Using Frame Controls in Compressor

Compressor has some very sophisticated technologies in the Frame Controls tab of the Inspector to enable it to perform high-quality resizing, retiming and deinterlacing of footage.

These are incredibly useful but it is very tempting to set everything to "best" when it is not at all necessary and causes a significant impact upon the processing time. So what this post aims to do is outline what each function does and when it should be used.

Firstly, if you are not resizing, retiming or deinterlacing footage, do not switch Frame Controls on at all.

Resizing controls
When an image is resized, new pixels need to be calculated. If you downsize an image, an area that was once 10 pixels could now be 2 pixels and vice versa if you are upsizing. Compressor goes through the resulting image and calculates what each pixel value should be based on the value at that point in the original image. These are not always integers. For example, a pixel at point (3,3) in the resulting image might correspond to point (2.8, 2.8) on the original. As there is no pixel at (2.8, 2.8), Compressor has to calculate a suitable value. The following options are different ways of calculating these pixel colors.

Fast (nearest pixel) - Compressor takes the value from the closest whole pixel in the original image. This is very fast but can result in significant visual artifacts and inaccuracies, particularly when resizing by a significant amount.

Suitable for: Minor size changes, situations where a fast encode is more important than a high quality one.

Better (linear filter) - This improves on the nearest pixel method by taking the values of four surrounding pixels and weighting them, increasing accuracy.

Suitable for: Most downconversions (e.g. 720p to NTSC).

Best (statistical prediction) - This uses more complex algorithms to maintain edge sharpness, at the expense of processing time.

Suitable for: Upconversions (e.g. NTSC to 1080p HD)

Anti-aliasing slider - This smooths rough edges but don't set it too high or you will visibly soften the image.

Suitable for: Smoothing jagged edges when upconverting footage (e.g. PAL to 720p HD)

Detail Level slider - This sharpens edges, increasing our perception of detail in the image. Use sparingly to avoid adding noticeable noise or jagged edges to the image.

Suitable for: Sharpening soft edges when upconverting footage.

Deinterlacing controls
To deinterlace, first select Progressive from the Output Fields drop-down menu. Use this instead of the Deinterlace filter in the Filters tab as it will provide greater output quality. Choose one of the following from the Deinterlace menu:

Fast (line averaging) - The two fields are blended into one frame. This, however, can result in strange motion artifacts because you are playing two fields at the same time that were originally offset in time.

Suitable for: Situations with very little motion.

Better (motion adaptive) - This method analyzes the fields and uses simple linear blends (equivalent to the Fast setting) on areas with no motion. For areas with motion it discards one of the fields and uses interpolation to build up a whole frame from the other field's data.

Suitable for: Most situations except those with extremes of motion. In most cases you will not get any benefits from choosing higher settings than this if you are working in Standard Definition (e.g. NTSC, PAL). The downside to this method is that you lose vertical resolution if there is a lot of motion in the image.

Best (motion-compensated) - This uses optical flow technologies to track the motion of objects and analyzes multiple frames at once. This data is then used to split the image into blocks (typical sizes are 4x4, 8x8 or 16x16) and then shift these blocks so as to align motion between the two fields. This is incredibly processor intensive and will take some time.

Suitable for: Situations with extremes of motion (e.g. sports).

Retiming controls

Fast (nearest frame) - If you slow down footage, Compressor will need to add extra frames. This option just duplicates the nearest existing frame. It is very speedy but it can result in very choppy / juddery motion. If you are speeding up footage however, the choppiness will probably not be noticeable.

Suitable for: Minor speed adjustments, fast motion

Good (frame blending) - This blends frames together to create "in-between" frames and is a lot smoother than the nearest frame method, at the cost of processing time. This is suitable for most applications unless you are performing extreme slow motion.

Suitable for: Most speed changes with the exception of extreme slow motion

Better (motion-compensated) - This uses complex algorithms to analyze a range of frames and interpolate (predict) what the in-between frames should look like based on their surrounding frames. It then creates these new frames from scratch. Obviously this is very processor-intensive.

Suitable for: Extreme speed changes or footage with large amounts of motion (sports for example)

Best (high quality motion-compensated) - Apple's documentation is a little unclear on what makes this particular option higher quality than the one below it. It could be that it analyzes more frames to produce the final result, it blends using a higher bit depth for greater accuracy, or it recreates every single frame instead of just the in-between ones for greater smoothness. Or maybe all of them.

Either way, this is incredibly processor intensive and should only be used if you cannot get good results with lower settings.

Suitable for: Extreme speed changes or significant frame rate increases (e.g. 23.98 to 59.94 fps)

Conclusion

So what does this tell us? It tells us that "Good" or "Better" should be adequate for most situations and it is not worth choosing "Best" unless your footage actually needs it. The best way of finding a happy medium is to start with everything on "Fast" and perform small test renders (10 seconds or so) for multiple areas of your movie. If you are not happy with the quality, go up to the next level and keep going until you reach a level of quality you are happy with.

Posted by Jon Chappell on Wednesday June 4 2008 9:44 PM to Video Editing, Software, Final Cut Studio
Post ID: 272


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