For Dumb LED strips: Every 3 LEDs will have its own current limit resistor; power & gnd are passed along in parallel to the next group of 3. Each group of 3 will draw ~20mA from 12V supply. A shorter length will thus draw less current. PWM/transistor can be used to control brightness.
For Smart LED strips, a control chip will control each LED/group of LEDs. Current draw will be similar. Software controls the brightness.
No external resistor is needed either way.
As an example, here is 74HC595 controlling AOI518 N-channel MOSFETs to fade strips with 9 LEDs (3 groups of 3) up & down.
http://www.crossroadsfencing.com/BobuinoRev17/
