I understand things for the most point, but can't a picture of the grass on a field be taken with a digital camera and get rendered in a video game?
Maybe I don't understand.
Technically yes, you could just go and find a patch of grass, take a high resolution picture and convert that picture to a game texture and apply that to a surface in the game (the floor) and then render the game at 1080p and have it look much the same as from 1080p film.
There's several problems with this however, one of them is resources. We have limited hardware resources in our computers and so we have a limited number of textuers we can use, really high quality textures that look real take more resources to store and process and so we can't use many of them.
The other problem is geometry, a picture taken of grass which is then used as a texture on a flat piece of ground has no 3D geometry so it only acts as a good approximation of grass at a distance, when you get close you notice it's just a picture of grass on a flat surface. to get tufts of grass that stick up, that you can maneuver your view point around in 3D space you need to make 3D grass, that makes the strain on resources even larger.
The fact is due to resource limitation we have to make an approximation of grass, something that's as close to believable as we can get using modern hardware, as graphics processing gets faster we can make closer approximations and the scene will look more like real life.
To give you some perspective, movies like Avatar do a great job of making 3D environments look believable, so we are capable of doing this, but the hardware they use to render the scene uses huge render farms, these are facilities use literally thousands of very powerful computers and even then spends days just rendering a few seconds of footage.
When we can squeeze all that processing power down into a single computer you can buy at PCworld, we'll have Avatar like grass in our games
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