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Pixels, 8-Bit vs 10-Bit & 4:4:4


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#1 Allan Kader

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Posted 13 January 2018 - 08:29 PM

Hey guys,

 

I'm currently trying to learn some basics about various aspects of color grading. As such, I have a few questions/statements I'm hoping some of you can help me with answering/confirming :)

 

  • 1 pixel contains the color channels Red, Green and Blue. - correct?

 

  • Each color channel in 1 pixel carries its own luma value (how bright or dark each color can become). And the luma value for each color channel in 1 pixel is between 0-255. If the luma value for each color channel in 1 pixel is 255 it will reduce white. - correct?

 

  • When talking about the bits of an image, you're talking about the bits per channel in 1 pixel. So this means e.g. 8 bits for the Red, Green & Blue channel. This also means that there are 256 different values/shades of color per channel - correct?

 

  • A 10-bit image will be able to reproduce more shades of color. E.g. in terms of gradient. And working with 10-bit footage means more color information to use for grading. - correct?

 

  • 4:4:4 stands for 4 pixels cross and another 4 pixels across below. - correct?

 

  • 4:4:4 is great for green screen as each pixel carries it's own chroma value. - correct?

 

I hope you guys can help :)

 

/Allan


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#2 Phil Rhodes

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Posted 13 January 2018 - 09:41 PM

There are two principal ways colour images are stored on computers.

 

The first is the familiar RGB. Usually, each pixel has three numbers in it. Set them all to maximum value, and you get white. If you want, say, red, set red to maximum value, and green and blue to zero.

 

The second is the less familiar system of component video, sometimes referred to as YUV or other related terms.

 

Component video separates luminance from colour. In the terminology, Y stands for the luminance data, and U and V the colour data.

 

It does this because the human eye sees colour information less sharply than it sees brightness information.

 

Separating the colour information allows it to be sent at a lower resolution. It is possible to have component pixels with Y, U and V values for every pixel, but it is more usual to send the U and V channels at a lower resolution.

 

This is called colour subsampling and saves data space without making a noticeable difference to image quality.

 

4:2:2 means that for every four Y pixels, two U and V pixels are sent. Effectively, the colour information is sent at half the horizontal resolution, compared to the luminance information.

 

Yes, this means that 4:4:4 is better for green screen, because the colour information is more detailed.

 

Bit depth is unrelated to colour subsampling, but yes, 10 bit images have 1024 possible values per channel, whereas 8 bit images have 256.

 

P


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#3 Allan Kader

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Posted 14 January 2018 - 02:49 PM

There are two principal ways colour images are stored on computers.

 

The first is the familiar RGB. Usually, each pixel has three numbers in it. Set them all to maximum value, and you get white. If you want, say, red, set red to maximum value, and green and blue to zero.

 

The second is the less familiar system of component video, sometimes referred to as YUV or other related terms.

 

Component video separates luminance from colour. In the terminology, Y stands for the luminance data, and U and V the colour data.

 

It does this because the human eye sees colour information less sharply than it sees brightness information.

 

Separating the colour information allows it to be sent at a lower resolution. It is possible to have component pixels with Y, U and V values for every pixel, but it is more usual to send the U and V channels at a lower resolution.

 

This is called colour subsampling and saves data space without making a noticeable difference to image quality.

 

4:2:2 means that for every four Y pixels, two U and V pixels are sent. Effectively, the colour information is sent at half the horizontal resolution, compared to the luminance information.

 

Yes, this means that 4:4:4 is better for green screen, because the colour information is more detailed.

 

Bit depth is unrelated to colour subsampling, but yes, 10 bit images have 1024 possible values per channel, whereas 8 bit images have 256.

 

P

 

 

Thanks for clearing that up :)

While I have you here, I also have a few more about SDI and HDMI.

 

  • SDI 3G-Level A can carry a higher frame rate over one cable - correct?
  • SDI 3G-Level B can carry a higher bit depth over one cable - correct?
  • Dual Link HD-SDI can achieve the same as one 3G SDI - correct?
  • Can a Dual Link SDI 3G carry 2K?
  • What are the benefits of using SDI over HDMI?

 

/Allan


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#4 Phil Rhodes

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Posted 14 January 2018 - 03:49 PM

This is all stuff you can Google, you know.
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#5 Allan Kader

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Posted 14 January 2018 - 04:42 PM

Thanks :) 


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Aerial Filmworks

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