How to model 2D rectangle with half circles (with continuity)?

[Anonymous]

I'm trying to model power and volume buttons for a mobile device, I didn't expect that it would take so much time to model such a simple shape, the shape should look like a pill from the side (or tall rectangle with half circles at its ends). Obviously it's easy without G2 or higher continuity, but to make it continuous it took me more than an hour messing around with it, I'm not sure if I did it right and if I can repeat the process without a lot of pain.

So yeah, I need an advice what would be the best strategy (or strategies) to achieve this shape? I tried adding a circle as a reference and doing curve filleting with G2 continuity on the rectangle, then deleting that fillet and doing free form blend, also tried manually editing CV points to be more similar to a circle.

I'm attaching an image of the shape I'm talking about (It's not continuous here but you get the idea).

[Anonymous · Accepted solution]

Hi mumblefluff,

Please refer to the video link http://sendvid.com/t4r29ha1. Is this what you are trying to achieve in Alias ? 

[Anonymous]

Interesting way, it looks way faster but probably a bit less accurate, so I will probably just gonna do it the long way. Thanks for suggestion anyway, your method might be useful in other cases.

[Anonymous]

Hi mumblefluff,

Could you describe how accurate you want it to be ? Kindly give an example... thank you.

[Anonymous]

You can see in the first image, gray curve is a half circle (I believe it's an equivalent of G1 continuity), blue curve is aligned using G2 (just one side). As you can see the difference is very subtle, it's very close to geometrical half circle, but there is just a little bit of smoothing where it goes to straight line. And of couse the size of this object is precise in milimeters, not random. This is what I needed. Takes some time but I guess when I need precision I need to invest some time 🙂

[Anonymous]

Hi mumblefluff,

There is a difference in between a normal half circle compared to an aligned G2 continuity half circle. And of course, the difference is subtle.... G1 only tangency, G2 tangency + curvature.

[Anonymous]

Yes, I understand that. I just needed some manual tweaking to make it more like half a circle, because only aligning it makes it quite deformed. I dragged aligned CV points to the left, and then I think I tweaked it just a little bit more.

[Anonymous]

You need to make the diameter of the half circles slightly smaller than the Distance from the two straight curves connecting them. Fast way is aligning the smaller circles to the straight curves.
Better way is to connect the half circles and the straight curves (which also should not be really straight, give them at least some acceleration towards the ends) with a third curves which will be aligned to them.

[Anonymous]

I'm not sure if you understood me correctly, that screenshot I added is how I did it and I think it's good for me, unless you see a problem with it? I'm not sure why do I need to make smaller diameter circle, also I didn't understand that part about third curves. That said, I'm probably ok because I'm happy enough with my result, unless you see something fundamentally wrong with it.

[Anonymous]

Hi mumblefluff,

There is nothing wrong with your method. There are many ways to achieve one thing in Alias...

Just make sure you get G2 continuity when you do a manual move on those CVs.

[Anonymous]

If ur just about exploring some shapes then yes ur Method is good enough.
But if u want to Archive a class a surfacing solution than i think its Not good enough.

First of all why would u want to align g2 to a flat curve? There is no change of curvature on a Flat line so it makes no Sense to align another curve g2 to it. It even makes your Situation worse. IT
1. Distorts ur circle (which never was a circle in the First place. Because u need at least 7 cvs in the Quarter of circle to Archieve a circle with a somewhat constant radius)
2. Creates a Huge Flat area in your circle because as u can even See in your Screenshot the 3Rd cv just Snaps to the Tangent line telling u there in no change of curvature.

So Like i Said u need to give ur straight lines a little Bit of curvature and u need a blendIng curve which connects them with ur half circles if u want a solution where u keep the constant Radius of a circle on the sides and a g2 connection to the top.

[Anonymous]

It's kinda difficult to get in my surfacing problem solving mind again because I didn't do anything related for a while now, so sorry but I'm not sure what you mean why I wan't to use G2 to straight line. But probably don't need class A anyway. To my understanding G2 just makes transition smoother. That method you're suggesting giving straigh lines curvature, I don't really understand how exactly is it done, but that's probably because I'm fairly green in this. I don't think that I need constant radius, I'm not sure it's even possible to make actual half circles continuous, because physics, unless you mean not full half circle, but with shortened ends, then I guess it's possible but I don't see what's the big difference, it's still manually made just the connection is in different place. That said, I'm dumb in this subject, so very likely I'm wrong.

[duaxiong]

A straight line has no Curvature.  

A circular arc has constant Curvature.

You cannot go from no Curvature to constant Curvature in a smooth, gradual manner;  G1 (Tangent) is the best you will be able to achieve with these conditions.

[Anonymous]

I understand that

[Anonymous]

Ok i Made some Screenshots. Sorry for the crappy quality. I took some pictures from the Screen with my mobile as im Not allowed to send out Data. Now in just have to Figure out how to make an attachment.

[duaxiong]

The example you showed in the 7nth post looks fine...it's technically G2.  

G1 or even G0 can be considered "Class A" as long as that's what is required or desired.

[Anonymous]

Sorry i disagree. Because a straight line and a distorted circle wont be accepted in any automotive design Departement im Pretty sure.

[duaxiong]

You are building a push button shaped for aesthetic reasons; it would be no different on a car radio.

Trust me, straight lines and distorted circles happen ALL the time in any automotive design department.

[Anonymous]

Thats exactly what im saying. A straight line and a distorted circle are NOT aesthetically pleasing just because they are g2 aligned to each Other.
And yes they May occur in Design Departements but only in early stages or if there is Not enough time and money left (which maybe the Case for Some things Like small Buttons). And What bold statement to tell that they happen in all automotive design.
I can just tell from personal experience that building Slot Like forms for Styling in the above way would Not be accepted By one big OEM in central Europe. Maybe its different for all Other car companies. I dont know...

[duaxiong]

Aesthetically pleasing is rather subjective;  It also depends on what you're building and why.

Personally, if you're going to make the arc slightly smaller and then G2 blend it with an additional curve, there's really no reason that corner needs to be an arc anymore....but that's just me.

I'm curious as to whether or not they would apply this requirement to fillets as well?