## Introduction to Motion Picture Formats

DPX - This format is used in Motion Picture and Effects industry that makes particular use of the extensive header information and the format's flexibility in being able to handle high dynamic range and logarithmic color values at a variety of bit depths using RGB or YCbCr pixel descriptions. It is based on, but largely supercedes, Kodak's Cineon format that has more a more film specific header.

One example of it's use would be when scanning film for use in post production. Each frame would be stored as an individual DPX file ranging from 2k (2048 pixels wide) to 8k (8192 pixels wide - for IMAX frames) at anything between 8 to 64 bits per color component. A sequence of these might then be processed using compositing software, altering the color or adding visual effects. Once complete they might then be recorded digitally to tape or projected back on to film.

The color values for each pixel are often stored logarithmically (particularly if the sequence is destined to be transferred back on to film) which more naturally reflects the density of how color information is stored in the emulsion on the original film. When viewed without alteration, logarithmic files appear to have very low contrast and requires a 'look up table' to translate the logarithmic image to something that resembles what you might see if the image was transferred back to film and projected in a cinema. Apart from making the image linear (like most typical computer images) and adjusting the gamma level this table sets where the black and white point lies.

For a 10 bit logarithmic image where each color component value ranges from 0 to 1023 the black and white points are normally set at 95 for black and 685 for white. What this means is that the logarithmic file stores color values that are lighter than what the linear version will display as pure white and darker than what it will display as pure black. This extra information therefore remains available for an effects artists who might wish to alter the brightness of the image after it has been stored as a DPX file.

As an example, had this information been lost, reducing the brightness of an image uniformly would result in highlights becoming darker, whereas with this extra information the highlights instead reduce in size and start showing details that were previously too bright to be seen. The latter is far closer to what happens in the real world.

The header can contain Film and/or Television specific data related to a production. For example the television header can contain a SMPTE time code so that shots exported as a DPX sequence from a production's edit can be easily replaced once any effects have been added. The film header holds information about the reel of film the frames originated from and various camera settings that were used while filming. All these details usually stay with the images as they are passed between post-production companies.

## Log Format

The color values for each pixel are often stored logarithmically (particularly if the sequence is destined to be transferred back on to film) which more naturally reflects the density of how color information is stored in the emulsion on the original film. When viewed without alteration logarithmic files appear to have very low contrast (leftmost image), and so require a 'look up table' to translate the logarithmic image to something that resembles what you might see if the image was transferred back to film and projected in a cinema (rightmost image). Apart from making the image linear (like most typical computer images) and adjusting the gamma level this table sets where the black and white point lies.

For a 10 bit logarithmic image where each color component value ranges from 0 to 1023 the black and white points are normally set at 95 for black and 685 for white. What this means is that the logarithmic file stores color values that are lighter than what the linear version will display as pure white and darker than what it will display as pure black. This extra information therefore remains available for an effects artists who might wish to alter the brightness of the image after it has been stored as a DPX file.

As an example, had this information been lost, reducing the brightness of a linear image uniformly would result in highlights becoming darker (leftmost image), whereas with this extra information the highlights instead reduce in size and start showing details that were previously too bright to be seen (rightmost image). The latter is far closer to what happens in the real world.

## DPX Properties

ImageMagick supports these DPX properties:

      dpx:file.copyright
      dpx:file.creator
      dpx:file.filename
      dpx:file.project
      dpx:file.version
      dpx:film.count
      dpx:film.format
      dpx:film.frame_id
      dpx:film.frame_position
      dpx:film.frame_rate
      dpx:film.held_count
      dpx:film.id
      dpx:film.offset
      dpx:film.prefix
      dpx:film.sequence_length
      dpx:film.shutter_angle
      dpx:film.slate
      dpx:film.type
      dpx:orientation.aspect_ratio
      dpx:orientation.border
      dpx:orientation.device
      dpx:orientation.filename
      dpx:orientation.x_center
      dpx:orientation.x_offset
      dpx:orientation.x_size
      dpx:orientation.y_center
      dpx:orientation.y_offset
      dpx:orientation.y_size
      dpx:television.black_gain
      dpx:television.black_level
      dpx:television.break_point
      dpx:television.field_number
      dpx:television.frame_rate
      dpx:television.gamma
      dpx:television.integration_times
      dpx:television.interlace
      dpx:television.padding
      dpx:television.time.code
      dpx:television.time_offset
      dpx:television.user.bits
      dpx:television.vertical_sample_rate
      dpx:television.video_signal
      dpx:television.white_level
      dpx:user.id

To determine which properties are associated with your DPX image, use this command for example:

      identify -verbose bluebells.dpx

To identify a particular property, try this:

      identify -format "%[dpx:television.time.code]" bluebells.dpx

Finally, to set a property:

      convert bluebells.dpx -define dpx:television.time.code=10:00:02:15 bluebells-001.dpx

## DPX Settings

Use -set to specify the image gamma or black and white points (e.g. `-set gamma 1.7`, `-set reference-black 95`, `-set reference-white 685`).

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