(en)HDR images are coded and distributed. An initial HDR image is received. Processing the received HDR image creates a JPEG-2000 DCI-compliant coded baseline image and an HDR-enhancement image. The coded baseline image has one or more color components, each of which provide enhancement information that allows reconstruction of an instance of the initial HDR image using the baseline image and the HDR-enhancement images. A data packet is computed, which has a first and a second data set. The first data set relates to the baseline image color components, each of which has an application marker that relates to the HDR-enhancement images. The second data set relates to the HDR-enhancement image. The data packets are sent in a DCI-compliant bit stream.

1.ApplicationNumber: US-201213468674-A
1.PublishNumber: US-8891863-B2
2.Date Publish: 20141118
3.Inventor: NINAN AJIT
HULYALKAR SAMIR

4.Inventor Harmonized: NINAN AJIT(US)
HULYALKAR SAMIR(US)

5.Country: US
6.Claims:
(en)HDR images are coded and distributed. An initial HDR image is received. Processing the received HDR image creates a JPEG-2000 DCI-compliant coded baseline image and an HDR-enhancement image. The coded baseline image has one or more color components, each of which provide enhancement information that allows reconstruction of an instance of the initial HDR image using the baseline image and the HDR-enhancement images. A data packet is computed, which has a first and a second data set. The first data set relates to the baseline image color components, each of which has an application marker that relates to the HDR-enhancement images. The second data set relates to the HDR-enhancement image. The data packets are sent in a DCI-compliant bit stream.

7.Description:
(en)CROSS REFERENCE TO RELATED APPLICATIONS
This Application claims the benefit of priority to related, Provisional U.S. Patent Application No. 61/496,445 filed on 13 Jun. 2011 entitled “High Dynamic Range, Backwards-Compatible, Digital Cinema” by Ajit Ninan, et al. hereby incorporated by reference in its entirety.


TECHNOLOGY
The present invention relates generally to digital cinema. More particularly, an embodiment of the present invention relates to the coding and transmission of high dynamic range (HDR) images in a format that is backwards compatible with existing digital cinema specifications.
BACKGROUND
As used herein, the term ‘dynamic range’ (DR) may relate to a capability of the human psychovisual system (HVS) to perceive a range of intensity (e.g., luminance, luma) in an image, e.g., from darkest darks to brightest brights. In this sense, DR relates to a ‘scene-referred’ intensity. DR may also relate to the ability of a display device to adequately or approximately render an intensity range of a particular breadth. In this sense, DR relates to a ‘display-referred’ intensity. Unless a particular sense is explicitly specified to have particular significance at any point in the description herein, it should be inferred that the term may be used in either sense, e.g. interchangeably.
As used herein, the term high dynamic range (HDR) relates to a DR breadth that spans the some 14-15 orders of magnitude of the human visual system (HVS). For example, well adapted humans with essentially normal (e.g., in one or more of a statistical, biometric or opthamological sense) have an intensity range that spans about 15 orders of magnitude. Adapted humans may perceive dim light sources of as few as a mere handful of photons. Yet, these same humans may perceive the near painfully brilliant intensity of the noonday sun in desert, sea or snow (or even glance into the sun, however briefly to prevent damage). This span though is available to ‘adapted’ humans, e.g., those whose HVS has a time period in which to reset and adjust.
In contrast, the DR over which a human may simultaneously perceive an extensive breadth in intensity range may be somewhat truncated, in relation to HDR. As used herein, the terms ‘visual dynamic range’ or ‘variable dynamic range’ (VDR) may individually or interchangeably relate to the DR that is simultaneously perceivable by a HVS. As used herein, VDR may relate to a DR that spans 5-6 orders of magnitude. Thus while perhaps somewhat narrower in relation to true scene referred HDR, VDR nonetheless represents a wide DR breadth. As used herein, the term ‘simultaneous dynamic range’ may relate to VDR.
Until fairly recently, displays have had a significantly narrower DR than HDR or VDR. Television (TV) and computer monitor apparatus that use typical cathode ray tube (CRT), liquid crystal display (LCD) with constant fluorescent white back lighting or plasma screen technology may be constrained in their DR rendering capability to approximately three orders of magnitude. Such conventional displays thus typify a low dynamic range (LDR), also referred to as a standard dynamic range (SDR), in relation to VDR and HDR.
Advances in their underlying technology however allow more modern display designs to render image and video content with significant improvements in various quality characteristics over the same content, as rendered on less modern displays. For example, more modern display devices may be capable of rendering high definition (HD) content and/or content that may be scaled according to various display capabilities such as an image scaler. Moreover, some more modern displays are capable of rendering content with a DR that is higher than the SDR of conventional displays.
For example, some modern LCD displays have a backlight unit (BLU) that comprises a light emitting diode (LED) array. The LEDs of the BLU array may be modulated separately from modulation of the polarization states of the active LCD elements. This dual modulation approach is extensible (e.g., to N-modulation layers wherein N comprises an integer greater than two), such as with controllable intervening layers between the BLU array and the LCD screen elements. Their LED array based BLUs and dual (or N-) modulation effectively increases the display referred DR of LCD monitors that have such features.
Such “HDR displays” as they are often called (although actually, their capabilities may more closely approximate the range of VDR) and the DR extension of which they are capable, in relation to conventional SDR displays represent a significant advance in the ability to display images, video content and other visual information. The color gamut that such an HDR display may render may also significantly exceed the color gamut of more conventional displays, even to the point of capably rendering a wide color gamut (WCG). Scene related HDR or VDR and WCG image content, such as may be generated by “next generation” movie and TV cameras, may now be more faithfully and effectively displayed with the “HDR” displays (hereinafter referred to as ‘HDR displays’).
As with the scalable video coding and HDTV technologies, extending image DR typically involves a bifurcate approach. For example, scene referred HDR content that is captured with a modern HDR capable camera may be used to generate an SDR or a VDR version of the content, which may be displayed on VDR displays or conventional SDR displays. In one approach, as described in U.S. provisional application 61/476,174 “Improved encoding, decoding, and representing high dynamic range images,” by W. Jia et al., herein incorporated by reference for all purposes, the SDR version is generated from the captured VDR version by applying a tone mapping operator (TMO) to intensity (e.g., luminance, luma) related pixel values in the HDR content. The Jia application also describes how an HDR image can be represented by a baseline SDR image, a ratio image, and a chroma residual.
The “Digital Cinema Systems Specification,” Version 1.2, Mar. 7, 2008, by Digital Cinema Initiatives, LLC, is referred to herein as the DCI System Specification, or simply as DCI, defines the technical specifications and requirements for the mastering, coding, and distribution of digital cinema content. DCI supports video streams at two resolutions: 2K (2048×1080) and 4K (4096×2160). Furthermore, DCI images are coded as 12-bit, X′Y′Z′ color channels, using the JPEG 2000 coding standard. This representation is inadequate to fully represent HDR content. Embodiments of this invention describe methods for the mastering, coding, and distribution of HDR content in formats that are backwards compatible with the existing DCI specification. As used herein, “JPEG-2000” refers to the JPEG-2000 compressor/decompressor (codec) standard of the Joint Motion Picture Experts Group (JPEG). As used herein, ‘TIFF’ refers to a Tagged Image File Format.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section. Similarly, issues identified with respect to one or more approaches should not assume to have been recognized in any prior art on the basis of this section, unless otherwise indicated.



BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the present invention is illustrated by way of example, and not in way by limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:
FIG. 1 depicts SDR digital cinema system work flow according to the DCI specification;
FIG. 2 depicts an SDR digital cinema codestream structure according to the DCI specification;
FIG. 3 depicts an example layered HDR encoding system according to an embodiment of the present invention;
FIG. 4 depicts an example HDR DCI codestream structure according to one embodiment of the present invention;
FIG. 5 depicts an example, backwards compatible, HDR DCI decoding system according to an embodiment of the present invention.
FIG. 6 depicts an example process flow to generate an HDR image according to an embodiment.



DESCRIPTION OF EXAMPLE EMBODIMENTS
Embodiments describe the coding and transmission of high dynamic range (HDR) images in a format that is backwards compatible with existing digital cinema specifications, such as those promulgated by Digital Cinema Initiatives (DCI). In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are not described in exhaustive detail, in order to avoid unnecessarily occluding, obscuring, or obfuscating the present invention.
Overview
An embodiment of the present invention codes and distributes high dynamic range (HDR) images. An initial HDR image is received. Processing the received HDR image creates a coded baseline image, which is compliant with the Digital Cinema Systems Specification of the Digital Cinema Initiative (DCI Systems Standard), and one or more HDR-enhancement images. The coded baseline image has one or more color components. The coded baseline image and the one or more HDR-enhancement images each provide enhancement information. The enhancement information allows reconstruction of an instance of the received HDR image using the baseline image and the HDR-enhancement images. A data packet is computed. The data packet has a first data set and a second data set, separated by an application marker. The first data set relates to the one or more baseline image color components, each of which has an application marker that relates to the HDR-enhancement images. The second data set is formed in relation to one of the HDR-enhancement images. The data packets are sent in a bit stream that conforms to the DCI Systems Standard. Upon receipt of the data packets, a DCI-compliant decoder may process the packets to reconstruct an instance of the initial HDR image, according to the information from the baseline image and the HDR-enhancement images.
Thus, an embodiment represents an original HDR image as a JPEG-2000 baseline image with components that are added to enhance the baseline image. The additional enhancement information components include a luminance ratio image and one or more optional chroma residual images. DCI-compliant decoders may all decode the baseline image. Decoders that have the capability to decode HDR images may use the baseline image, along with the added enhancement components to decode the full HDR image, e.g., an instance of the original HDR image. An embodiment multiplexes the base layer and the HDR-enhancement component data into a single, backwards compatible, DCI-compliant stream.
DCI System Specification
FIG. 1 depicts a simplified diagram of a digital cinema system workflow according to the current DCI systems specification. DCI specifies the structure of a Digital Cinema Distribution Master (DCDM) which is used to exchange image, audio, and metadata. Once a DCDM is compressed, encrypted and packaged for distribution, then it is considered a Digital Cinema Package (DCP) ( 115 ). DCDMs use a hierarchical image structure that supports both 2K ( 105 ) and 4K ( 110 ) resolution master files. A studio can choose to deliver either a 2K or a 4K master. Both 2K ( 120 ) and 4K ( 125 ) projectors can decode both 2K and 4K distributions. A 4K projector receiving a 2K file can optionally upscale it. A 2K projector receiving a 4K master extracts and uses the data for decoding the 2K output from the 4K distribution. Up-sampling 2K content or down-sampling 4K content is not a requirement for any of the DCI-compliant projectors.
Uncompressed DCDM files use a TIFF-like file format where image frames are coded using 12-bit unsigned integers in the X′Y′Z′ color space. For compression, DCI uses the JPEG 2000 standard (ISO/IEC 15444-1).
As depicted in FIG. 2 , each compressed frame of a DCI distribution consists of tiles. In a 2K distribution, each frame has exactly 3 tiles, each tile containing a Tile-part Header (TH) 210 and data from one color component ( 206 - 0 , 206 - 1 , and 206 - 2 ). In a 4K distribution, each frame has 6 tile parts. The first 3 tile parts can be used by a 2K decoder to reconstruct a 2K output. The remaining 3 tile parts ( 208 - 0 , 208 - 1 , and 208 - 2 ) contain additional data to decompress each of the 4K color components.
To accommodate HDR images in a DCI compliant milieu, an HDR DPCM master may be created. Both an SDR and an HDR instance of content, such as a movie, are thus produced, which essentially doubles the size of the files with which the content is stored and/or transmitted, and concomitantly increases the bandwidth consumed in such transmission. An embodiment of the present invention provides an HDR-capable DCI-compliant system that is backwards-compatible with conventional DCI systems, which thus obviates dual production, storage and transmission of both an SDR version and an HDR version of the same content.
Example Enhancements
FIG. 3 depicts an example HDR DCI-like encoding system 300 , according to an embodiment of the present invention. An HDR image or video sequence is captured, e.g., using HDR camera 305 . The HDR sequence may also be created or synthesized, such as with techniques that relate to computer generated images (CGI). Following capture, the captured image or video is processed by a mastering process to create a target HDR image 312 . The mastering process may incorporate a variety of processing steps, such as: editing, primary and secondary color correction, color transformation, and noise filtering. The HDR output 312 of this process represents the director's intent on how the captured image will be displayed on a target HDR projector or display.
The input HDR image may be in any color space that supports a high dynamic range color gamut. In an example embodiment, the input HDR image is an RGB image. In an example, each pixel in the input HDR image comprises floating-point pixel values for all channels (e.g., red, green, and blue color channels in the RGB color space) defined in the color space. In another example, each pixel in the input HDR image comprises fixed-point pixel values for all channels (e.g., 16 bits or higher/lower numbers of bits fixed-point pixel values for red, green, and blue color channels in the RGB color space) defined in the color space. Each pixel may optionally and/or alternatively comprise down-sampled pixel values for one or more of the channels in the color space.
The mastering process may also output a corresponding SDR image 317 , representing the director's intent on how the captured image will be displayed on a legacy SDR projector or display. The SDR output 317 may be provided directly from mastering circuit 310 or it may be generated by a separate HDR-to-SDR converter 315 , such as tone mapping operator (TMO). The input SDR image may also be in any color space that supports a standard dynamic range color gamut. In an example embodiment it may be in YCbCr or YUV color space.
In an example embodiment, the HDR 312 and SDR 317 signals are input into Divider 320 which produces a ratio image 322 . Ratio image 322 represents the pixel by pixel ratios of the HDR luminance component (say, Yh) over the SDR luminance component (say, Ys). Depending on the color space representation of both the HDR and SDR inputs, Divider 320 may also perform inverse gamma operations, color transformations, or other preprocessing operations (not shown).
Because of the large dynamic range of the pixels in the Ratio image 322 , a Log unit 330 transforms image 322 into a Log ratio image 332 , which subsequently may also be processed by a Ratio image encoder 340 , which outputs an encoded ratio image 342 . In an alternative implementation, the Log operation may be performed directly on the luminance (or luma) signals Yh and Ys, and division in 320 can be replaced by subtraction.
HDR signal 312 and the Ratio image 322 are also input into Divider 325 . If the HDR to SDR transformation maintained the original color balance and there is no color clipping, then a multiplication of the SDR image 317 with the Ratio image 322 should match the input HDR image color by color. However, in practice, there are differences in the chrominance (or chroma) channels between the HDR and SDR signals. Given the ratio image 322 , Divider 325 remaps the chroma channels of HDR signal 312 into an approximation ( 327 ) of the chroma channels of SDR input 317 . Thus, subtractor 355 , which computes the chroma difference between signals 327 and the SDR signal 317 , captures chroma information that might have been lost during the HDR to SDR process 315 . This chroma residual may be encoded and compressed further by Residual encoder 335 to yield coded residual 337 .
SDR signal 317 is also coded using a JPEG 2000 DCI encoder ( 345 ) to output the baseline, DCI-compliant, image 347 .
In an embodiment of the present invention, each frame in the original HDR input 312 is coded using three images: a baseline CDI-compliant image, a coded Ratio image, and two coded Chroma residual images. Note that according to another embodiment of this invention, if the values of the residual chroma images are too small, then they can be removed and not be included as part of the enhancement images.
FIG. 4 depicts how these three images are multiplexed and packetized into a DCI-compliant bitstream ( 350 ) according to an embodiment of the invention. JPEG 2000 coding syntax allows an encoder to insert application specific markers into the bitstream. A JPEG 2000 compliant decoder which does not recognize the marker, will simply ignore all data following it. However, a JPEG 2000 decoder which recognizes the marker can extract the additional information and process the data as needed. The use of such markers allows embodiments of this invention to construct a single, fully compliant, DCI stream, that supports both SDR and HDR playback.
Denote by APP SEQ ( 410 ) a JPEG 2000 marker that in a DCI stream may identify HDR-specific data. Given a 4K DCI bitstream, FIG. 4 depicts an example of how to embed HDR-enhancement information in to the 4K packets. Since each packet 208 corresponds to bitstream information for a specific color channel (X′, Y′, or Z′), one can apply a single HDR-enhancement application marker to all of the HDR-enhancement information. In one implementation, the coded ratio image data ( 415 - 0 ) may be embedded into the 4K — 0 packet ( 208 - 0 ), as part of the baseline X′ data ( 405 - 0 ), the coded residual for the first color component (CC) ( 415 - 1 ), say Cb, may be embedded into the 4K — 1 packet ( 208 - 1 ), as part of the baseline Y′ data ( 405 - 1 ), and the coded residual for the second color component ( 415 - 2 ), say Cr, may be embedded into the 4K — 2 packet ( 208 - 2 ), as part of the baseline Z′ data ( 405 - 2 ).
Under this scheme, a legacy DCI projector will ignore the HDR-enhancement data and will simply decode an SDR 4K or 2K bitstream. However, as shown in FIG. 5 , an HDR DCI decoder can extract all the HDR-enhancement information and reconstruct and display an HDR bitstream.
It should be appreciated that the example packet structure in FIG. 4 is independent on how the Ratio image and the Chroma residual images are coded inside the packets. For example, luma ratio and chroma residuals can be transformed and coded in a device-independent color space, as the baseline images. Furthermore, instead of using a single application marker, two or more separate and distinct markers may be used.
The packet structure shown in FIG. 4 is also applicable to other possible configurations. In an example implementation, the HDR-enhancement data may be multiplexed with the three 2K packets. In an example implementation, the HDR-enhancement data may be spread across both the 2K and 4K data packets.
FIG. 5 depicts an example implementation of a DCI compliant, backwards-compatible, decoding system according to the methods of this invention. DCI packets are demultiplexed and decoded according to the existing DCI specification. A legacy decoder will ignore application-specific HDR-enhancement markers and extract a baseline SDR stream 512 . However, an HDR DCI decoder will recognize the APP SEQ markers ( 515 ) and extract the additional, HDR-enhancement information, such as the coded Ratio image 518 and the coded chroma residuals 517 . Following decoding ( 520 and 530 ) which extracts a Log ratio image and converts it back to pixel data ( 532 ), the ratio image is multiplied by both the extracted SDR image ( 512 ) and the chroma residuals ( 525 ) to derive a first estimate of the HDR signal ( 542 ) and missing chroma information ( 537 ). These two signals are added together in adder 545 to create a reconstructed HDR signal 547 . The baseline SDR signal 512 and the HDR-enhancement information ( 517 and 518 ) may be in different color spaces and chroma formats. Thus, an embodiment performs or accommodates additional color correction, color transformation, and post-processing operations. Following the extraction of the HDR 547 and SDR 512 signals, a DCI decoder may also perform additional display-specific color management processing.
FIG. 6 illustrates an example process flow according to a possible embodiment of the present invention. In some possible embodiments, one or more computing devices or components such as an HDR image encoder may perform this process flow. The HDR image encoder may be implemented by adding one or more new processing blocks to and/or modifying one or more existing processing blocks in, a standard-based image encoder such as a JPEG image encoder. In block 602 , the HDR image encoder receives a high dynamic range (HDR) image. In an example embodiment, the HDR image is one of a fixed-point image or a floating-point image. In an example embodiment, the HDR image is encoded in one of JPEG, JPEG-2000, MPEG, AVI, TIFF, BMP, GIFF, or another image format.
In block 604 , the HDR image encoder also receives a tone-mapped (TM) image that was generated based on the HDR image. The TM image comprises one or more color alterations that are not recoverable from the TM image with a luminance ratio image. In an example embodiment, at least one of the one or more color alterations in the TM image is caused by one of clippings (e.g., in R, G, or B pixel values), or alterations of hues at one or more pixels.
In block 606 , the HDR image encoder computes luminance ratio values, on an individual pixel basis, by dividing luminance values of the HDR image with luminance values of the TM image on the individual pixel basis.
In block 608 , the HDR image encoder applies the luminance ratio values to the HDR image to create a re-mapped image.
In an example embodiment, the HDR image encoder converts at least one of the re-mapped image and the TM image from one color space to a different color space.
In block 610 , the HDR image encoder determines residual values in color channels of a color space based on the re-mapped image and the TM image. If the original color is altered, at least one of the residual values is non-zero. In an example embodiment, the color space is an YCbCr color space; the color channels of the color space comprise a Cb color channel and a Cr color channel. The residual values in the color channels of the color space are calculated as differences between first pixel values, as derived from the re-mapped image, in the color channels and second pixel values, as derived from the TM image, in the color channels.
In block 612 , the HDR image encoder outputs a version of the TM image with HDR reconstruction data. The HDR reconstruction data is derived from the luminance ratio values and the color-channel residual values.
In an example embodiment, the HDR reconstruction data comprises a residual image with quantized values derived from the luminance ratio values and the residual values in the color channels of the color space. The HDR reconstruction data may further comprise parameters specifying ranges of the quantized values.
In an example embodiment, the HDR reconstruction data is stored in an application segment of an image file with the TM image as a base image in the image file. In an example embodiment, the image file is in a JPEG-HDR format.
In an example embodiment, the HDR image encoder may perform one or more sanity checks on the HDR image, for example, before the HDR image is manipulated by a tone mapping operator (TMO) or a user. In an example embodiment, the HDR image encoder replaces zero, one, or more color-channel zero-values in the TM image with values smaller than a threshold value. This threshold value may be 1, 2, 3, . . . , 10, 11, etc. in various possible embodiments.
In an example embodiment, any tone mapping operations with any TMO and/or any color alterations on any number of pixels in the TM image may be performed in the process of generating the TM image.
In an example embodiment, the HDR image encoder applies a color space conversion to at least one of the HDR image, the TM image, or the re-mapped image.
In an example embodiment, luminance residual values between the TM image and the re-mapped image are all zeros. For example, in a color space (e.g., YUV) with a luminance channel (e.g., Y) and two color channels (e.g., Cb and Cr), differences in luminance values between the TM image and the re-mapped image (e.g., already, or alternatively after a color space conversion, in the color space) may be all zeros.
Example Computer System Implementation
Embodiments of the present invention may be implemented with a computer system, systems configured in electronic circuitry and components, an integrated circuit (IC) device such as a microcontroller, a field programmable gate array (FPGA), or another configurable or programmable logic device (PLD), a discrete time or digital signal processor (DSP), an application specific IC (ASIC), and/or apparatus that includes one or more of such systems, devices or components. The computer and/or IC may perform, control or execute instructions relating to HDR image coding and decoding, such as those described herein. The computer and/or IC may compute any of a variety of parameters or values that relate to HDR coding. The image and video dynamic range extension embodiments may be implemented in hardware, software, firmware and various combinations thereof.
Certain implementations of the invention comprise computer processors which execute software instructions which cause the processors to perform a method of the invention. For example, one or more processors in a display, an encoder, a set top box, a transcoder or the like may implement HDR DCI coding and decoding methods as described above by executing software instructions in a program memory accessible to the processors. The invention may also be provided in the form of a program product. The program product may comprise any medium which carries a set of computer-readable signals comprising instructions which, when executed by a data processor, cause the data processor to execute a method of the invention. Program products according to the invention may be in any of a wide variety of forms. The program product may comprise, for example, physical media such as magnetic data storage media including floppy diskettes, hard disk drives, optical data storage media including CD ROMs, DVDs, electronic data storage media including ROMs, flash RAM, or the like. The computer-readable signals on the program product may optionally be compressed or encrypted.
Where a component (e.g. a software module, processor, assembly, device, circuit, etc.) is referred to above, unless otherwise indicated, reference to that component (including a reference to a “means”) should be interpreted as including as equivalents of that component any component which performs the function of the described component (e.g., that is functionally equivalent), including components which are not structurally equivalent to the disclosed structure which performs the function in the illustrated example embodiments of the invention.
EQUIVALENTS, EXTENSIONS, ALTERNATIVES AND MISCELLANEOUS
Example embodiments that relate to applying HDR image coding in digital cinema are thus described. In the foregoing specification, embodiments of the present invention have been described with reference to numerous specific details that may vary from implementation to implementation. Thus, the sole and exclusive indicator of what is the invention, and is intended by the applicants to be the invention, is the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction. Any definitions expressly set forth herein for terms contained in such claims shall govern the meaning of such terms as used in the claims. Hence, no limitation, element, property, feature, advantage or attribute that is not expressly recited in a claim should limit the scope of such claim in any way. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.