Tag Archives: sharpness

Flicker, jaggies and moire in down converted 4K.

I kind of feel like we have been here once before. That’s probably because we have and I wrote about it first time around.

A typical video camera has a special filter in it called an optical low pass filter (OLPF). This filter deliberately reduces the contrast of fine details in the image that comes from the cameras lens and hits the sensor to prevent aliasing, jagged edges and moiré rainbow patterns. It’s a very important part of the cameras design. An HD camera will have a filter designed with a significant contrast reduction on parts of the image that approach the limits of HD resolution. So very fine HD details will be low contrast and slightly soft.

When you shoot with a 4K camera, the camera will have an OLPF that operates at 4K. So the camera captures lots of very fine, very high contrast HD information that would be filtered out by an HD OLPF. There are pro’s and con’s to this. It does mean that if you down convert from 4K or UHD to HD you will have an incredibly sharp image with lots of very fine high contrast detail. But that fine detail might cause aliasing or moiré if you are not careful.

The biggest issue will be with consumer or lower cost 4K cameras that add some image sharpening so that when viewed on a 4K screen the 4K footage really “pops”. When these sharpened and very crisp images are scaled down to HD the image can appear to flicker or “buzz”. This will be especially noticeable if the sharpening on the HD TV is set too high.

So what can you do? The most important thing is to include some form of anti-aliasing to the image when you down scale from 4K to HD.  You do need to use a scaling process that will perform good quality pixel blending, image re-sampling or another form of anti-aliasing. A straight re-size will result in aliasing which can appear as either flicker, moire or a combination of both. Another alternative is to apply a 2 or 3 pixel blur to the 4K footage BEFORE re-sizing the image to HD. This seems a drastic measure but is very effective and has little impact on the sharpness of the final HD image. Also make sure that the sharpening on your TV is set reasonably low.

I previously wrote about this very same subject when HD cameras were being introduced and many people were using them for SD productions. The same issues occurred then. Here are the original articles:

Getting good SD from HD Part 1.

Getting good SD from HD Part 2.

Remember to take a look in the TECH NOTES for info like this. There’s a lot of information in the XDCAM-USER archives now.


The Difference Between Detail Correction and Aperture.

Just to clarify the differences between Detail settings and the Aperture setting.

Detail has a sub set of settings including: frequency, level, crispening, knee aperture, black and white limit. These sub settings all affect the amount and level of detail correction applied.

Aperture is a completely separate type of adjustment.

Detail works on contrast. The higher the contrast in an image, the sharper it appears. A bright sunny day will look sharper that a dull cloudy day because there is more contrast. detail works by increasing contrast by adding black or white edges to any parts of the image where the contrast changes rapidly, for example the edge of an object silhouetted against the sky. This increases contrast still further, making the image appear sharper. The crispening setting sets the contrast threshold at which detail gets added, level adjusts the amount.

Aperture is a simple high frequency boost. As fine details and textures are normally represented by high frequencies within the image, boosting high frequencies can help compensate for the natural fall off in lens and sensor performance at higher frequencies. This helps enhance textures and other subtle, fine details within the image look clearer.

Neither setting will increase the cameras resolution. Both make the image “appear” sharper. Detail correction IMHO is very un-natural looking and electronic, while careful use of aperture can help sharpen the image without necessarily looking un-natural.