How Video Bit Depth Affects File Size

Recently, after reading an article explaining the basics of high-dynamic range media and illustrating the impact of different codecs and formats on file size, Sam Russell took exception to how I calculated the change in file size as image bit-depth increases.

While Sam agreed that file size increases as bit-depth increases, he felt I was exaggerating the results. So, given the challenge to prove my thinking, I did more research and discovered that Sam was right.

WHAT IS BIT-DEPTH

Bit-depth, whether for stills or video, determines the number of color or gray-scale values in an image. Bit-depth is expressed as powers of 2:

• A bit-depth of 1 = 2 to the power of 1 = 2 colors
• A bit-depth of 2 = 2 to the power of 2 = 4 colors
• A bit-depth of 3 = 2 to the power of 3 = 8 colors
• A bit-depth of 8 = 2 to the power of 8 = 256 colors
• A bit-depth of 16 = 2 to the power of 16 = 65,536 colors

Tech-Ease, at the University of South Florida, provided some great examples of this.

(Image courtesy: University of South Florida.)

Here’s an image at a bit-depth of 1; that is, showing only two colors. (Remember, white is a color.)

(Image courtesy: University of South Florida.)

Here’s an image at a bit-depth of 2; a total of four colors (black, gray, gold and white.)

(Image courtesy: University of South Florida.)

Here’s an image at a bit-depth of 8; a total of 256 colors. Notice the smoother shading in the gradients.

CALCULATING FILE SIZE

The BBC, in a series of articles explaining technology to teenagers, defines the formula to calculate file size as bit-depth changes:

• Horizontal pixels * vertical pixels * bit-depth = file size

This assumes that the media is uncompressed and that the frame size and frame rate remain the same. When I calculate this for different bit depths, I get a consistent growth in file size.

As bit-depth increases by 2, file size increases by 25%. And, thankfully for the amount of storage they would require, video files above 16-bit don’t yet exist.

NOTE: Keep in mind that your results will vary depending upon how much the codec you are using compresses the file. This chart assumes uncompressed media to better show the relationship in file sizes.

If you are interested, here’s the math behind the chart. Feel free to check this yourself.

Thanks, Sam, for suggesting I look at this again. And, yes, I’ve correct the original article.

EXTRA CREDIT

• Here’s the article from Tech-Ease. It is a fast and well-illustrated read.
• Here’s the article from the BBC. Again, this is a quick read.

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