Difference between revisions of "Reference:Isosurface"
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Revision as of 19:06, 15 March 2012
Details about many of the things that can be done with the isosurface object are discussed in the isosurface tutorial section. Below you will only find the syntax basics:
isosurface { function { FUNCTION_ITEMS } [contained_by { SPHERE | BOX }] [threshold FLOAT_VALUE] [accuracy FLOAT_VALUE] [max_gradient FLOAT_VALUE] [evaluate P0, P1, P2] [open] [max_trace INTEGER] | [all_intersections] [OBJECT_MODIFIERS...] }
Isosurface default values:
contained_by : box{-1,1} threshold : 0.0 accuracy : 0.001 max_gradient : 1.1
function { ... }
This must be specified and be the first item of the
isosurface
statement. Here you place all the mathematical functions that
will describe the surface.
contained_by { ... }
The contained_by
object limits the
area where POV-Ray samples for the surface of the function. This container can either be a
sphere or a box, both of which use the standard POV-Ray syntax. If not specified a
box {<-1,-1,-1>, <1,1,1>}
will be used as default.
contained_by { sphere { CENTER, RADIUS } } contained_by { box { CORNER1, CORNER2 } }
threshold
This specifies how much strength, or substance to give the
isosurface
. The surface appears where the function
value
equals the threshold
value. The default threshold is 0.
function = threshold
accuracy
The isosurface finding method is a recursive subdivision method.
This subdivision goes on until the length of the interval where POV-Ray finds a surface
point is less than the specified accuracy
. The default value is 0.001.
Smaller values produces more accurate surfaces, but it takes longer to render.
max_gradient
POV-Ray can find the first intersecting point between a ray and
the isosurface
of any continuous function if the maximum gradient of the function
is known. Therefore you can specify a max_gradient
for the function.
The default value is 1.1. When the max_gradient
used to find the
intersecting point is too high, the render slows down considerably. When it is too
low, artifacts or holes may appear on the isosurface. When it is way too low, the surface
does not show at all. While rendering the isosurface POV-Ray records the found gradient values
and prints a warning if these values are higher or much lower than the specified
max_gradient
:
Warning: The maximum gradient found was 5.257, but max_gradient of the isosurface was set to 5.000. The isosurface may contain holes! Adjust max_gradient to get a proper rendering of the isosurface.
Warning: The maximum gradient found was 5.257, but max_gradient of the isosurface was set to 7.000. Adjust max_gradient to get a faster rendering of the isosurface.
For best performance you should specify a value close to the real maximum gradient.
evaluate
POV-Ray can also dynamically adapt the used max_gradient.
To activate this technique you have to specify the evaluate
keyword
followed by three parameters:
- P0: the minimum max_gradient in the estimation process,
- P1: an over-estimating factor. This means that the max_gradient is multiplied by the P1 parameter.
- P2: an attenuation parameter (1 or less)
In this case POV-Ray starts with the max_gradient
value P0
and dynamically changes it during the render using P1
and P2
.
In the evaluation process, the P1 and P2 parameters are used in
quadratic functions. This means that over-estimation increases more
rapidly with higher values and attenuation more rapidly with lower
values. Also with dynamic max_gradient
, there can be artifacts and holes.
If you are unsure what values to use, start a render without evaluate
to get
a value for max_gradient
. Now you can use it with evaluate
like this:
- P0 : found max_gradient * min_factor
min_factor being a float between 0 and 1 to reduce themax_gradient
to a minimum max_gradient. The ideal value for P0 would be the average of the found max_gradients, but we do not have access to that information.
A good starting point is 0.6 for the min_factor - P1 : sqrt(found max_gradient/(found max_gradient * min_factor))
min_factor being the same as used in P0 this will give an over-estimation factor of more than 1, based on your minimum max_gradient and the found max_gradient. - P2 : 1 or less
0.7 is a good starting point.
When there are artifacts / holes in the isosurface, increase the min_factor and / or P2 a bit. Example: when the first run gives a found max_gradient of 356, start with
#declare Min_factor= 0.6; isosurface { ... evaluate 356*Min_factor, sqrt(356/(356*Min_factor)), 0.7 //evaluate 213.6, 1.29, 0.7 ... }
This method is only an approximation of what happens internally, but it gives faster rendering speeds with the majority of isosurfaces.
open
When the isosurface is not fully contained within the contained_by object,
there will be a cross section. Where this happens, you will see the surface of the container.
With the open
keyword, these cross section surfaces are removed. The inside of the isosurface
becomes visible.
Note: Using open
slows down the render speed, and it is not recommended to use it with CSG operations.
max_trace
Isosurfaces can be used in CSG shapes since they are solid finite objects
- if not finite by themselves, they are through the cross section with the container.
By default POV-Ray searches only for the first surface which the ray intersects. But when using an
isosurface
in CSG operations, the other surfaces must also be found. Therefore,
the keyword max_trace
must be added to the isosurface
statement.
It must be followed by an integer value. To check for all surfaces, use the keyword all_intersections
instead.
With all_intersections
POV-Ray keeps looking until all surfaces are found.
With a max_trace
it only checks until that number is reached.