Why can't VFX artists get enough of this shiny ball?

What makes us think that an impossibly big, fictional monster is actually real? A lot of things go into making a CG asset sit properly in the real world, and indeed into making a real-life object sit properly within a CG environment, but perhaps the most important one of all, is light.
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What do you think makes a VFX shot look real?

If your plate photography is lit from left to right and your CG asset is lit from right to left it will instantly look wrong, if one is lit slightly brighter than the other, or if the shadows vary in darkness, if one has fewer light sources illuminating it than the other or if the light doesn't move across it as it does with other elements in the shot, it will look wrong.
So how do VFX artists make it look right?
well, before the invention of this Chrome Ball and HDRI, it was an extremely laborious, and not always fruitful process.

One method was to survey all the light sources on the set and then set up virtual light sources in the CG environment in approximately the same positions, attempting to match their intensities and colors manually.

Another way was to use a gray ball or a shiny ball on the set and use it as a guide to light your CG object in the same manner, so when you combine it with the plate photography it should look right.

and another technique is called Environment mapping, this technique requires taking a panorama from the position where you want to place the CG object, then calculate the position of each pixel in relation to the CG object you what to place in the shot and the brightness of each pixel or Pixel Value (this is a number ranging from 0, which is black to 255, which is white), this can also be done with photographs.

VFX artists would mostly use a combination of all of these techniques and even then, it was a very time-consuming process.

The Modern method is to use the chrome ball and HDRI's to create radiance maps.

If you look at this image of a Chrome ball you'll notice that in its reflective surface you can see almost 360 degrees around the ball.
In the reflection, you'll also notice parts that are completely saturated with light (they're at a value of 255 which is white) but the light's color may not actually be white.
If I shine a bright yellow light in your eyes you will see it as a blinding white light, even though its color is actually yellow. Objects that are illuminated by this light will therefore gain a yellow tone and not, a white one.
Here is where a HDRI comes in handy.
HDRI stands for High Dynamic Range Image and it's basically a combination of photographs with different exposure lengths ranging from long to short.
A long exposure means the lens is open for longer, so more light is captured and darker areas become lighter and lighter areas become completely saturated.
With a short exposure, the lens opens and closes very quickly so only a small amount of light is captured, dark areas are now indistinguishable but previously saturated areas are now visible and their color can now be distinguished.
By combining all these images we can create a radiance map for everything around the ball, where no area is completely black or completely white.
By putting this information into the computer and basically Texture Mapping the radiance information around the CG object you've created, you can mimic the lighting conditions that were on set, in your CG 3D space.
However, light conditions on set are constantly changing, especially if the set is outdoors, this is why every time the director says "Cut!" The VFX team will spring into life, capturing as much light information as possible before the next take.

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