This might be a very stupid question, but there is one thing I have always wanted to know, why is it that there is always one line in a surface.
Take this for an example, a loft between two circles and the line:). Its not a big deal but just always wanted to know, there must be some reason for it?

Hi olio - that is the "seam edge" of that surface.

Basically every NURBS surface is made up of a rectangular grid of control points.

You can kind of think of it similar to a sheet of paper, but it is kind of like rubber paper since it can be pulled and stretched.

A piece of paper has 4 edges on it, and if you want to make a cylindrical tube out of your piece of paper you will roll it so that 2 opposite edges are touching. Because the opposite edges are touching along the same spot it will look like 1 edge, and that is what produces the kind of edge that you show there.

- Michael

Hi Michael,

Thanks for informing me!, like I said I always wanted to know that.

It doesn't bother me much but, sometimes seeing the seem edge can be visually disturbing, when showing a model, it there a way to turn it off or is that not possible technically.

Thanks.

Hi olio, if you don't want to see it you can select it and use Edit/Hide to hide it.

In the current release hidden edges will kind of pop back into being unhidden if you edit the object, but in the next beta they will stay hidden through editing operations.

- Michael

While we're on the subject of knowing things :) I've always wondered why cad layer systems are limited to 256 layers.

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Hi Danny, I'm not really sure that is such a universal limitation...

But it is not unusual to see things that were originally designed a while ago to be limited to 256 items or sometimes 65536 items.

The reason for these particular numbers is that a single byte of information can hold one of 256 values, and 2 bytes of information can hold one of 65536 different values.

Back when memory was very tight, data was often trimmed down to only take up as little space as possible, so for example your system decided to use only one byte to hold the layer index of an object which made it only possible to have 256 different possible values (ranging from 0 to 255).

- Michael

Very details information professor :)

You can also move the top and bottom point of the seam line to give a twisting or untwisting effect to you lofts which can be useful.

As Michael explained - BYTE and WORD data types were used often back in the early days of programming.

I remember one of my early programs "Digital Address Book" - I designed the data records of that program to fit exactly into a floppy disk sector (256 bytes) - LOL - those were the days! ;-)
(I forget now how many sectors fit onto a formatted 5.25" disk back in the day but I remember being pleased with the idea of having 800+ addresses & contacts on a single disk. :-P )

...And most likely WHY most CAD programs >stick< with that 256 layer limitation still today is to be compatible with the "industry leader" AutoCAD et al...

If companies didn't care about maintaining compatibility with AutoCAD (and removing that "AutoCAD Compatible" bullet from their product promo materials), we would have probably seen packages with more than 256 layers years ago.

-Will

Hi Will, actually as far as I know AutoCAD has not been limited to 256 layers for quite a long time.

It looks like with R14 and higher there is no limit, and for R13 and prior (not sure how far back) it was limited to 32767 layers (which is a 15 bit limit, sometimes the high bit is reserved for a negative/positive sign bit).

So I think this is one of those rare areas that you can't really blame on AutoCAD...

Some programs that have been around for a while probably have this limit more to maintain compatibility with their own older versions of the same program.

- Michael

Another Q, I have always wanted to know, maybe a bit OT, but whenever I see a tutorial on texturing, people always make the texture size 512 px or 1024 or 2048... Now I know this has something to do with bytes, but does this way of working really do anything special, is this just habit from old way of working when this really meant something?

Most graphics cards work faster when the image size is a power of 2 (128, 256, 512 etc.)

Imagine the low level code require for accessing images that were exactly 497 x 363!

Even though that may be all you need out of your texture, it's faster for the processor if it was working with 512 x 512 blocks with some wasted space in them.

Power of 2 multiplication or division is a simple shift left or shift right (respectively):

256 << 1 == 512
512 >> 1 == 256

Instead of having to call a (typically CPU costly) divide or multiply opcode, you can get away with a simple bit shift which typically only takes a single CPU cycle.


The choice between using say 256 x 256 or 1024 x 1024 is arbitrary and usually depends on how close the camera may get to the textured surface (we've all seen the 3D games where the walls get blocky when you move too close to them...)

OR

If the same texture map is being used by more than one object - you'll want to have more "space" to work with using a larger image map - it all boils down to image quality vs available texture memory - the larger the image, the better it will look...

-Will

Hi olio, that has to do with the way that video cards work.

If you were going to use that texture for a "full render" calculation like a ray-trace render or something like that, then it would not really make any difference.

But if the texture is to be displayed by your video card in real time, there are some optimizations that it can do for textures that are a power of 2 like Will mentions. I'm not sure that this is very significant anymore with more modern cards though.

Originally it used to be that 3D video cards would actually only support textures that had a size of a power of 2 (like one of: 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048) and nothing else, so especially things that were oriented towards producing textures to be used inside of games would be really focused on that.

- Michael

Thanks guys, for very unselfish sharing of knowledge. thanks.