Understanding the Video Aspect Ratio
The Video Aspect Ratio is the Ratio between the Width and Height of an Image as it is being displayed on your screen.
Today's display devices are mostly divided into two categories, 4:3 (fullscreen) and 16:9 (widescreen).
Zoom Player when set to the "Derived" Aspect Ratio mode (Under "Advanced Options / Playback / Video / Aspect Ratio") should automatically compensate for the Video's Aspect Ratio, as long as it was properly encoded (some non-commercial entities at times, improperly encode video content, resulting in the image appearing stretched or squished).
Widescreen Displays, Cropping vs. Black Bars:
An important Aspect Ratio issue you is how to handle 4:3 (fullscreen) content on a 16:9 (widescreen) screen. By default, Zoom Player will Crop the top/bottom parts of the screen so that the image will cover the entire screen. Some users may prefer that Zoom Player will add black bars to the left/right sides of the screen and squeeze the height of the video so none of it is cropped. To do this, you must uncheck "Advanced Options / Playback / Video / Aspect Ratio - Restrict AR Adjustments to Height in Fullscreen / Zoom mode".
Dealing with Non-Square Pixels:
Another thing to pay attention to is display devices which have a non-square pixel. For example, a display which is 48x27cm (16:9) with a resolution of 1500x1000 pixels. Under such conditions, the pixel is not a square, so Zoom Player must be made aware of this and allowed to compensate.
If you would open the "Advanced Options / Playback / Video / Aspect Ratio" dialog, you will see an "Aspect Ratio - Relative Stretch Calculator". Using this calculator you can specify the dimensions of your screen (in cm or inches) and the actual resolution of the display mode (in pixels) and by pressing the "Calculate Source Relative Stretch" button, Zoom Player will automatically feed the compensation value into the "Source Relative Stretch" entry. Next you will need to enable the "Use Source Relative Stretch" check box on this dialog.
Automatically applying Aspect Ratio changes in DVD Mode:
DVD Content has only 3 Aspect Ratio modes, regardless of the actual content of the DVD. Even if a DVD has an Aspect Ratio of 1:2.35, some of the black bars are actually encoded into the video.
The 3 DVD Aspect Ratio modes are:
1. Full-Frame (4:3)
2. Letterbox Widescreen (4:3 with all the black bars encoded into the video)
3. Anamorphic Widescreen (16:9 with some of the black bars encoded into the video)
Zoom Player can automatically detect each Aspect Ratio change and act upon it in a series of events. Open "Advanced Options / Playback / DVD / Auto AR" and you will see that the dialog is split into 3 categories as listed above.
For each category, Zoom Player can automatically set a Video Positioning preset, a Blanking Preset, an Aspect Ratio mode and run an external program.
Video Positioning presets ("Advanced Options / Playback / Video / Presets - Position") allow you to define where the video display rectangle is displayed on your screen, Blanking Presets ("Presets - Blanking") allow you to blank-out part of the screen, Aspect Ratio modes allow you to compensate for special cases where you would want a non-standard Aspect Ratio and the execution of an external programs allows you to perform special effects such as calling an external application that will close the blinds, turn off the lights, etc...