Below is an answer I got from Mark (the creator of Apophysis) about the options in Render to Disk:
"This following used to be in the help file (for a long
time it was the
only topic there). It'll be back eventually...
The Sample density parameter determines the number of times that the flame
algorithm is iterated. This parameter has the most obvious effect on the
quality of an image and images rendered using a low sample density will
look very grainy. The exact value to use may depend on the flame that you
are rendering: thin, wispy flames will need a higher sample density than
tight, solid looking flames. The default value of 200 is a good place to
start.
Sample density has a large effect on rendering time: the higher the sample
density the longer the image will take to calculate. It is adjusted
automatically for image size and zoom factor, so larger images will take
longer to render, as will images that have been zoomed in.
The Filter radius parameter controls the strength of a blurring filter
that is applied to the image. The filter works in conjunction with
oversampling to anti-alias the image. The default value of 0.4 works well
in many cases, but you can reduce it if you think the image needs to be
sharper. With larger values the image will look more obviously blurred.
The effect of a certain filter radius value might change with a change in
the oversampling factor.
The Oversample parameter controls the degree of oversampling used for a
render. Oversampling means that the flame is calculated at a larger size
than intended for the final image, and then resized down. Another way of
looking at oversampling is to think of the image as being calculated on a
finer grid than the pixel resolution. An oversample value of 1 means that
no oversampling occurs, a value of 2 means that the flame is calculated at
twice the size of the final image, and so on. Oversampling helps to add
detail and anti-alias the image. The default value of 2 works well in most
cases but you might try a higher value if aliasing is very obvious in the
flame that you are rendering (this can be the case if it has very fine
lines or straight edges). Increasing the Oversample value has a large
effect on the amount of memory required to render an image."
Mark
More advice from Mark:
My question was why it was going to take so long rendering a flame at 2560 x
2048 with the sample density at 2000.
"Yes the size combined with the sample density and the
number of trips if you're liming. You don't usually
have to go as high as 2000 for the sample density: if
it's a tight flame you won't see much difference from
a much lower density. Because the sample density is
scaled with the size of the image, you can do test
renders at a smaller size and see if the sample
density is high enough, and not have to change it when
you do a larger render."
Trying out the different settings for rendering.
 Default Sample Density: 200 Filter Radius: 0.4 Oversample: 2 |
 Sample Density: 200 Filter Radius: 0.4 Oversample: 4 |
 Sample Density: 400 Filter Radius: 0.4 Oversample: 2 |
 Sample Density: 400 Filter Radius: 0.4 Oversample: 4 |
 Sample Density: 200 Filter Radius: 0.1 Oversample: 2 |
 Sample Density: 200 Filter Radius: 0.2 Oversample: 2 |
|
Sample Density: 200 Filter Radius: 0.3 Oversample: 2 |
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