how would i draw petal or leaf?

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I would use Photoshop, import an image of a leaf to your canvas. Save. Then map it to a plane or face in your software (make it roughly of the same proportions as your canvas containg your leaf).

Most 3D packages allow you to apply an opacity or transparency map/mask so...

Now create a second map, all parts you want to see of the leaf fill in with white colour, all of the background make black.

apply this map to the opacity, or transparency slot in your material editor.

Now if you hit render, you will only see your leaf and you don't need to mess around trying to model one.

Create a separate bump, spec, displacement map to make it look more realistic. If you're going for close ups, you may need to model some of the geometry too.

Attached is a very rough example using just a diffuse map.

Do you wish in 3D? Time to wake-up this old thread: http://www.cadtutor.net/forum/showthread.php?t=10002

ran that lisp routine, pretty cool! It would take me 100 years to figure all that out.