Digital Cinema Resolution – Current Situation and Future requirements

Entertainment Technology Consultants Digital Cinema Resolution – Current Situation and Future requirements By: Matt Cowan Report Objectives This report is intended to discuss the resolution and related issues that affect image quality for digital cinema applications, to relate it to current film resolution characteristics and current and potential future digital offerings. Introduction Resolution is often discussed in the context of digital cinema and digital film processing. The visual attributes described as resolution are often confused with other image attributes such as sharpness and detail. Resolution numbers are often quoted for original negative capability, but seldom related to distribution prints. In practice, the resolutions used for digital post production for films are lower than original negative film is capable of. As a result, a discussion on the required resolution for digital cinema is not straightforward, but must consider a number of perhaps conflicting and not fully defined parameters in coming to the answer. Seeing is believing, and it is more than just resolution numbers that will determine how filmic the image appears. Factors such as color management and dynamic range, which are important contributors to filmic image presentation are not considered in this report. Resolution, Sharpness, and Detail In general usage, the meanings of resolution, sharpness, and detail are often interchanged. Modulation Transfer Function (MTF) another technical term that relates to the same issues. !∀ Resolution is a technical term, relating to the ability to image fine spatial frequencies through an optical system. When used as a figure of merit for an optical system, it refers to the results of measurements using test patterns and usually measuring the limiting resolution (often specified as 5% modulation) !∀ Sharpness is a descriptive term that relates to the amount of picture information available to the viewer. Sharpness is a combination of not only limiting resolution, but how much contrast is available at various resolutions below limiting resolution. Sharpness measures have been devised which relate to the area under the MTF curve. !∀ Detail is a term with similar meaning to sharpness, and us used by viewers to describe relatively fine information in the image. Details as we see them do not usually occur on the scale of a pixel, but rather on a larger scale of several pixels, and as such are coarser than the threshold of resolution for the visual system. A sharp picture is often more detailed. ©2002 Entertainment Technology Consultants Modulation Transfer Function (MTF) MTF is used to give a complete description of the contrast vs. spatial performance of an optical system. It is usually a 2 dimensional plot that shows the degradation in contrast ratio (or modulation) against increasing spatial frequencies (or higher resolution). A perceived image sharpness metric has been devised which is roughly proportional to the square of the area under the curve. Therefore a lower resolution image with higher modulation in the mid spatial frequencies may appear as “sharper” than a high resolution image with substantial roll off in modulation at higher frequencies. MTF - Release Print from Negative - I/P - I/N 1 0.8 0.6 Film 0.4 Limiting Resolution 0.2 0 0 10 20 30 40 50 60 Resolution - lp/mm on film Plot of “typical” MTF of release print. Data is from 1996 from a Quantel publication, and references film stocks prior to the current Vision family The MTF curve plot gives the limiting resolution, and the shape of the curve at lower resolutions indicates the sharpness to be expected in the image – where a higher curve indicates a sharper image. Sharpness and Film With the capabilities inherent in digital manipulation of images, it is possible to expand the amplitude of mid spatial resolutions, and provide an image with a very high degree of sharpness. If applied, this needs to be done very lightly, otherwise, the end result would not be pleasing. Film is a “soft” medium, and as such excessive sharpness will create an edgy texture to the picture, and in many cases, make it look like television. The management of sharpness is a critical issue to the look and feel of the image and must in practice be carefully used. Resolution Units From the point of view of the Human Visual System (HVS), resolution is measured in line pairs per degree of arc. This is a relative unit, and obviously relevant to the HVS’s mechanisms for managing resolution and contrast. The measurement is viewer centric. In the physical world, resolution is measured as spatial resolution in line pairs per millimetre. This is an absolute measurement, and must be converted to relative terms to determine the relationship to the HVS capabilities. It is the unit used for film and optical elements. It relates to Issues in Resolution 2 ©2002 Entertainment Technology Consultants the dimensions of the actual film stock being measured. To convert from line pairs per millimeter to lines across the film, multiply by the dimension of the film for line pairs per picture height, and further multiply by 2 to convert line pairs to lines. (A line pair or cycle is 2 lines on the resolution chart – one black and one white.) The display industry (from television) has used the measure “lines per picture height” or “TV lines per picture height”. This unit relates everything to the screen, and viewing distances are defined in screen heights to determine relative resolution. Defining geometry in screen height terms allows relatively easy conversion to line pairs per degree for reference to the human visual system. Human Visual System Issues – What can the human process? 1. Threshold of visual acuity: This is a term given to the limit of the human visual system to resolve high contrast test patterns. It is often used as a single number to define this limit. For a “normal” viewer, the limit is 30 line pairs per degree. (One line pair is a black and a white resolution element, or effectively 2 pixels) Some researchers place it as high as 45 lp/degree – we’ll use 30 lp/degree for this report. USAF 1951 resolution test target, showing diminishing line spacing on groupings. This is used to visually ascertain resolution in optical systems or visual systems. This type of target can be used to determine the threshold of visual acuity, by setting up a known viewing geometry, and converting the lp/mm from the chart to lp/degree for HVS application 2. Anisotripic Resolution: Spatial resolution on the current digital projectors is different in horizontal and vertical directions due to the anamorphic projection lens. (Cinemascope 2.35:1 is also anamorphically projected) To some, this seems a waste, because the HVS is itself isotropic – it has the same resolution limits in vertical and horizontal directions. In fact, the HVS will perceive substantial benefit in image sharpness and resolution by the extra pixels in the vertical direction. 3. Vernier Acuity: This is the ability of the human visual system to notice that a line is not continuous, but has a slight offset. The limit of vernier acuity is about 10 times that of Issues in Resolution 3 ©2002 Entertainment Technology Consultants normal acutity – or about 300 lp/degree. This attribute allows our visual system to be very precise about aligning images. 4. Contrast Sensitivity Function: The resolution limits of the HVS are related to black and white test patterns. In normal viewing situations, we do not live in a world of black and white patterns, but lower contrast. If we devise a test where we reduced the contrast of our test pattern, we will see that the eye has different resolution limits for different levels of contrast. The following pattern demonstrates this. Spatial Frequency Contrast Sensitivity Function pattern showing that visual sensitivity to contrast is highest at lower resolutions than the threshold of acuity. This is noted by the envelope of modulation that looks (sort of) like a Gaussian curve. If the pattern is observed from close up, the Gaussian shape moves right, if viewed from further away, it moves left. The peak on this curve represents the spatial frequency where contrast discrimination is most sensitive. (View from at least 5’ away on a 17” monitor with image at 150%) Contrast Sensitivity Function provides useful information on the human visual system. It shows that the HVS is most sensitive to information that appears at 4 to 9 lp/degree, or 1/7th to 1/3 of the threshold of visual acuity. This means that the important attributes of the display system are largely governed by performance in the spatial frequencies in this area. These drive the perception of sharpness, and represent the reason that a (relatively) small number of pixels making up an image does a surprisingly good job, if the pixel system maintains good modulation at the mid resolution. Relationship of Pixels on screen and Visual Acuity If resolution is defined in lines per picture height (or line pairs per picture height), then it is useful to consider the viewing geometry in terms of seating distance from the screen in units of picture height. The following table calculates the visual resolution in terms of line pairs per degree, for comparison to the HVS threshold of acuity. Issues in Resolution 4 ©2002 Entertainment Technology Consultants Table of pixel resolution against screen heights for a flat (1.85:1) presentation. (note 2 pixels = 1 line pair) Viewing distance Pixel resolution Line pairs/degree for pixel resolution (screen heights) 1280 x 1024 2048 x 1024 4096 x 2048 (1.5x anamorph. lens) (no anamorph) (no anamorph) H V H V H V 1 6.04 8.94 8.94 8.94 17.87 17.87 2 12.08 17.87 17.87 17.87 35.74 35.74 3 18.11 26.81 26.81 26.81 53.62 53.62 4 24.15 35.74 35.74 35.74 71.49 71.49 Notes: 1. Grey cells in the table indicate situations where pixels resolution is less than the limit of visual acuity. Limit of visual acuity is about 30 lp/degree. 2. Normal theatre places the front row about 1 picture height back from the screen. 3. “Best viewing” is normally 2 to 2.5 picture heights back from screen. 4. Anamorphic lens provides anisotripic pixel resolution in 1280 x 1024 mode – i.e. higher resolution vertically than horizontally 5. Perceived resolution in an anisotropic situation is approximately the average of horizontal and vertical resolutions. From this table, we can see that even a 4K projector does not satisfy the HVS requirement for simple limiting resolution until the viewer is almost 2 screen heights away from the screen. Pixelization and Sampling Theory A very strong factor in digital vs. film displays is that the resolution elements on a digital projector are rigidly aligned, and every frame maintains this same structure. This structure becomes effectively a fixed pattern which can interfere with the actual image content, by providing a highly digital structure on the image. Film on the other hand has resolution elements (grains) which are randomly scattered across the film frame, and each frame has a different randomness. This eliminates any fixed structure to the image, and provides a different look/feel from digital. Film makers will enhance the moving grain structure to develop a gritty look to the image for artistic reasons (e.g. Ocean’s 11). Theory would say that the digital display must have a finer structure than the film structure if it is to emulate the look/feel of the random grains. Nyquist sampling theory would recommend at least 2 pixels per resolution element, and Kodak’s publications go further and recommend 3 to 5 pixels per resolution element. Resolution of film The controversy rages over the ultimate resolution of film. Datasheets, tests and popular belief and actual practice don’t always agree. Popular belief is that film is a “4K” medium. The current Vision family of film stocks, for example Kodak Vision 200T color negative film - 5274, is specified by Kodak to exceed 4000 lines of resolution (horizontally across the frame), and independent tests (ITU tests) have confirmed that this resolution is captured on the negative. As the film is duplicated through the generations of inter-negative, inter-positive, and release print, this resolution suffers from generation loss, through cascading MTF of each film generation, and through inaccuracies in the method of printing from one generation to the next. Issues in Resolution 5 ©2002 Entertainment Technology Consultants ... - tailieumienphi.vn