Well I am trying to learn how to place CG objects into real photos and make it believeable. This is my first try at this type of thing so I decided to start as simple as possible... a sphere! :)  So I downloaded this pic off of google.....

I wanted to add the sphere to the closest table. So using the pic as a background image, and looking through the cam, I added a plane for the table (to catch shadows), the sphere on the plane, and 2 area lights from the windows. Also added a small back light for ambience.

I set OSA to 11, area lights to 11x9 samples, and raytracing on. And got this......

It's looking pretty good, but I want it better. Can you guys help me out? Tell me what you see when you look at this image that keeps you from believeing the sphere is really there. Any crits are welcomed please, no experience required :) I would like to learn all the in's and out's of this before I do a more complex project.

I think there's a problem with the direction of the area light shining "through" the right hand window. Note in the photo the light (and shadows) from that window are parallel to those from the other window. You have the light at near right angles to the left one. This makes the highlight and shadows of the sphere different to the rest of the room. Hence it's not believable.

Nice first try though, scale and position seem OK to me.

Thanks for the feedback, rfornasari. I'm not sure I am following you though. The only shadows visible from both windows is the table, and I can clearly see two shadows at different angles, although the angles will be different at the sphere. If both light sources ran parallel, wouldn't that be the exact opposite of the way the light is entering the room? I think I understand your thinking in that all the light is coming from the same direction(the sun) but I'm not sure if that would work in this type of setting. I  will experiment with your idea and rotate both my area lights to point in the same direction (parallel) and see what I get. Thank you for your compliment on correct scaling, although I didn't really have to scale anything :) I also want to lower the value of the back light on the sphere it looks a little bright I think.

Well, not useful feedback for you, but I`d be happy to know how did you mount your shadows onto an existing table... Some 2D program? What function is used to achieve such an effect? Sorry, I have totally no experience in 2D.

Hey Silgrin.  In the shader's panel there is a button "OnlySh" so it renders the plane transparent except for the shadows. Next I want to try to do reflections! But I want to try rfornasari's idea first.

Ill have another update for this pic in a few I've also downloaded some more pics to drop stuff in :)

Ok I tried turning the light for the right-hand window parallel with the left one, like this...

And got this when rendered.....

I still think I prefer the first image, what do you think? I'm going to move on to an actual object (maybe a coffee cup or somthing) and try to add reflections. There just isnt enough light in this scene to see much.

Nice work!

I do think this image is more realistic than the first one. I think you're right in saying there's not enough light in the scene to show up much.

I haven't done much 3d compositing like this but I've done a bit of 2d work. Little things like the direction of the shadows (a difference of 10 degrees will stand out) or the position of a highlight in comparison to other things in the scene determine whether an image is real. In compositing 2d images even getting the quality of the light to match for the two images is important in making the final image believable. Many people wouldn't be able to tell you what's wrong with a scene but their experience of the real world will let them understand intuitively when there are mistakes.

Of course if you are creating an art work this is of lesser importance and you use what you need to produce your final effect. However, you did say you wanted to achieve realism, and I think that's much harder.

it would look better if you created a custom hdri map of an area like that on a sunny day, then placed the sphere on the plane in a box with holes in thoes two areas.  other than that you might want to use a scene that has more objects in the middle so you can get a more percise idea of where the shadows would be where the sphere is.  there are also some radiosity problems, the bottom of the ball would be blacker and the top should be lighter. 

another important thing is the angle of the shadows on the ball, try moving the area lights a little left of the table