Abstract:
A printer contains a list of objects to be printed under control of a controller. Different resolutions for the objects are included in the list of objects to be printed. In one embodiment, each item has an associated resolution indicating a minimum resolution for printing the item, unless the item before it has the same resolution. In such a case, the previous resolution is used to render ensuing objects for printing until an object requires a different resolution. A rendering module renders the object for printing as a function of the listed resolutions and resolutions supported by the printer. If the desired resolution is lower than that supported by the printer, the lowest resolution of the printer is used to render the object.

Description:
FIELD OF THE INVENTION 
     The present invention relates to printing, and in particular to identifying the resolution of objects in an object list to be printed. 
     BACKGROUND OF THE INVENTION 
     Many printers, such as laser printers and ink jet printers, commonly receive data in page description language form before starting the printing process. With a page description language (PDL), a page is represented using graphics, text, and images that can be specified to be located anywhere on the page, and in any order, using successive commands of the language. The PDL commands reflect the way the page is composed. However, before printing, the received data must be rasterized so a page print mechanism (“print engine”) in the printer can print the page while moving the print media (e.g, paper) uniformly in one direction, usually top to bottom. 
     Rasterization is the process of converting data that represent a page or portion of a page into a series of individual dots or pixels across the printed page to form a raster scanline, producing successive scanlines one after another down the page. The complete set of scanlines for a page is the raster data for the page. Rasterization is performed in the page printer by a rasterizer or other processor. 
     PDL commands for the page are converted to an intermediate representation called an “object display list” prior to rasterization. An object display list is a sequence of objects to be printed along with various print commands. The object display list is a compact representation of the page to be printed. 
     In many cases, a page to be printed is divided into several strips, such as horizontal strips of data. An object display list is created for each strip, which can be separately rasterized. By so dividing the page, the processing and memory requirements to provide rasterization are reduced. All the information for a particular band is contained in the display list commands for that band. 
     Strips contain many different types of objects to be printed, such as fonts, vectors, bit maps and others. Each strip is rendered and printed by a print engine. Rendering is normally performed to a resolution that is selected by a user. A user can enter the resolution for a page on a front panel of a printer. Higher resolutions require larger quantities of data to be rendered, which can require more time, storage and disk bandwidth. If a resolution of 1200 dpi is selected, that resolution is used for printing regardless of what is on the page. If an object having a resolution of 150 dpi raster is to be printed, it is scaled up to 1200 dpi and rendered. It is desired to minimize the amount of data that needs to be rendered. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a controller for a printer according to an embodiment of the present invention. 
         FIG. 2  is a block diagram of a object display list incorporating resolution opcodes according to an embodiment of the present invention. 
         FIG. 3  is a flow chart showing functions implemented by the controller utilizing the resolution opcodes in the object display list according to an embodiment of the present invention. 
         FIG. 4  is a flow chart showing functions implemented by a renderer for converting an object display list to raster data for directly printing by a print engine according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description and drawings illustrate specific embodiments of the invention sufficiently to enable those skilled in the art to practice it. Other embodiments may incorporate structural, logical, electrical, process, and other changes. Examples merely typify possible variations. Individual components and functions are optional unless explicitly required, and the sequence of operations may vary. Portions and features of some embodiments may be included in or substituted for those of others. The scope of the invention encompasses the full ambit of the claims and all available equivalents. The following description is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims. 
     The functions described herein are implemented in software in one embodiment, where the software comprises computer executable instructions stored on computer readable media such as memory or other type of storage devices. Further, such functions correspond to modules, which are software, hardware, firmware of any combination thereof. Multiple functions are performed in one or more modules as desired, and the embodiments described are merely examples. 
     A control system for a printer is shown at  100  in  FIG. 1 . Control system  100  comprises a processor  110  coupled to a memory  120  and a program memory  130 . In some embodiments, the program memory  130  is a read only memory, while the memory  120  is a random access memory that can be both read from and written to. Many different types of memories may be utilized for both memories, and the embodiments described are not intended to be limiting. 
     Memory  120  is the working memory of the control system, containing information to be printed. In one embodiment, memory  120  comprises a memory area for storing object display lists  135 . The memory may also store further information, such as PDL files, or other information as desired. Program memory  130  contains modules of computer instructions comprising a controller  145 , and a renderer  150 . Controller  145  and renderer  150  instructions are executed on processor  110  to perform functions of creating and managing object display lists, and rendering them to a raster form for printing as controlled by printer engine  125 . In one embodiment, all elements of the controller  100  are integrated into a printer, such as on a circuit board within the printer. In further embodiments, the elements may be located in a separate computer system coupled to the printer. Further, the object display list  135  may be created from a received PDL by processor  110 , or may be received directly from a separate computer system. 
       FIG. 2  shows a block diagram of a page consisting of multiple strips each having an object display list at  210 . Object display lists are used to identify objects to be printed. Many different types of printers, such as ink jet and laser printers utilize object display lists. In some printers, an object display list is a list of objects in a strip of a page as indicated by strip 1  at  215 , strip 2  at  220  through stripN at  230 . In one embodiment, the strip is 128 bits high, and there are approximately 50 strips per page. Pages are broken into strips to make it easier to store and print the page, and minimize requirements for printer resources. 
     Further detail of strip 1   215  is shown in an example expanded object display list  232  starting with a resolution opcode at  235 . The value of opcode  235  is 600 in this example, and corresponds to a desired resolution of 600 dots per inch (dpi) for a following object  240 . At  245 , another resolution opcode  245  is set to 1200. Both ensuing objects  246  and  247  in the object display list  232  are thus associated with a resolution of 1200 dpi. A further resolution opcode  248  follows object  247 , and sets the desired resolution to 300 for following objects. If there are no further resolution opcodes prior to the last object in the list, object  249 , all the objects ensuing from opcode  248  will have an associated resolution of 300 dpi. The expanded object display list for stripN is shown in block form at  250 . 
     In one embodiment, the object display lists are provided to a rasterizer  255  that corresponds to the rendering module  150 . The rasterizer renders each object display list to convert them to raster scans. Raster scans identify individual pixels in the strip, and are sent to a print engine  260  for printing on printer  270 , or stored for later printing, possibly with compression. 
       FIG. 3  is a flowchart of functions to be performed to generate an object display list having different resolutions. As mentioned above, the functions need not all be integrated into a printer. In fact, generation of the object display list is performed in a separate system and provided to the printer in a further embodiment. 
     A page to be printed is received at  310 . The page may be in the form of a printer definition language, a bit map image, or other form of page to be printed. The received page is then converted to objects that span the page at  315 . The objects are then broken based on strip boundaries at  320 . At  330 , an object display list is created for each strip until the page is complete. Resolution Opcodes are then inserted for objects in each object display list at  340 . In further embodiments, the strips are generated separately from the printer controller to minimize the resources required. Opcodes are positioned such that they can be obtained prior to rendering of the object. 
     As the resolution for each object is identified, if the resolution is different from the resolution for the preceding object, the new resolution opcode is inserted at  340 , and is associated with the object. If the resolution for the object is the same as the resolution for the preceding object, then no new opcode is inserted. 
     In further embodiments, objects, such as font objects are created with a resolution already identified. Thus, when creating the object display list, resolutions are automatically available with the object. In still further embodiments, the object display list already exists, and resolutions are added to the list dependent on the types of objects in the list. 
       FIG. 4  illustrates the rendering process for converting an object display list to raster data for directly printing by a print engine. At  410 , an object display list for a strip to be printed is received. The object display list is searched for resolution Opcodes at  415 . At  420 , the resolution for the strip is set to the highest found resolution. In one embodiment the highest resolution for a strip is stored and updated as objects are inserted such that  415  and  420  do not require parsing the object display list. If the resolution is less than the minimum printer resolution, the resolution is set to the minimum printer resolution at  425 . If the resolution is greater than the maximum printer resolution, then the resolution is set to the maximum printer resolution at  425 . 
     At  440 , a check is made for more strips on the page to be printed. If there are more strips, the object display list for the next strip is obtained at  450 , and the process is repeated for that list, starting at  415 . If there were no more strips on the page, rasterization at the resulting resolution for the strip is performed at  460  for direct printing at  470 . In some embodiments, rasterization and printing at  460  and  470  may occur in parallel with the resolution determinations for succeeding strips. 
     In one embodiment, printers supporting high resolutions, such as 2400 dpi recognize that some objects such as fonts do not need to be rendered at such a high resolution, but that a lower resolution such as 1200 dpi is sufficient. For even lower end printers, 600 dpi is likely sufficient, particularly if the renderer uses a very good scaling algorithm for fonts. In this case, a page with fonts may or may not drive up the resolution depending on the quality decided by the printer. 
     Most vector objects would work best at higher resolutions, and depending on halftoning methods used, the full resolution of the printer may or may not be desired. For raster objects, 600 dpi is generally considered enough if 8-bit data is preserved, while 1200 dpi or 2400 dpi is desired if the data is dithered to 1-bit. 
     When the strip is rasterized, the resolution that is used is the highest or largest resolution present on the object display list for the strip. In one embodiment the resolution opcodes have been limited to those resolutions allowable by the print engine, while in another embodiment any resolution is allowed and the limitation is made after the object display list is closed but before rasterization begins. The resolution chosen ensures that no object is rasterized at a lower resolution than is required for its chosen quality as well as being as small as possible such that rasterization can be performed quickly. In some embodiments the print engine cannot switch between multiple resolutions on a per-strip basis. This can be solved in one embodiment by inserting hardware scaling after the rasterization but before the print engine. In another embodiment all the strips are examined and the greatest resolution present is that chosen for all strips. 
     The rendered strip may be provided to the print engine and printed while other objects are still being rendered in some embodiments. In further embodiments, only full strips are provided, while in still further embodiments, an entire page is provided to the print engine. In still further embodiments, a resolution for the entire page is selected based on the required resolutions for all the strips on the page. For printers capable of varying resolution during a strip, the resolutions may be based on resolution opcodes for individual objects in the object display list. 
     Providing for varying resolutions in an object display list allows the smallest resolution required to be used for each strip. When lower resolutions are utilized, less time is spent rendering, and the print engine may operate faster than if all of a page were printed at a higher resolution.