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Graphics have never been my force, so i decided to close this gap. After learning the basics of Inkscape and SVG, i'm trying to give more "reality" to my drawings. I have a circle which i want to make appear as a realistic sphere. After following this tutorial and playing with gradients and blurs, this was the result:
What i want is mainly a polished surface, or even if the matter has the same consistency as it is, at least an improved border. How to do it? Thank you.
This answer is rewritten because it contained much clutter which was gradually fixed due the given comments. In addition Inkscape 1.0 works a little better than older versions. The comments are now obsolete. The older revisions are readable and the comments are valid for before 2021 versions.
Draw a plausible greyshade sphere by applying a radial gradient to a circle. Make also a duplicate of the circle and fill it with solid color:
Place (align or have point snaps ON) the grey version above the colored version. Let the grey version have blending mode = Luminosity.
You can as well have the colored version on top and let it have blending mode = Color. The result is the same.
The white area can present a gloss which is too bright to be shown as colored in the RGB system. If you want the brightest area as colored you cannot have full white in the gradient. Or you can if the used blending mode is = Multiply.
In the next version the blending mode is still = Luminosity, but the bright spot is substantially darker. The colored version in the bottom has got a brighter color to compensate the darkening:
There's another answer which doesn't use blending with grey gradient for shading, the gradient contais all needed colors. That, of course, gives the best control, but you cannot experiment easily with different colors nor use imported images as surface color.
The shadow can be made as blurred grey or black ellipse. Here's 2 elementary versions:
Placing the shadow and coloring it realistically is as tricky as creating the shading of the sphere. The shadow must be in accordance with the direction of the light, how the light is projected (=does it generate sharp shadow edges and is it uniform) and how much there's diffuse ambient light. I skip the details.
I guess you may want glossy reflections. Making them needs an assumption of what's around the sphere. Some cliches like a curved window or ellipse, of course, can be drawn quite easily, but there can be more. They are beyond the scope of this answer.
Photos as surface
You must import an already spherically distorted photo as PNG with transparent background. Then you use it as the color. Inkscape works in 2D, it has no idea how a photo should be distorted for right 3D geometry appearance.
My two cents.
In my opinion, you have some problems.
You are not wrapping the lower end in a shadow.
The main gradient should consist of 3 color nodes.
The first one, the center one is not white. It is the actual color I want the object to be. That makes the color more solid.
The last one is pretty dark. Take a look at this: How can I make the color white look as white as possible? on how to achieve a good gradient.
And I added an additional node, first at the center. Playing a bit with it gives me control of the shadow. It is not at the center but displaced to the dark side... Luke...
This is how the gradient looks:
I added a glossy reflection (if you need a glossy material), it is just a white ellipse with a transparent gradient. It could be a curved rectangle or any shape. This shape corresponds to the shape of the reflected light source. Take a look at these two posts regarding the materials:
Additionally really control the position of the shadow, yours do not match the position of the highlight.
The height of this shadow gives the illusion of camera position regarding the floor.
The biggest thing I see is the lack of a shadow on the sphere, realistically your shadow does not match the strength of its highlight. your gradient goes from a white > skyblue (or whichever). If you can adjust the gradient do white > skyblue > really dark blue (like almost black blue), it might help. Remeber, the stronger the light, the stronger the shadow. As if its 12 noon and not a cloud in the sky, the shadows are darker and crisper, if a cloud is blocking the light the objects become more dull (grayish), and so the shadows lighten up as well.
Another area is the edge of the sphere, its too sharp so it does not blend in with the background. if you have a blur tool, blur the edges. The way the cast-shadow edges are blurred.
Extra realism: to really make it feel real, get some textures online and overlay them on your object, like metal texture, etc.
Here's a little sample I did in photoshop trying to fix that.
