![]() ![]() ![]() What is grayscale and monochrome? Monochrome printing only requires a small amount of black ink, while grayscale printing mixes black and color ink or toner to make up different shades of gray. Why do photographers shoot in black and white?īlack and white photography removes any distraction of color and helps the viewer focus on other aspects of the photo, such as the subject, the textures, shapes and patterns, and the composition. Si vous ne voyez pas la fenêtre des options de démarrage, appuyez sur le bouton d'alimentation et maintenez-le enfoncé pendant 10 secondes supplémentaires. Vous devriez voir la fenêtre des options de démarrage si vous maintenez le bouton d'alimentation enfoncé pendant 10 secondes. (9) have proposed a square-root nonlinearity where 0.0 < Y < 1.0 and 0.0 < A < 10.0.How do I restart my MacBook air when the screen is black? There have been several efforts to quantitatively model this nonlinear response by a lightness function A, which is related to incident luminance. The contouring of the luminance image part of the split image becomes noticeable for 5 bits.Īs discussed in Section 2.4, it has been postulated that the eye responds logarithmically or to a power law of incident light amplitude. Gray scale contouring in these pictures is apparent in the ramp part of the split image for 6 or fewer bits. In Figure 5.3-1, the luminance signal of the image has been uniformly quantized with from 8 to 256 levels (3 to 8 bits). The source image is a split image in which the left side is a luminance image and the right side is a computer-generated linear ramp. Finally, any camera or monitor noise tends to diminish the effects of contouring.įigure 5.3-1 contains photographs of an image linearly quantized with a variable number of quantization levels. Also, the photographic film that is often used to record the experimental results is highly nonlinear. As noted in Section 3.5.3, most television cameras and monitors exhibit a nonlinear response to light intensity. In most studies, however, there has been some question as to the linearity and calibration of the imaging system. ![]() Other investigators have reached similar conclusions. Goodall (5) performed some of the earliest experiments on digital television and concluded that 6 bits of intensity quantization (64 levels) were required for good quality and that 5 bits (32 levels) would suffice for a moderate amount of contouring. There have been many experimental studies to determine the number and placement of quantization levels required to minimize the effect of gray scale contouring (5-8). Consideration is limited here to linear quantization of companded pixel samples. A given nonlinear quantization scale can be realized by the companding operation of Figure 5.1-3, in which a nonlinear amplification weighting of the continuous signal to be quantized is performed, followed by linear quantization, followed by an inverse weighting of the quantized amplitude. The minimal number of quantization bits required for basic PCM coding to prevent gray scale contouring is dependent on a variety of factors, including the linearity of the image display and noise effects before and after the image digitizer.Īssuming that an image sensor produces an output pixel sample proportional to the image intensity, a question of concern then is: Should the image intensity itself, or some function of the image intensity, be quantized? Furthermore, should the quantization scale be linear or nonlinear? Linearity or nonlinearity of the quantization scale can be viewed as a matter of implementation. For a reduced number of quantization levels, the first noticeable artifact is a gray scale contouring caused by a jump in the reconstructed image brightness between quantization levels in a region where the original image is slowly changing in brightness. The eye is only capable of judging the absolute brightness of about 10 to 15 shades of gray, but it is much more sensitive to the difference in the brightness of adjacent gray shades. For a subjective assessment, B is lowered until quantization effects become unacceptable. If image quality is to be judged by an analytic measure, B is simply taken as the smallest value that satisfies the minimal acceptable image quality measure. Where B represents the number of code bits allocated to each sample.Ī bit rate compression can be achieved for PCM coding by the simple expedient of restricting the number of bits assigned to each sample. Because uniform-length code words are used for each image sample, the number of amplitude quantization levels is determined by the relationship In the literature, the binary code is often called a pulse code modulation (PCM) code. Monochrome images are typically input to a digital image processor as a sequence of uniform-length binary code words. This section considers the subjective and quantitative effects of the quantization of monochrome and color images. ![]()
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