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I can create 3d ball (spherical) shape with GIMP but now I need round surface that's curved inward. How can I do with GIMP?

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    GIMP really hasn't 3D tools except mapping flat images onto some preset 3D surfaces. Your best option is to use an existing image of a reflector or paint your own after watching a while a photo of a reflector. There are distortion tools if you want to stretch some image or text to make it look like it's on a curved surface, but it's up to you to get it right. The same is true for painted lights and shadows. Both are useful to create the right illusion of a curved surface. In a 3D program you'll get this all nearly automatically with very little effort, but getting it right in 2D needs skill.
    – user287001
    May 7 at 20:50
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    (continued) Check this image. i.stack.imgur.com/1jvOe.png It's drawn in few minutes in a simple freeware CAD. It's a fake - as a spherical piece it only resembles a paraboloid, but it can well be good enough for illustration purposes. The concave side has got a mapped familiar looking screenshot. It actually makes it look cancave. Without any recognizable surface image it could as well be an elliptical egg i.stack.imgur.com/eJX86.png (except the rounded edge.)
    – user287001
    May 7 at 22:07
  • Thanks for your nice idea.
    – WailenB
    May 7 at 22:17

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This is a supplement for my comment, which said that the shading of the concave surface of a round paraboloid could as well be used for an elliptical egg. This ambiquity - convex or concave, the shading is the same - has been well known forever and one can easily find much more flashy examples of it.

Here's a construction example of a paraboloid lookalike:

enter image description here

In the right there's a flattened copy of the three layers of the paraboloid. It's the top layer in the layer list.

The layers are moved apart in the right. The concave surface really is like an elliptical egg - it's an elliptical selection filled with a radial gradient.

The edge is a vertical linear gradient in another elliptical selection which is so little off from the previous one that only a narrow edge is visible, most of it is hidden by the concave surface layer.

The backside of the reflector is a selection filled with a darker gradient. The selection was originally a circle, but a part of it was deleted to keep the left side of the shape behind the foreground part.

The brightness ranges of the gradients are all fine-tuned with the curves. It's difficult to guess in the fly the right gradients.

NOTE: The shading was made in the easiest way without any connection to the environment. If it must fit into the light conditions of a bigger painting, the task really starts to demand some talent and well sharpened drawing skills. Drawing it as an isolated item is possible for everyone.

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