Bill Cosby -- Alleged Sexual Assault Victim Files Police Report ... Will Trigger Full LAPD Investigation

The woman who claims she was sexually assaulted by Bill Cosby when she was 15 is filing a police report with the LAPD Friday afternoon TMZ has learned ... and that will trigger a formal investigation.

Judy Huth is -- at the time of this post -- on her way to the PD, along with her new lawyer, Gloria Allred.

Huth just sued Cosby, claiming he got her drunk and then took her to the Playboy Mansion where he sexually assaulted her. Cosby filed legal docs Thursday, claiming she's a liar who tried to sell the story to the tabloids a decade ago and then tried extorting the comedian in return for her silence.

Cosby also notes ... the alleged incident occurred 40 years ago ... well beyond the statute of limitations.

TMZ broke the story ... even if the statute of limitations has run, they will investigate if someone files a police report -- which she is now doing. The cops say they conduct these investigations because they don't know what twists and turns will occur ... possibly unearthing new alleged victims.