Objects in POV-Ray can either be solid, empty or filled with (small) particles.

A solid object is made from the material specified by its pigment and finish statements (and to some degree its normal statement). By default all objects are assumed to be solid. If you assign a stone texture to a sphere you'll get a ball made completely of stone. It's like you had cut this ball from a block of stone. A glass ball is a massive sphere made of glass.

You should be aware that solid objects are conceptual things. If you e. g. clip away parts of the sphere you'll see that the sphere is empty, i. e. you'll clearly see that the interior is empty and it just has a very thin surface.

This is not contrary to the concept of a solid object used in POV-Ray. It is assumed that all space inside the sphere is covered by the sphere's material. Thus there is no room for any other particles like those used by fog or halos.

In order to be able to put solid objects inside a halo (this also holds for fog and atmosphere) a test has to be made for every ray that passes through the halo. If this ray travels through a solid object the halo will not be calculated. This is what anyone will expect.

The problem arises when the camera ray is inside any non-hollow object. In this case the ray is already traveling through a solid object and even if the halo's container object is hit and it is hollow, the halo will not be calculated. There is no way of telling between these two cases.

POV-Ray has to determine whether the camera is inside any object prior to tracing a camera ray in order to be able to correctly render halos when the camera is inside the container object. There's no way around doing this.

The solution to this problem (that will often happen with infinite objects like planes) is to make those objects hollow too. Thus the ray will travel through a hollow object, will hit the container object and the halo will be calculated.

Note that the above is also true for atmosphere and fog.

Imagine you want to create an object that's partially made of glass and stone. You'd use something like the following merge because you don't want to see any inside surfaces.

What's wrong with this, you may ask? The problem is that there is no way of telling what the interior of the actual object will look like. This is not a problem of POV-Ray, it's a general problem. You can't define the interior of any object in a surface based model. You would have to create some (complex) rules to decide what the interior will look like. Is it made of stone? Is it made of glass? Is it made of some bizarre mixture between glass and stone? Is it half stone and half glass? Where is the boundary between the two materials and what does it look like?

You will not be able to answer any of the above questions by just looking at the above object. You need more information.

If you wanted to create an object made half of stone and half of glass you would have used the following statements.

This example is correct because there is one object made only of stone and one made only of glass.

You should never use objects whose interior is not well defined, i. e. there must not be different textures for the object having different refractive (and translucent) properties. You should be aware that this holds only for the lowest layer if you use layered textures.

Picture each blob component as an object floating in space. This object is filled with a field that has its maximum at the center of the object and drops off to zero at the object's surface. The field strength of all those components are added together to form the field of the blob. Now POV-Ray looks for points where this field has a given value, the threshold value. All these points form the surface of the blob object. Points with a greater field value than the threshold value are considered to be inside while points with a smaller field value are outside.

There's another, simpler way of looking at blobs. They can be seen as a union of flexible components that attract or repel each other to form a blobby organic looking shape. The components' surfaces actually stretch out smoothly and connect as if they were made of honey or something like that.

If you have a single blob component then the surface you see will just look like the object used, i. e. a sphere or a cylinder, with the surface being somewhere inside the surface specified for the component. The exact surface location can be determined from the blob equation listed below (you will probably never need to know this, blobs are more for visual appeal than for exact modeling).

For the more mathematically minded, here's the formula used internally by POV-Ray to create blobs. You don't need to understand this to use blobs. The formula used for a single blob component is:

This formula has the nice property that it is exactly equal to the strength parameter at the center of the component and drops off to exactly 0 at a distance from the center of the component that is equal to the radius value. The density formula for more than one blob component is just the sum of the individual component densities:

The geometry of a box.

Where <CORNER1> and <CORNER2> are vectors defining the x, y, z coordinates of the opposite corners of the box.

A height field is essentially a one unit wide by one unit long square with a mountainous surface on top. The height of the mountain at each point is taken from the color number or palette index of the pixels in a graphic image file. The maximum height is one, which corresponds to the maximum possible color or palette index value in the image file.

The size and orientation of an un-scaled height field.

The mesh of triangles corresponds directly to the pixels in the image file. Each square formed by four neighboring pixels is divided into two triangles. An image with a resolution of N*M pixels has (N-1)*(M-1) squares that are divided into 2*(N-1)*(M-1) triangles.

Four pixels of an image and the resulting heights and triangles in the height field.

The resolution of the height field is influenced by two factors: the resolution of the image and the resolution of the color/index values. The size of the image determines the resolution in the x- and z-direction. A larger image uses more triangles and looks smoother. The resolution of the color/index value determines the resolution along the y-axis. A height field made from an 8 bit image can have 256 different height levels while one made from a 16 bit image can have up to 65536 different height levels. Thus the second height field will look much smoother in the y-direction if the height field is created appropriately.

The size/resolution of the image does not affect the size of the height field. The un-scaled height field size will always be 1* 1. Higher resolution image files will create smaller triangles, not larger height fields.

There are six or possibly seven types of files which can define a heightfield, as follows: