I keep getting numpties telling me that 4K 4:2:0 8bit footage does not translate to 4:4:4 10bit on an HD timeline, don’t take my word for it lets hear from renowned expert Barry Green…
Barry “One excellent benefit of downconverting UHD/4K footage to 1080 HD in post is that you can realize an increase in proportional color resolution and a notable increase in bit depth. The AG-DVX200 records 4K or UHD footage at 8 bits per pixel and utilizes 4:2:0 color sampling. After downconversion, the resulting footage has 10 bits per pixel and 4:4:4 color sampling! Yes, you can convert 3840×2160 8-bit 4:2:0 recorded footage into 1920×1080 10-bit 4:4:4 footage in post.
To understand the color sampling advantage, you’d have to first understand that the camera re- cords its footage in 4:2:0 color sampling. That means (simply put) that there is one color sample for every 2×2 block of pixels. In any given 2×2 block of pixels there are four different “bright- ness” samples, but they all share one “color” sample. Effectively, within the 3840 x 2160 frame, there is a 1920 x 1080 matrix of color samples, one for every 2×2 block of pixels. During the downconversion to HD, each block of 2×2 brightness samples are converted into one HD pixel, creating a 1920 x 1080 matrix of brightness pixels. This 1920 x 1080 “brightness” (luminance) matrix can be effectively married to the originally-recorded 1920 x 1080 “color” matrix, result- ing in one individual and unique color sample for each and every brightness pixel. The result is 4:4:4 color sampling at high-definition resolution.
In terms of pixel depth, the original recorded footage is quantized and recorded at an 8-bit depth, providing for up to 256 shades per pixel. Other formats, like Panasonic’s own AVC-Intra, quantizes and records at a 10-bit depth, for up to 1,024 shades per pixel. Having deeper bit depth provides the ability for finer shading and more subtle transitions, especially apparent on smooth gradients (such as in a clear blue sky).
Generally 8-bit cameras perform fine for most images, but extensive image manipulation in post can reveal the limitations of 8-bit encoding and cause “banding” and “stair-stepping” from one shade to the next. 10-bit footage minimizes those effects because there are up to four shades for every one shade in 8-bit footage. When downconverting UHD/4K footage to 1080p HD, you also get the benefit of converting 8-bit pixel depth into 10-bit pixel depth! Since each 2×2 block of UHD/4K pixels will be summed to- gether to create a single 1×1 pixel in 1080p HD, the individual pixel values and gradations from the source footage can be retained in the downconverted footage.
Imagine a smooth gradient of medium gray, gradually getting brighter from left to right. In 8-bit pixel data, a medium gray might be represented by a pixel value of 128, and the next brighter shade might be 129. In 10-bit pixel data, that same medium gray (128) might be repre- sented by a pixel value of 512 (128 x 4) and that brighter shade (129) might be represented in 10- bit by a value of 516 (129 x 4). The obvious difference here is that an 8-bit camera can’t represent any difference between 128 and 129, but the 10-bit camera (looking at the exact same gradient) could represent a smoother transition between 512, to 513, 514, 515, and then eventually 516. Having 10 bits of data provides for the ability to retain and discern between finer shades of grey (or color). So what happens when we downcon- vert our 8-bit UHD footage to 10-bit 1080p HD footage? As each 2×2 block of pixels is summed together, those subtle differences in shade are retained, and we end up being able to represent shades that the 8-bit footage couldn’t have.”
HDW : Remember this only works as long as you setup a 10bit timeline.
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