How 1080p and 1080i Differ


A few days ago I found myself talking to a colleague about the differences between 1080p and 1080i. Inspired by that conversation, I have decided to write about these differences.

Note beforehand that both these resolutions have 1080 horizontal lines. 1080p is a frame-based (or progressive-scan) video where the frame rate is expressed in terms of frames per second. 1080i is a field- based (or interlaced-scan) video whose frame rate is expressed in terms of fields per second. In this context, a field is either all the odd lines of a frame or all its even lines.

1080p most common frame rates are 24, 25, and 29.97 frames per second while 1080i most common field rates are 50 and 59.94 fields per second. 1080p at 25 frames per second would correspond to 25 full bitmaps of 1920 x 1080 pixels. 1080i at 50 fields per second, on the other hand, represent 50 bitmaps of 1920 x 540 pixels - as if you were shooting 50 pictures per second but storing only half of the bitmaps every time – sometimes you store the odd lines and sometimes the even lines.

There is a lot of difference between 25 full pictures and 50 halves, although common sense may tell you otherwise. These 50 halves are not simply 25 pictures divided in half, but 50 different pictures with half of their content thrown away. Interlacing make mundane operations on frames such as video scaling and rotating, video pausing, screenshot capturing, and reverse playback quite complicated. Video encoding is also more complicated for interlaced video because the codec does not work with full frames.

The biggest downside of 1080p is its substantially lower frame rate, but the deinterlacing problems that may arise from 1080i resolution generally make it a worse choice. Although most TV transmissions are interlaced, plasma and LCD screens are progressively scanned, making a process known as deinterlacing necessary. Bad deinterlacing - likely to occur in very fast-paced scenes - may produce undesired effects such as the one demonstrate by the image below.

Bad deinterlacing

At the end, which combination of frame resolution and scan type you use is a choice that depends on how much you can spend and which options are available. However, I don’t see any reason to opt for 1080i over 1080p if you can freely pick between them.