What is Bit Depth?
Since all medias (sounds, images and video) are now digital, their storing de facto is too. IT is comfortable with these as they are based on a treatment and handling exclusively digital.
To understand about Bit Depth you need to understand What is digital image?
A digital image is nothing more than data—numbers indicating variations of red, green, and blue at a particular location on a grid of pixels.
bit : digital unit
A bit is the smallest unit of data in digital imaging. Each pixel in a digital image is represented by a number of bits. More bits translate into more tones, gray scale and color, represented per pixel in a digital image. The number of pixels represents the two-dimensional height and width of an image. The number of bits represents a third dimension describing how light, dark or colorful each pixel is. This dimensional aspect results in the term Bit Depth.
It’s a digital world, its unit : the bit, There is only 2 states : on/off ; black/white; full/empty. And the quantity of bit used for recording,simulate, restore, can be colossal (this is the purpose of the Codecs).
Digital images are produced in bitonal, grayscale or color formats. The difference between the formats is determined by the number and the type of information each bit records per pixel. Every bit represents two options; 1 or 0, on or off.
TERMINOLOGY
Every color pixel in a digital image is created through some combination of the three primary colors: red, green, and blue. Each primary color is often referred to as a "color channel" and can have any range of intensity values specified by its bit depth. The bit depth for each primary color is termed the "bits per channel." The "bits per pixel" (bpp) refers to the sum of the bits in all three color channels and represents the total colors available at each pixel. Confusion arises frequently with color images because it may be unclear whether a posted number refers to the bits per pixel or bits per channel. Using "bpp" as a suffix helps distinguish these two terms.
Example
Most color images from digital cameras have 8-bits per channel and so they can use a total of eight 0's and 1's. This allows for 28 or 256 different combinations—translating into 256 different intensity values for each primary color. When all three primary colors are combined at each pixel, this allows for as many as 28*3 or 16,777,216 different colors, or "true color." This is referred to as 24 bits per pixel since each pixel is composed of three 8-bit color channels. The number of colors available for any X-bit image is just 2X if X refers to the bits per pixel and 23X if X refers to the bits per channel.
COMPARISON
The following table illustrates different image types in terms of bits (bit depth), total colors available, and common names.
CONCLUSION
Bit Depth means the color information stored in one pixel. Only when one pixel stores some colors, then the image can show us a cat, a dog, a flower, or something else. The bigger the number of the bit depth, like 1-bit, 2-bit, 4-bit, 8-bit, 16-bit, etc. the more color information one pixel can store, more precisely the whole image would be described, and consequently more storage the image would take up.
1 Bit — 2 colors (Monochrome) 8 Bit — 256 colors (Low Color) 16 Bit — 65536 colors (High Color) 24 Bit — 16777216 colors (True Color)
Why?
The color of each pixel is the combination of the three primary colors: red, green, and blue. Each primary color is often called a color channel and can have any range of intensity values specified by its bit depth. The bit depth for each primary color is termed the bits per channel. The bits per pixel (bpp) refers to the sum of the bits in all three color channels and represents the total colors available at each pixel. Here are some specific examples, which maybe help you understand those words above easily.
Now we have pretty much get the Bit Depth clear. And in the following content related to Bit Depth, we will take the 8-bit per channel as an example to help us.
And at this time, we can also calculate how much storage one frame, or one image will take up. The formula, which we have mentioned above, is,
Like we have said, we take the 4K resolution (4096 x 2160) and 8-bit per channel as examples. So for a 4K video file, the size of its one frame would be 4096 x 2160 x 8 = 70778880 bits.
Well, bits are less likely to see out there. It would be much better to convert into megabytes (MB).
1 megabytes = 1024 kilobytes;
1 kilobyte = 1024 bytes;
1 byte = 8 bits;
So 70778880 bits = 70778880/(8 x 1024 x 1024) MB = 8.4375 MB. That is to say, for 4K video file with 8-bit Bit Depth, the size of one frame is 8.4375 MB.
Now we can answer the question of how to calculate video file size, and here is the video file size formula,
Please let me explain to you the meaning of each item in this formula. Time refers to how long your video is; Frame per Second, or called FPS, means how many frames will be played per one second for this video; Pixel per Frame, or the Resolution, and Bit Depth have been talked about above.
Imagine there is a 4K video with 30 minutes, 60FPS, 4096 x 2160 pixels, and 8-bit Bit Depth, then its size would be (in MB),
30 x 60 x 60 x 4096 x 2160 x 8 ÷ 8 ÷ 1024 ÷ 1024 = 911250 MB;
OK, let's convert it to gigabyte (G), and it would be,
911250 ÷ 1024 = 889.89GB.
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