Your projector has 1024 by 768 pixel native resolution and is essentially a 720p projector. However, this projector has a native 3 x 4 aspect instead of the 16 x 9 ratio that is used for HDTV, therefore when displaying HD video there will be black bars at the top and bottom of the image and the displayed HDTV image will only be about 1024 x 575 pixels, which is not true HDTV resolution, but better than standard definition. With a PC you can use the VGA connection with the PC set to match the projector's the native 1024 x 768 resolution and this will provide best results. Do not confuse a component video connection on a projector (which on your projector supports the HD input signal formats of 720p and 1080i, but not 1080p) with the composite video input which is strictly for standard definition signals and uses a single connector that is usually yellow in color. The single yellow video plus red and white audio connectors on your projector are thus for standard definition only. Also the S-Video connector on the projector is strictly for standard definition. When using a HDTV source with your projector you will need to connect it using a component video connection. However your specific model of projector uses the 15-pin connector for both VGA (for PC connection) and for also for component video from the HD video connection. In order to use it as a component video connection you will need a special cable. See page 22 of the owner's manual for info on the connection and page 25 describing the menu for selecting the video input. You may be able to use the following VGA-to-RCA video cable plus a triple Female/Female RCA adapter to, in effect, provide the projector with a standard component video input configuration (with one connector for Y, one for Pb and one for Pr signals and these are usually color coded green, blue and red with component video cables). You would then use a standard component video cable (with 3 connectors or 5 connectors if stereo audio is included) from the HD video source to the projector (actually from the HD source device to the component video adapter cable you have made up as described above. Below are links for where you can purchase the cables and adapters.
VGA to Component Video (RCA) adapter cable -
F/F RCA coupler (a triple version for component video) - HERE
Standard component video cables:
6 ft. video only (no audio) -
6 ft. video and audio - HEREwww.monoprice.com
has many other component video cables in other lenghts and also higher quality cables.
Most Blu-ray players up until fairly recently had such component video connectors (but some new models do not) and you will typically also find component video connections on Directv, Dish Network and Cable TV HD set top boxes and HD-DVRs. You will probably need to go into the settings menu on the Blu-ray player, sat. box or cable box or other HD video source device and set its output resolution to 720p.