"'... tried to make some 2m² grass patch but the trimming in't really clean as he is in the preview ..."
The Edge Boundary Checking
is removing elements within one item that are "based out of area used for distribution" - having any "overhanging faces" under one element, these will be fully removed or not, depending if the element is based inside/outside of the distribution area.
If any gaps appear, a recommendable way to fix these areas would be to adjust Distribution Map
settings - Density
or Offset X
values. That should fix it (as shown on attached screen-shots).
As mentioned above, there is no "physical cut" performed while using the Edge Boundary Checking mode (as this would be extremely resource demanding). This "trimmed part" is treated by the render engine as a kind of "Matte object" internally (this way render engine ignores it for its calculations).
"it doesn't follow a fine surface, and we have "flying part of the pattern""
I would suggest you to check the Direction
settings under the Surface
tab (probably is set to 100). In case of large clumps I would recommend you to set it to Surface's Normal
(0). Specially for these purposes, you can take advantage of Scale to fit sloped area
option. You can find more about these setting in our on-line reference guide