Interlacing is a technique used by video recorders and television sets to ensure a smooth and fluid display on screen. It splits each frame into two sets of fields, an upper field and a lower field. Each field carries half the picture in horizontal rows, which when added together creates one interlaced frame. When viewing these frames in motion they are played too quickly for the eye to pick up any trace of interlacing, however, if you pause the video the effect becomes more prominent, growing even more if you happen to export a frame and view it on a computer screen.
What does Interlacing Look Like?
To demonstrate the effect of interlacing I’ve exported a frame from a video I shot in 2006. I was lucky enough to capture some real lightning from a vantage point in the Utah hills looking down on the valley below. If this scene looks familiar you may remember it from my first ever video tutorial here on the site – how to create a lightning bolt in Photoshop and add it to an image. Anyway, the interlaced frame captured on my video camera is below, if you would like to download the whole PAL standard definition interlaced frame then click here.
You can see the interlacing effect above at its most intense within the lightning bolt and the trees on top of the closest hill. It leaves the lightning bolt looking like there are sections missing and the trees jagged and rough – very unnatural and not photo realistic at all.
Remove Interlacing in Photoshop
Photoshop has a filter that was designed to do video work like this – and it’s called, not surprisingly, de-interlace. To use it go to the Filter menu > Video > De-interlace and you should see a filter that looks like this-
You essentially have two options to choose from which makes it easy to try all three combinations for the greater success. If you want to try a few remember not to apply multiple filters, or multiple instances of the same filter, to the same image. That’s likely to leave the it looking battered. Instead use the undo command or history panel to remove the filter and try again.
The Options
Here’s what the options inside the De-interlace filter actually do.
Eliminate – Photoshop is asking what fields you want to eliminate, odd or even? This equates in video terms to upper or lower fields. If you exported the frame with interlaced lower fields then choose odd, or for upper fields try even. My favourite approach is to try both and use the one that looks the best, you’ll soon know the difference. In the Utah image we’re using here, if I select the wrong elimination option of odd field the lightning bolt completely disappears as you can see below – definitely not the effect I’m after!
Create New Fields By - To remove the interlaced fields from the image Photoshop has to create new fields that better match the image. Here you can choose to create the new fields by duplication, in which the pixels around the area to be replaced are duplicated, or by a method of interpolation, in which Photoshop evaluates the pixels around the interlaced content and creates new pixels based on an average of the ones that are already there.
I nearly always use interpolation but if you see that method picking up unwanted colours or noise then try duplication. In the following example I used odd fields and interpolation to remove the interlacing from the video frame.
You’ll notice the lightning is bold and bright, and the trees on the hilltop are smooth, if not a little too much so. You’ll also notice that when you apply the filter the image seems to move a little, that’s nothing to worry about and just a consequence of the two fields (odd and even) containing different content in the first place.
I hope you now understand more about the wonders and importance of de-interlacing video frames before using them in your photographs and artwork.