That is, the highest dynamic range that the device would be capable of reproducing if all other constraints are eliminated.įor instance, a bit-depth of 12 for a CCD tells you that the maximum dynamic range of the sensor is 4096:1, but the captured dynamic range is likely to be much less once noise is taken into account (most 12-bit sensors have on average a dynamic range around 1,000:1 only). The bit depth of a capturing or displaying device gives you an indication of its dynamic range capacity. Bit depth and dynamic range are indeed separate concepts and there is no direct one to one relationship between them. ^ But aren't we confusing Dynamic Range with bit depth here? See Greg Ward's HDR Image Encodings page for an excellent overview of HDR formats. There are various formats available to store HDR images, such as Radiance RGBE (.hdr) and OpenEXR (.exr) among the most commonly used. When an image comes from a single capture with a standard camera, it will remain a Low Dynamic Range image, regardless of the format used to store it. It is important to note, though, that storing an image in a 32-bit HDR format is a necessary condition for an HDR image but not a sufficient one.
Unlike 8- and 16-bit images which can take a finite number of values, 32-bit images are coded using floating point numbers, which means the values they can take is unlimited. 96 bits per pixel for a color image) are considered High Dynamic Range. By using 16 instead of 8 bits, you will gain precision but you will not gain dynamic range.ģ2-bit images (i.e. The need to adapt the output image file to the dynamic range of the display is the factor that dictates how much the dynamic range is compressed, not the output bit-depth. Note that converting a RAW file involves applying a tonal curve that compresses the dynamic range of the RAW data so that the converted image shows correctly on low dynamic range monitors. 48 bits per pixel for a color image) resulting from RAW conversion are still considered Low Dynamic Range, even though the range of values they can encode is significantly higher than for 8-bit images (65536 versus 256). 24 bits per pixel for a color image) are considered Low Dynamic Range.ġ6-bit images (i.e. However, the number of bits itself may be a misleading indication of the real dynamic range that the image reproduces - converting a Low Dynamic Range image to a higher bit depth does not change its dynamic range, of course. Most often, the distinction is made depending on the number of bits per color channel that the digitized image can hold. Whether an image may be considered High or Low Dynamic Range depends on several factors. In this sense, HDR images are scene-referred, representing the original light values captured for the scene.
This means that the value of a pixel from an HDR image is proportional to the amount of light measured by the camera. floating-point values stored with 32 bits per color channel.Īnother characteristics of an HDR image is that it stores linear values. Therefore, an HDR image is encoded in a format that allows the largest range of values, e.g. An HDR (High Dynamic Range) image stores pixel values that span the whole tonal range of real-world scenes. The Dynamic Range of real-world scenes can be quite high - ratios of 100,000:1 are common in the natural world. The scanline below represents the log base 10 of the luminance, so going from 0.1 to 1 is the same distance as going from 100 to 1000, for instance.Ī scene showing the interior of a room with a sunlit view outside the window, for instance, will have a dynamic range of approximately 100,000:1. To make it easier to represent values that vary so widely, it is common to use a logarithmic scale to plot the luminance. The human eye can accommodate a dynamic range of approximately 10,000:1 in a single view. The luminance of the sun itself is approximately 1,000,000,000 cd/m2. While the luminance of starlight is around 0.001 cd/m2, that of a sunlit scene is around 100,000 cd/m2, which is a hundred millions times higher. The range of luminance human vision can handle is quite large. In photography and imaging, the dynamic range represents the ratio of two luminance values, with the luminance expressed in candelas per square meter.
Dynamic range is a ratio and as such a dimensionless quantity.
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