There are many window cornice ideas out there, so the question is, where do you start?
Let's narrow down the options. Instead of thinking about 'all those possible shapes I could use', consider this.
There are two basic styles - all cornices fit into one or the other.
These are shapes where the base depth is constant across the width. It doesn't mean the base can't have a shape. The cornice doesn't have sides, (or 'ears', as one of my clients once described it).
The simplest form of this is a straight base, so you get a rectangle (top illustration). This shape is one of the most useful of all cornice designs. It's also one of the most neglected. Because it's simple, it tends to be overlooked. ("Surely there must be a more interesting shape we can use?")
Always consider using a straight cornice, even if you finally select something else.
While it's unusual for the top edge of a cornice to be shaped (there are exceptions), shaping the base is much more common.
If you're using a plain fabric, then shaping the base can give interest while keeping the design simple. For patterned fabrics, always check that the shape is suitable for the pattern. If the pattern is pronounced, then it can often dictate what the shape should be.
The sides can be shallow or pronounced. Deep sides work best if you have floor length drapes. If your drapes are sill length then keep the sides of your cornice designs in proportion.
As with straight cornices, the fabric you're using may influence your choice of shape.
Plain fabrics mean you can have any shape you want.
Patterned fabrics will have a strong influence on your window cornice ideas. The importance of that influence is increased with the strength of the pattern.
Large patterns - strong flowers against a pale background, for example - can often dictate the shape of your cornice.
When you've designed a shape you think will be suitable, always lay the fabric over it to make sure it all works.
Cornice designs can also have an effect on how you arrange the widths. The correct method is to have a width centralized, and then work out to the sides. But sometimes a strong pattern can work best if the pattern is centralized, rather than the widths.
So in this illustration, if you follow the correct method and place the first width in the center and work out, the pattern placement just doesn't look right.
By moving the widths right or left as shown, you can obtain a more balanced result.
When the top of a window is shaped, you can often solve the apparent problem of a cornice by shaping the cornice to follow the window shape. (See the bottom cornice in the illustration 'Cornices with sides'.) Arched windows are the most common of these situations.
The safest way to check your window cornice ideas is to make a mock up of the shape in card, and attach it to the window. This type of design will inevitably break some of the 'rules', such as the main part of the shape being one fifth of the drop of the drapes. These rules should probably be looked on as guides, which should be ignored when necessary.
Once again, I'm going to emphasize this: to achieve professional results you need to pay attention to details. And the shapes and curves you use for your window cornice ideas are no exception.
Once you get your basic shape, see if you can improve it. Without going into the mathematical theory, curves look best if they are constructed using gentle sweeps, rather than primitive shapes.
For example, the upper design shown here might be your first attempt. But notice how refining the curves you get a much more pleasing shape.
This is one area where experience can help, but don't fixate too much on it. Once you have the basic shape you want, try a few alternatives, then settle on the one you like best.
It's generally the case that if you're reasonably happy with a shape for your cornice, it will look so much better when it's upholstered in the fabric.
So don't spend hours agonizing over it!