Bit Depth

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Introduction

You will hear terms such as "8-bit", "16-bit", "24-bit", "32-bit", "64-bit" and "96-bit" with reference to digital images. This article will clarify what those things mean.

Digital images consist of millions of pixels, and each pixel describes one or more color channels. Grayscale images need only one channel (a value of 0 could represent pure black, 255 could represent pure white, and the values in-between would then represent shades between black and white), while RGB color images need three channels - one describes red, one green and one blue. Each channel describes only an intensity, so there is nothing inherently green about a number which describes a pixel from the green channel; colors derive from the interaction between all three channels in the RGB color model.

A single pixel could represent more than three channels, for example it could contain information about an alpha channel (which describes transparency) or an infra-red channel (which some scanners support).

The higher the bit depth, the more precisely a color can be described, at a cost of requiring longer computation, more RAM and more storage space.

Bits Per What?

Bit depth is expressed as a value which describes either the number of bits per pixel (BPP), or bits per channel (BPC). The very popular JPEG format typically saves images with a precision of 8 bits per channel, using three channels, for a total of 24 bits per pixel. The TIFF format supports various bit depths, for example 32 bits per channel for a total of 96 bits per pixel.

When describing bit depth, state what you're describing to leave no room for ambiguity. For example, if someone says they have a "32-bit" image, does that mean the image has 32 bits per channel, or does it have 4 channels at 8 bits per channel?

Precision

What difference does bit depth make? The more bits are available to describe a color, the more precisely you can describe that color.

  • A precision of 1 bit per channel means that there is only 1 bit to describe the value. A bit can only be 0 or 1, so you can only represent two values, which typically would mean black or white.
  • A precision of 2 bits per channel means there are two bits available to describe a color. Since each bit can be 0 or 1, and there are two of them, they can represent 4 possible values:
    [00] = 0
    [01] = 1
    [10] = 2
    [11] = 3

    If we use 0 to represent black and 3 to represent white, there are two additional shades of gray which can be described.

  • A precision of 8 bits per channel means there are 8 bits which can represent 256 values:
    [0000 0000] = 0
    [0000 0001] = 1
    (...)
    [1111 1110] = 254
    [1111 1111] = 255

    If we use 0 to represent black and 255 to represent white, 254 shades of gray can also be described. This is what JPEG files use - 8 bits per channel, with 3 channels. It is sufficient to be used for most ready-to-view photographs in the sRGB color space without visible posterization, so you can use it when saving photographs ready to be viewed over the internet. It is not suitable as an intermediate format nor as a final format if there is a chance you might need to tweak the photograph later on, as you run the risk of introducing posterization artifacts, depending on the strength of adjustments. 8-bit precision is not enough to represent a high dynamic range scene in a linear way without posterization, i.e. you theoretically could use 8 bits of precision to describe a high dynamic range scene linearly, but the numbers would be so far apart that heavy posterization would occur. For instance, if a photograph captures a sunny day in the park, and if we assume that black should be 0 and white should be 1 000 000, we could map 0 to 0 and 255 to 1 000 000, but then there would only be 254 values left for describing all the remaining 999 999 shades of the original scene.

  • A precision of 16 bits per channel (16-bit integer) means there are 16 bits which can represent 65536 values:
    [0000 0000 0000 0000] = 0
    [0000 0000 0000 0001] = 1
    (...)
    [1111 1111 1111 1110] = 65534
    [1111 1111 1111 1111] = 65535

    Digital cameras typically capture light in 12-bit or 14-bit precision (and due to noise and imprecise electronics the lowest bits are of dubious quality). 16 bits per channel are enough for most photography needs, including for use in intermediate files (if you want to pass an image from one program to another without data loss).

  • The values of a 16-bit floating point image, also known as half-precision floating point, are spread in a way more suitable to sampling light than in 16-bit integer. This is so for various reasons: human vision is more sensitive to small changes in dark tones than to small changes in bright ones; our eyes respond to light in a logarithmic way (light must be 10 times more intense in order for us to see it as twice as bright); and specular highlights which can be the the brightest elements in a scene (the sun reflecting off a door knob) need not be described as accurately as all the other tones. In 16-bit floating-point notation, values are distributed more closely in the (lower) darker tones than in the (higher) lighter ones, thus allowing for a more accurate description of the tones more significant to us.
  • A 32-bit floating point image can represent 4.3 billion values per channel, and requires roughly twice the disk space as a 16-bit image. Few programs support 32-bit images.

One of the reasons bit depth affects mostly shadows is due to the way colors are stored. Each color is defined by a mixture of red, green and blue. Using an 8-bit image and the color orange as an example, many values are possible when describing bright orange, but the number of samples available to describe dark orange drops to very few, i.e. only the lowest 3-4 bits from each channel can be used to describe dark orange, which means only 16 possibilities exist. The higher the bit-depth, the more colors can be described, and posterization avoided.

Gamma Encoding

Gamma encoding can be used when saving image files, meaning that values are modified in such a way that more can be allocated in the shadow range than in the highlight range, which better matches the human eye's sensitivity. This means that an 8 bit JPEG can display as much as log2((1/2^8)^2.2) = 17.6 stops of dynamic range, which indeed exceeds the 14 stops of the current best cameras, which explains why you sometimes can see a camera's shadow noise even in an 8-bit JPEG. However, due to the non-linear distribution, we lose precision compared to the raw file recorded in a linear way by the digital camera. Practically this is not a problem when the output file is the definitive one and will not be processed anymore, however a photo can be vastly improved when saved as raw data and processed using a state of the art raw processing program, such as yours truly - RawTherapee.

After RawTherapee

Once you have adjusted a photo in RawTherapee and are ready to save, you are faced with a choice of output format, per-channel bit depth, color space and gamma encoding. If you plan to post-process your photos after RawTherapee in a 16-bit-capable image editing program, it is better to save them in a lossless 16-bit format. RawTherapee can save images in 16-bit integer precision (denoted as "TIFF (16-bit)" in the Save dialog) as well as 16-bit floating-point precision (denoted as "TIFF (16-bit float)"). Uncompressed TIFF at 16-bit integer precision is suggested as an intermediate format as it is the fastest to save and is widely compatible with other software. 32-bit files are roughly twice the size and not well supported by other programs.