The new HTC One M8 comes with the Motion Launch feature to let you wake up, unlock to BlinkFeed, open camera app and more, by some gestures and pressing buttons. To quickly open camera app on HTC One M8, you just need to pick and hold your phone in landscape mode and simply press the volume button to launch the camera app.
If the camera app is not launched via pressing the volume button, do the following 2 steps to make it work and function:
1. Go to Settings >> Display & Gestures, and make sure "Auto Rotate Screen" is enabled.
2. Go to Settings >> Display & Gestures >> Motion Launch Gestures and make sure "Motion Launch Gestures" is enabled.