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Posterization
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Posterization or posterisation of an image is the conversion of a continuous gradation of tone to several regions of fewer tones, causing abrupt changes from one tone to another.[1] This was originally done with photographic processes to create posters. It can now be done photographically or with digital image processing, and may be deliberate or an unintended artifact of color quantization. Posterization is often the first step in vectorization (tracing) of an image.
Cause
[edit]The effect may be created deliberately, or happen accidentally. For artistic effect, most image editing programs provide a posterization feature, or photographic processes may be used.
Unwanted posterization, also known as banding, may occur when the color depth, sometimes called bit depth, is insufficient to accurately sample a continuous gradation of color tone. As a result, a continuous gradient appears as a series of discrete steps or bands of color — hence the name. When discussing fixed pixel displays, such as LCD and plasma televisions, this effect is referred to as false contouring.[2] Additionally, compression in image formats such as JPEG can also result in posterization when a smooth gradient of colour or luminosity is compressed into discrete quantized blocks with stepped gradients. The result may be compounded further by an optical illusion, called the Mach band illusion, in which each band appears to have an intensity gradient in the direction opposing the overall gradient. This problem may be resolved, in part, with dithering.
Photographic process
[edit]Posterization is a process in photograph development which converts normal photographs into an image consisting of distinct, but flat, areas of different tones or colors. A posterized image often has the same general appearance, but portions of the original image that presented gradual transitions are replaced by abrupt changes in shading and gradation from one area of tone to another.
Printing posterization from black and white requires density separations which is then printed on the same piece of paper to create the whole image. Separations may be made by density or color, using different exposures. Density separations may be created by printing three prints of the same picture, each at a different exposure time that will be combined for the final image.
Modern uses of posterization also include thresholding or color segmentation that transform the image into a simplified version with fewer, highly contrasting colors.[3]
Applications
[edit]Typically, posterization is used for tracing contour lines and vectorizing photo-realistic images. This tracing process starts with 1 bit per channel and advances to 4 bits per channel. As the bits per channel increase, the number of levels of lightness a color can display increases.
A visual artist, faced with line art that has been damaged through JPEG compression, may consider posterizing the image as a first step to remove artifacts on the edges of the image.[citation needed]
Studies show that using posterization can significantly improve the ability to locate landmarks or subjects in outdoor photography.[3]
Video posterization
[edit]Temporal posterization is the visual effect of reducing the number of frames of video, while not reducing the total time it takes the video to play. This compares to regular posterization, where the number of individual color variations is reduced, while the overall range of colors is not. The motion effect is similar to the effect of a flashing strobe light, but without the contrast of bright and dark. Unlike a pulldown, the unused frames are simply discarded, and it is intended to be apparent (longer than the persistence of vision that video and motion pictures normally depend on). An animated GIF often looks posterized because of its normally-low frame rate.
More formally, this is downsampling in the time dimension, as it is reducing the resolution (precision of the input), not the bit rate (precision of the output, as in posterization).
The resulting stop-go motion is a temporal form of jaggies; formally, a form of aliasing. This effect may be the intention, but to reduce the frame rate without introducing this effect, one may use temporal anti-aliasing, which yields motion blur.
Compare with time stretching, which adds frames.
See also
[edit]References
[edit]- ^ Chao, Cheng-Kang Ted (15 July 2021). "PosterChild: Blend-Aware Artistic Posterization" (PDF). Computer Graphics Forum. 40 (4): 87. doi:10.1111/cgf.14343. Retrieved 19 April 2024.
- ^ "HDTV World Glossary". CNET Networks. 2007. Retrieved 2007-06-06.
- ^ a b Tendyck, Christian; Haddad, Andrew; Boutin, Mireille (2018). "Photo-unrealistic Image Enhancement for Subject Placement in Outdoor Photography". arXiv:1807.06196 [cs.CV].
- Langford, Michael. The Darkroom Handbook. New York: Dorling Kindersley Limited, 1981. 245-249.
- Jasc Software. Paint Shop Pro Help, 1998.
External links
[edit]
Look up posterization or posterize in Wiktionary, the free dictionary.
Media related to Posterization at Wikimedia Commons
Posterization
View on Grokipediafrom Grokipedia
This article is about the visual effect in imaging. For the basketball slang, see posterizing (slang).[1]
Posterization is a visual effect in imaging where a continuous range of tones or colors in an image is quantized into a limited number of discrete levels, resulting in abrupt transitions or bands rather than smooth gradients, often evoking the bold, simplified aesthetic of traditional posters.[1][2]
This phenomenon can occur intentionally as an artistic technique or unintentionally as a digital artifact, and it has roots in mid-20th-century photographic processes that converted continuous-tone originals into high-contrast reproductions using separation negatives of varying densities.[1] The term's first known use dates to 1950.[1] In modern contexts, posterization is commonly achieved through digital tools that reduce bit depth—such as converting an 8-bit image (with 256 levels per channel) to fewer effective levels—or apply quantization algorithms to simulate limited color palettes.[3]
Key causes of posterization include reductions in bit depth, which limit the available tonal steps; image compression algorithms that discard subtle color variations to save data; and editing operations like histogram stretching or curves adjustments that amplify quantization errors, leading to visible "steps" in gradients such as skies or shadows.[3] For instance, an 8-bit image stretched beyond its native dynamic range can produce comb-like spikes in its histogram, manifesting as color banding.[3] In video processing, posterization often arises from similar issues, including low-bit-depth encoding or aggressive compression in formats like streaming media, where smooth motion gradients break into stepped artifacts.[4]
As an intentional effect, posterization is employed in graphic design, advertising, and fine art photography to create dramatic, stylized visuals with enhanced contrast and simplified forms, as seen in software like Adobe Photoshop where users can apply posterize filters to limit colors to as few as 2–4 levels per channel.[5] Conversely, when unintended, it degrades image quality and is mitigated by working in higher bit depths (e.g., 16-bit, offering 65,536 levels per channel) or applying dithering to blend tonal steps.[3] In professional digital imaging and film production, avoiding posterization is critical for preserving perceptual smoothness in gradients.[6]
Fundamentals
Definition
Posterization is the conversion of a continuous gradation of tone or color in an image or video into distinct regions with fewer discrete tones, resulting in a flat, poster-like appearance where smooth transitions are replaced by abrupt changes between color bands.[7] This effect simplifies the visual complexity of the original content, emphasizing bold, simplified areas that mimic the limited color palette of traditional posters.[3] The term encompasses both intentional and unintentional applications: intentional posterization is an artistic technique deliberately employed to stylize images by reducing color variety for dramatic or illustrative purposes, such as in graphic design or fine art photography.[8] In contrast, unintentional posterization arises as an undesired artifact that degrades image quality, often manifesting during editing or reproduction processes.[8] Key outcomes of posterization include "banding," where subtle color gradients appear as visible steps or separations rather than seamless blends, and "contour lines," which highlight the edges between these discrete tone regions.[3] For instance, a smooth sky gradient in a landscape photograph might transform into stepped blocks of color, creating unnatural horizontal or vertical lines that disrupt the natural flow.[8]Visual Characteristics
Posterization manifests as flat regions of uniform color across an image, where continuous tonal variations are replaced by discrete blocks or bands, creating a banded or stepped appearance particularly evident in areas that originally featured smooth gradients. These abrupt transitions between color levels eliminate subtle nuances, resulting in the loss of fine details and the emergence of artificial edges or contours that delineate the boundaries of these flat zones. In perceptual terms, the effect alters the viewer's experience by simplifying complex visual information into a more graphic, illustrative form, often evoking the bold, simplified aesthetics of traditional print media.[3][8] For instance, in portraits, skin tones that typically exhibit gentle, natural gradations may appear unnaturally flat or patchy, with visible steps in shading that disrupt the organic flow and lend a stylized, almost cartoonish quality reminiscent of screen-printed posters like Shepard Fairey's "Hope" image of Barack Obama. Similarly, landscapes such as skies or foliage lose their airy smoothness, transforming into blocky patches of color that highlight major tonal shifts but obscure intermediate hues, thereby shifting the image toward an abstract, poster-like illustration rather than photorealism. This transformation is especially pronounced in high-gradient regions, where the human eye expects fluid changes, leading to a perception of artificiality.[8][3] Regarding image quality, posterization can paradoxically enhance visual impact in low-detail areas by amplifying contrast and making shapes more defined, which aids in emphasizing compositional elements without overwhelming subtlety. However, it significantly degrades realism in areas with rich gradients, such as shadows or highlights, producing a blocky or muddy look that diminishes overall fidelity and can make the image appear low-quality or washed out if the effect is severe. These perceptual shifts arise from reductions in effective bit depth, which limit the available color steps, though the visual outcome prioritizes stylistic boldness over naturalistic accuracy.[9][10][3]Causes
Bit Depth Reduction
Bit depth reduction is a fundamental cause of posterization in digital imaging, occurring when the precision of color or intensity representation is diminished, resulting in fewer discrete levels than required to smoothly render continuous gradients. This quantization process maps a wide range of input values to a limited set of output levels, introducing errors that appear as abrupt transitions or bands, particularly noticeable in areas of subtle tonal variation such as skies, shadows, or skin tones. For instance, converting an image from 8-bit to 4-bit per channel severely restricts the tonal palette, making smooth fades appear as distinct steps due to the coarse sampling of the color space.[11][12] The mathematical foundation of this phenomenon lies in the exponential relationship between bit depth and available tones: the number of discrete levels per channel is , where is the bit depth. In an 8-bit grayscale image, this yields levels, typically adequate to approximate continuous tones without visible artifacts under standard display conditions. Reducing to 4 bits, however, provides only levels, leading to pronounced quantization banding as the eye detects steps larger than the just-noticeable difference in luminance. This limitation becomes critical in high-dynamic-range scenes, where insufficient bit depth fails to capture the full tonal continuum.[13]Compression and Processing Artifacts
Lossy compression algorithms, such as JPEG, introduce posterization through quantization steps that map continuous tonal gradients to discrete levels, particularly in smooth areas like skies or shadows, resulting in visible blocky contours during encoding and decoding.[14] This effect is exacerbated when low quality factors are used, as coarser quantization tables discard subtle color variations, leading to abrupt tone jumps that mimic posterization.[15] For instance, in JPEG, the discrete cosine transform followed by quantization primarily affects low-frequency components in uniform regions, amplifying the discontinuity.[16] Image processing operations like resizing, filtering, and dithering can further propagate or intensify these tone discontinuities if not calibrated properly. For example, mismatched gamma correction during processing—such as applying linear scaling to gamma-encoded images—can alter luminance distribution and compress tones. Improper dithering, intended to mitigate quantization errors, can instead amplify artifacts if the noise pattern correlates with existing gradients, leading to patterned discontinuities rather than uniform blending.[17] These artifacts commonly arise unintentionally in bandwidth-constrained environments, such as web image delivery or video streaming, where aggressive compression is applied to reduce file sizes. In streaming services, codecs like H.264 or AV1 at low bitrates quantize luma and chroma channels insufficiently for smooth transitions, causing banding in flat areas during real-time transmission.[18] Bit depth reduction often underlies these issues, enabling the quantization steps that manifest as posterization in processed outputs.[19]History
Origins in Analog Photography
Posterization emerged as a deliberate darkroom technique in the mid-20th century, with the term's first known use in 1950, building on earlier high-contrast printing methods from the 1920s and 1930s that inspired its development.[1] This process enabled photographers to transform continuous-tone originals into high-contrast images with distinct tonal bands, mimicking the bold, simplified aesthetics of commercial posters. The term's first known use dates to 1950, initially describing darkroom methods involving multiple high-contrast positives and negatives to limit tonal ranges for poster production. It involved exposing the negative onto high-contrast lithographic film or paper multiple times, creating a series of separations that isolated specific tone ranges—typically four to five levels—before recombining them into a final print with flattened colors and sharp edges. Such methods were particularly valued in advertising, where the effect enhanced visual impact and readability from a distance, bridging the gap between photographic realism and graphic design.[20] Key innovations centered on the use of separation negatives with varying densities, especially in color printing workflows. Photographers produced multiple negatives by adjusting exposure and development times to capture shadows, midtones, and highlights separately, often filtering through red, green, and blue screens for additive color synthesis. These separations were then printed onto sensitized materials, allowing printers to assign solid colors to each band, resulting in the characteristic poster-like appearance. A significant advancement came in 1970 with the invention of Agfacontour film, which revolutionized color posterization by enabling precise equidensity separations, used in fields like astronomy and NASA photography.[20] This approach drew inspiration from established lithographic and silkscreen production techniques, where photographic stencils or plates were hardened via light exposure to block ink in non-image areas, facilitating mass reproduction of vibrant, low-tone designs—though posterization specifically focused on tonal quantization rather than general printing. Early photo-emulsion methods, patented in 1906 and refined by the 1920s, contributed to these broader workflows.[21][22] In the post-World War II era, particularly from the 1950s onward, posterization gained prominence in commercial advertising, as photographers adapted continuous-tone images for large-scale lithographic and silkscreen posters that demanded simplified forms for street-level visibility. This period saw widespread adoption in Europe and the United States, influenced by the flat-color style of late-19th-century artists like Henri de Toulouse-Lautrec, whose hand-drawn lithographs inspired the integration of photographic bases into modern designs. For example, Czech designer Ladislav Sutnar incorporated high-contrast photographic elements via photomontage into posters during the 1920s, influencing the bold, tone-reduced visuals that became central to posterization in promotional work by the 1950s and 1960s.[23]Developments in Digital Imaging
The transition to digital imaging in the 1980s marked a pivotal shift for posterization, as bitmap graphics and limited display hardware necessitated color reduction techniques to fit images into constrained palettes. Early algorithms for color quantization, such as those developed by Paul Heckbert in his 1982 paper on frame buffer displays, enabled the compression of full-color images to 8 bits per pixel or fewer, often producing posterized results with distinct color bands to approximate continuous tones on devices like early personal computers. This era's bitmap imaging, prevalent in software like MacPaint (1984), inherently involved palette reduction that could induce posterization as both an artifact and an intentional stylistic choice for efficiency. Adobe Photoshop's launch in 1990 introduced dedicated tools for posterization, including the Posterize adjustment under Image > Adjustments, allowing users to specify tonal levels (typically 2–255) for controlled color reduction directly in raster editing workflows. The open-source GNU Image Manipulation Program (GIMP), first released in 1996, further democratized access with its Colors > Posterize function, which intelligently reduces colors while preserving image structure, building on quantization methods to support real-time previews on increasingly affordable hardware. From the 2000s, posterization integrated into interactive media, with video games adopting it for stylized rendering; for instance, cel-shading techniques in titles like Jet Set Radio (2000) used edge detection and color banding to mimic hand-drawn animation aesthetics on consumer consoles. In web graphics, the era's proliferation of broadband and tools like Adobe Flash enabled artistic posterization for dynamic effects, though early web formats like GIF (standardized in 1987 but widely used in the 2000s) often forced palette limitations leading to banding in animations and banners. Affordable computing, exemplified by GIMP's free availability and cross-platform support, facilitated real-time posterization in user-generated content, from web design to game modding, as processing power allowed iterative adjustments without high costs. Key milestones in digital color management addressed posterization risks during conversions. The International Color Consortium (ICC), founded in 1993, released its first profile specification in 1994, standardizing device-independent color spaces to minimize banding artifacts in workflows across monitors, printers, and software by ensuring smooth gradient mappings. In the 2020s, AI-driven enhancements transformed posterization into advanced stylized rendering; for example, the 2021 PosterChild algorithm uses multi-label optimization and palette-based recoloring to automatically extract blended regions from images, preserving details while enabling artistic control in tools like Adobe Firefly.[24]Techniques
Analog Photographic Methods
In analog photography, posterization was achieved through darkroom techniques that intentionally limited the tonal range of an image to create bold, discrete bands of color or gray, mimicking the flat appearance of posters. This process relied on high-contrast materials and precise control over exposure and development to separate continuous tones into a few distinct levels, often 3 to 5 steps.[25] The core darkroom workflow began with creating multiple high-contrast internegatives or positives from an original negative, using orthochromatic lith film to introduce density variations. Typically, three exposures were made: one correctly timed, one underexposed to capture highlights, and one overexposed for shadows, each developed in a lith developer to produce sharp tonal breaks with minimal gradation. These were then contact-printed or enlarged onto additional lith film to generate a set of negatives, aligned using registration marks such as edges of the subject or punch holes for precise overlay. The final print involved sequential multiple exposures of these negatives onto a single sheet of photographic paper in an enlarger, with exposure times adjusted (e.g., 4 seconds for highlights, 7 for midtones, and 10 for shadows at f/11) to build layered tones without blending.[26][27] During printing, dodging and burning techniques were employed to further force tone separations by selectively controlling light exposure on the paper. Dodging involved holding masks or tools to block light from highlight areas, preventing overdevelopment and preserving bright bands, while burning extended exposure to shadow regions to deepen dark tones and sharpen edges between levels. This hands-on manipulation ensured the image's contours stood out in flat, poster-like blocks rather than subtle gradients.[25] For lithographic poster production, printing processes incorporated halftone screening and color separation to adapt photographic images for mechanical reproduction. Color separation required photographing the subject through red, green, and blue filters to yield three black-and-white negatives, each capturing a primary color channel; these were then halftoned using fine screens (e.g., 65-85 lines per inch) to convert tones into variable-sized dots for ink adhesion on lithographic plates.[28] Chemical developers, such as those with high alkalinity (pH 12-14) like glycin-based lith formulas, were tuned for extreme contrast, accelerating development in highlights while retarding midtones to emphasize separations.[29] Key materials included orthochromatic films like Kodalith or Ilford Ortho, which were sensitive only to blue and green light, inherently limiting spectral response and enhancing contrast for tonal banding. Printing papers, such as variable-contrast resin-coated types (e.g., Ilford Multigrade), allowed further adjustment of tone steps during exposure, while fixers and opaques ensured clean edges in the final high-contrast output. These elements collectively enabled photographers to craft posterized effects precursor to digital tools.[26][27]Digital Processing Techniques
Digital processing techniques for creating posterization in still images rely on computational methods to intentionally reduce color gradations, producing a stylized effect reminiscent of graphic art. These approaches typically involve quantizing pixel values across color channels to limit tonal variations, often to 4-8 levels per channel, which creates sharp transitions between color bands.[30][31] This quantization is achieved through thresholding algorithms that map continuous intensity values to discrete steps, effectively compressing the dynamic range while preserving overall structure.[3] To enhance visual contours, these techniques frequently incorporate edge detection, which identifies boundaries between regions and accentuates them with outlines. For instance, edge detection algorithms like Sobel operators can be applied post-quantization to detect gradients in intensity, overlaying thickened lines for emphasis.[32] In Adobe Photoshop, the Posterize adjustment layer implements per-channel quantization by specifying tonal levels, allowing non-destructive application via layers.[30] Complementing this, the Poster Edges filter combines quantization with edge detection, drawing black lines along detected boundaries adjustable by thickness and intensity.[32] Open-source tools like ImageMagick provide command-line options for batch processing, using the-posterize operator to enforce limited levels per channel on multiple images.[31]
Advanced variants extend basic quantization with palette control and region-specific processing. Gradient mapping remaps the image's luminance or individual channels to a predefined gradient with few stops, restricting colors to those in the gradient for a controlled posterized look.[30] Selective application to image regions is facilitated by masking mechanisms, such as layer masks in Photoshop, which isolate the effect to targeted areas like foreground elements. More sophisticated methods, such as fuzzy logic-based quantization combined with bilateral filtering, classify pixels into categories (e.g., dark, gray, bright) using membership functions and assign discrete values, preserving edges through edge-aware smoothing for smoother transitions at low levels like three per channel.[33] Similarly, blend-aware posterization employs K-means clustering to extract a compact palette (4-15 colors) and multi-label optimization to partition and recolor regions, ensuring perceptual blends without explicit thresholding.[34]