This invention relates, generally, to the field of digital film recorders, and more particularly to an improved digital film recorder which may provide increased recording speed while minimizing the introduction of visible and undesirable artifacts associated with the recording of a quantized image.
In the fields of computers and electronics, a series of sophisticated tools have been developed which permit individuals to display and manipulate images on a personal computer or work station. In particular, a number of image scanners and other input devices have been developed which allow full-color, photographic-quality images to be incorporated into computer applications.
Along with such image scanners, new software packages have also been developed with which a user may generate, on a computer, full color images consisting of charts, graphics, and the like, which may then be incorporated into printed documents, viewed on a video display, or projected as a form of electronic slide show.
Improvements have also been made in the performance of color display devices. These new display systems have the capability to render electronic images recorded at resolutions of 1024.times.768 pixels or higher. By using software which can develop and process images in a 24-bit color space, these input and display systems allow users to work with images made up of over 16.7 million colors.
While much use may be made of such high resolution and high color density scanned and computer-generated images in association with high quality display devices, it is often desirable to transform such screen images into hard copy representations which may then be used or manipulated further using traditional photographic means. Positive prints and slides are two such examples of the kinds of hard copy output which are often desirable in commercial, professional and engineering environments. Therefore, to transfer and record the high quality images generated on a computer and displayed on its screen to a more permanent and fixed form, film recorders have been developed.
Early efforts at film recorders were often simple variations on the theme of pointing a camera at a cathode ray tube or other suitable display device and then taking a picture. However, film recorders have since been developed which are capable of producing a more accurate end product by assembling an image on photographic film in a digital manner. Such digital image manipulation and recording techniques provide more precise and repeatable control to the user and offer a system which can produce a more accurate and pleasing finished photographic product.
An example of one such digital film recorder is a device described in U.S. Pat. No. 4,855,940 issued to Thomas L. Richardson, et al. on Aug. 8, 1989 and assigned to the Polaroid Corporation of Cambridge, Mass.; the teachings of which are incorporated herein by reference. In the '940 patent, Richardson describes a method and apparatus for defining and photographing computer images using a photographic exposure adaptor and providing an interface between a computer having a CPU and a film printer. Unfortunately, the apparatus disclosed by Richardson suffers from a number of limitations which reduce its overall usefulness. These limitations and their solutions are the focus of the present invention.
In particular, as discussed in Richardson, a digital film recorder may produce an exposure on a plate of light-sensitive film of an image to be recorded by repeatedly scanning each line of the image, which is displayed on a cathode ray tube or other image-producing device. This repetitive scanning is accomplished with a beam which is modulated in a binary manner such that the cumulative exposure of the light sensitive film by the beam produces a gray scale image. More particularly, given a known film speed, in order to produce an image which is capable of resolving 256 gray levels, at least 256 scans must be performed, with at least one scan for each possible gray level. Assuming that each scan will take approximately 50 milliseconds to expose a line of linear film, the complete recording of a 640.times.480 pixel image made up of 256 grey levels will take almost half a minute. Using a similar scan rate, the same recording at a resolution of 2048.times.1366 pixels would take over a minute.
Extrapolating further to a color system, then, we find that if the recording of a color image having a color resolution of 8 bits per primary color is desired, at least 256 scans must be performed for each red, blue and green color component yielding a minimum of 768 scans in order to produce a fully exposed 24-bit color image. It is noted that although 24-bit color is not the same as continuous analog color, it is, in fact, accurate enough for all but the most critical work.
Therefore, based on the above assumptions, at an output resolution of 2048.times.1366 pixels, a recording will require over four minutes, which is far too long for high volume output. In actual practice more than 256 per color may be required, since most film, color or monochromatic, has non-linearities in the properties of each recording layer. In addition, to match the typically non-linear input space of the input image, additional corrections, such as "gamma corrections" will also often need to be made. There will, therefore, be the need to compensate for these non-linearities in the exposure process by exposing some levels more than once in order to get 256 levels of nearly linear output.
Since the total scanning time is a function of (1) the number of exposure levels multiplied by (2) the number of lines scanned divided by (3) the number of lines scanned per second, one way to speed up the scanning process is to reduce the number of exposure levels which are recorded by the film recorder. However, while reducing the number of exposure levels from 256 per color component (or 256 in total for a monochrome image) to a smaller number will result in a faster overall recording time, quantizing from an input of 256 levels, which requires eight bits of information, to an output of fewer levels will result in an increased quantization error, and the increased production of visual artifacts. In addition, since in practice more scans must be performed for each output level in order to properly expose the photographic film as well as to compensate for various non-linear qualities, such as the film or the image input space, the use of fewer scans will result in a photographic film which receives less total exposure and is not properly compensated for its non-linearities.
Therefore, it has been determined that the need exists for an improved method and apparatus for digital film recording which overcomes the above noted limitations and which provides for decreased scan times by reducing the number of scans required for each color component, as well as reducing the required scan rate, while significantly reducing the problems associated with image quantization and maintaining sufficient image illumination.