributed in the top of the mesh instead of bottom (button just near the dropdown menu where you choose vertex group to affect).


Use texture to control particles' density

In the settings of the Particle system create new texture:

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For simple setup Blend texture may be used. Moving control points of the ColorRamp will make density of the particles change.

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The last thing is that by default this texture affects only time, not density. So in the Influence scroll check Density (you can leave Time if you need it).

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  • $\begingroup$ thank you very much for this very helpful and informative reply, i'm gonna study them now :) $\endgroup$ – ideorium Dec 16 '15 at 7:27
  • $\begingroup$ yes weight paint method did work very well for me too , thanks again.. I haven't tried the texture method yet tho, I kinda preferred the randomness of hand painted weight paint $\endgroup$ – ideorium Dec 16 '15 at 7:58
  • $\begingroup$ Mr Zak have you experienced success with Blender Internal Render .... with a image sequence to control location/density? Not Cycles Render. $\endgroup$ – atomicbezierslinger Dec 18 '15 at 3:09
  • $\begingroup$ @atomicbezierslinger If I remember correctly, that was the BI where I've done the test file presented in the answer. However, I tried that in BI now and it works for me. Still I'm not sure what do you mean saying "image sequence". I used simple texture for particles - either procedural or image file. $\endgroup$ – Mr Zak Dec 18 '15 at 13:26
  • $\begingroup$ In the Cycles particles, then texture panel, [Type] field Image or Movie, Image Panel ... [Source] field. There are 4 choices on the drop down menu in the Blender GUI. Generated, Movie, [Image Sequence], Single Image. Perhaps [Single Image] is the most common. I selected more than one image, which is an animated texture, image sequence. Because I want to generate particles from an image sequence, rather than a single image. You can try this out by selecting multiple images in an new particle texture example. ..... Have you had any success with an image sequence? $\endgroup$ – atomicbezierslinger Dec 18 '15 at 17:12

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