Until I joined AbsoluteWrite, I had never heard of the writing term "filtering" before. But it turns out to be a technical term for something fairly simple in creative writing.
She saw him cross the street and approach.
He looks and sees her weave in and out of the crowd.
The children notice that their toys are missing.
None of those sentences are noticeably weird, right? There's nothing glaringly wrong about them.
Filtering is when you place a character between the detail you want to present and the reader.
In other words, filtering is distancing the detail or image from the reader. I tend to do this in first drafts (and, ahem, maybe in parts of third drafts...), usually when I can't think of a way to introduce something in a more graceful way. I've noticed filtering more now that I know what it is, usually in third person POVs.
It was pointed out in several posts on AW that filtering can come across as lazy writing because, duh, in 3rd person POV, it should be obvious who the viewpoint character is. Therefore, if, say, a woman happens to be in a cafe or something and looks out the window and sees the guy she despises crossing the street and coming toward the same cafe, then obviously, she sees him.
Therefore, no need to say, "She saw him cross the street." Edit. Just write: "She gritted her teeth because Danny was crossing the street, his stride leading him to the door of the cafe."
This way, the reader sees him crossing the street, too, instead of watching her watch him cross the street. Get it?
Filtering aids in that old adage of "show, don't tell."
Here are some good sources on "filtering:"
An Introduction to Filtering
So, have you heard of filtering? Do you do it?