{"id":256,"date":"2006-10-03T17:45:30","date_gmt":"2006-10-03T08:45:30","guid":{"rendered":"https:\/\/regex.info\/blog\/photo-tech\/color-spaces-page5"},"modified":"2006-10-03T13:36:39","modified_gmt":"2006-10-03T04:36:39","slug":"color-spaces-page5","status":"publish","type":"page","link":"https:\/\/regex.info\/blog\/photo-tech\/color-spaces-page5","title":{"rendered":"Digital-Image Color Spaces, Page 5: Chromaticity Diagrams"},"content":{"rendered":"\n\n<style type='text\/css'>\na.btn    { background-color: #555; border: solid 1px #888; padding: 2px 5px }\nspan.now { background-color: #533; border: solid 1px #888; padding: 2px 5px; font-weight: bold }\n<\/style>\n<div style='display: block; background-color: #444; padding: 7px; border: solid 2px gray; margin-bottom:40px'>\n<b>Article:<\/b>\n<a class='btn' href='\/blog\/photo-tech\/color-spaces-page0\/'>Table of Contents<\/a> &nbsp; &nbsp; &nbsp; <b>Page:<\/b>\n<a title='Introduction' class='btn' href='\/blog\/photo-tech\/color-spaces-page1\/'>1<\/a> &middot;\n<a title='Test Images' class='btn' href='\/blog\/photo-tech\/color-spaces-page2\/'>2<\/a> &middot;\n<a title='Color Mis-Management' class='btn' href='\/blog\/photo-tech\/color-spaces-page3\/'>3<\/a> &middot;\n<a title='Color Management' class='btn' href='\/blog\/photo-tech\/color-spaces-page4\/'>4<\/a> &middot;\n<span class='now'>5<\/span> &middot;\n<a title='Design Tradeoffs'          class='btn' href='\/blog\/photo-tech\/color-spaces-page6\/'>6<\/a> &middot;\n<a title='Recommendations and Links' class='btn' href='\/blog\/photo-tech\/color-spaces-page7\/'>7<\/a>\n<small>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;This is the fifth page of a seven-page article<\/small>\n<\/div>\n\n\n<p>\n\nWhen talking about color spaces, it seems to be obligatory to show a\nchromaticity diagram, so here is a common CIE xy chromaticity diagram:\n\n<\/p>\n\n<div style='margin-top:20px' class='img_center'><img loading=\"lazy\" decoding=\"async\" src=\"\/i\/cs\/chromaticity.jpg\" width=\"452\" height=\"500\" alt='Chromaticity Diagram showing the sRGB and\nAdobeRGB color spaces'\/><br\/><big><b>xy Chromaticity Diagram<\/b><\/big><br\/><small>(with grossly inaccurate colors)<\/small><br\/>Showing sRGB and AdobeRGB color spaces<\/div>\n\n<p>\n\nThis kind of diagram is often seen, but not so often understood, so here\nare some notes:\n\n<\/p>\n<ol>\n\n<li><p>The shark-fin shaped blob is meant to represent the chromaticity\nthat the human eye can see. &#8220;Chromaticity&#8221; is color without\nregard to brightness; each point on the graph represents a specific\nspectrum (relative combination of visible wavelengths of light). Such light\ndoesn't become a &#8220;color&#8221; until perceived by the eye.\n\n<\/p><p>\n\nKeep in mind that the eye's perception of a light's color is strongly\ninfluenced by the light's relative brightness. (For example, relatively\nunbright white is perceived as gray, and relatively unbright orange is\nperceived as brown.) Brightness is not taken into account with this graph,\nwhich is concerned only with chromaticity.\n\n<\/p><\/li>\n\n<li><p>The two triangles are meant to represent the range of chromaticities\nthat the sRGB and AdobeRGB color spaces can encode.\n\n<\/p><p>\n\nNote that the size of the green area is exaggerated by the mathematical\napproach taken with this plot (much like the size of Greenland is\nexaggerated in a standard <a class='quiet'\nhref='http:\/\/en.wikipedia.org\/wiki\/Mercator_projection'>Mercator\nprojection<\/a> map). The difference between sRGB and AdobeRGB is not as\npronounced as this plot might at first suggest. This lack of <i>perceptual\nuniformity<\/i> across the plot is a common criticism of xy chromaticity\ndiagrams (but that's an issue about the plot, not about the color spaces\nthe plot is attempting to illustrate).\n\n<\/p><\/li>\n<li><p>\n\nThe triangles are the range of <i>chromaticities<\/i> that the two color\nspaces can represent; keep in mind that the range of <i>brightnesses<\/i>\nacross which those chromaticities can be encoded in the color space is not\nat all represented by this graph.\n\n<\/p><p>\n\nThe total range of colors (chromaticity\/brightness combinations) that a\ncolor space can encode is called its &#8220;<a\nhref='http:\/\/en.wikipedia.org\/wiki\/Gamut'>gamut<\/a>,&#8221; but again, an\nxy chromaticity diagram like this shows only two dimensions of a\nthree-dimensional issue.\n\n<\/p><\/li>\n\n<li><p>The colors that you see in the plot are necessarily approximations.\nThe graph is presented via a digital image &mdash; an image with the sRGB\ncolor space, so <i>by definition<\/i> anything outside the small triangle\nsimply can't be represented properly because the color space <i>by\ndefinition<\/i> has no way to encode those colors.<\/p><\/li>\n\n<li><p>Even those colors inside the sRGB triangle are approximations,\nbecause the relative brightness is inconsistent across the graph. Since\nthis is all just an illustration to begin with, I opted for a prettier\ndisplay rather than the much-harder-to-understand correct display. (This\npage on <a href='http:\/\/www.techmind.org\/colour\/rogues.html'>bad\nchromaticity graphs<\/a> shows what they really should look like.)<\/p><\/li>\n\n<li><p>The top and sides of the fin are labeled with marks ranging from 380\nto 700, which are wavelengths of light, in nanometers. The colors around\nthat edge (the edge of the fin except the bottom edge) are the\n<i>monochromatic<\/i> or <i>spectral<\/i> colors &mdash; colors made up of a\nsingle wavelength of light. Although these select colors are the building\nblocks of <i>all<\/i> color, they are rarely found in nature.\n\n<\/p><p>\n\nOstensibly, these are the colors of a rainbow, but because one sees a\nrainbow only in the context of what's behind it, you don't see\nmonochromatic colors when looking at a rainbow. You do see them with a\nprism, and from a laser.<\/p><\/li>\n\n<li><p><a name='Monochromatic'>You'll<\/a> notice that the <b>entire<\/b> set\nof monochromatic (&#8220;rainbow&#8221;) colors are outside both the sRGB\nand AdobeRGB triangles. Neither color space can represent these deep, rich,\npure colors; they are simply omitted from the color space. The closest\ncolors within the triangles are <i>close<\/i>, but they are different. Many\nwould argue that they are not so different that most people could tell on\nour consumer monitors or printers, but in any case, remember that the\ncolors you see in the image are just an approximation.<\/p><\/li>\n\n<li><p>The point marked &#8220;D65&#8221; is the <i>white point<\/i> for\nboth color spaces. You might be surprised that there's not a standard\ndefinition for the color &#8220;white,&#8221; so the white point of a color\nspace is the <i>its<\/i> definition of white. The white called\n&#8220;D65&#8221; is roughly equivalent to late-afternoon shade on a clear\nday (although evaluate this description in the vague, er, light of the next\nparagraph). Some color spaces have dramatically different white points,\noften due to historical conventions within a particular field.\n\n<\/p><p>\n\nAs an aside, it might also be surprising to learn that many lights that we\ngenerally consider to be white have fairly strong tints to them. Our\neyes\/brain compensate quite well (&#8220;chromatic adaptation&#8221;), so we normally\ndon't notice. The color of sunlight, for example, changes markedly as the\nsun's height above the horizon, and weather, change. The color of light in\ndirect sun is remarkably different than the color of light in the shade.\nYet a &#8220;white&#8221; shirt (objects generally have no color; they merely reflect\nlight) on a passerby indeed seems white, even though it's usually presented\nto our eyes via some very light shade of blue or red.\n\n<\/p><p>\n\nBecause a digital camera's sensor doesn't compensate the way our eye does,\nit sees the light as it is, tint and all. We must therefore set the\ncamera's &#8220;white balance&#8221; setting to let it know the tint of the\nambient light so that it can attempt the same compensation that our brains\ndo automatically. If you're not familiar with the problems of not setting\nthis correctly, I have a <a\nhref='\/blog\/2006-06-28\/208'>short writeup on white\nbalance<\/a> that might be informative.<\/p><\/li>\n\n\n<li><p>An interesting feature of this kind of plot is that the colors\nbetween any two points are made up of the various combinations of the two\nendpoint colors. Similarly, colors within a triangle of three points are\nmade up of the various combinations of the colors of those three points.\nThis last effect means that the <i>range<\/i> of chromaticities of these\ncolor spaces is defined by three points (apply named the color space's\n<i>red point<\/i>, <i>blue point<\/i>, and <i>green point<\/i>). These points\nare the exact shade of each color that is considered &#8220;pure&#8221;\nwithin the context of the color space.\n\n<\/p><p>Here's a small Java application which allows you to <a href='http:\/\/www.cs.rit.edu\/~ncs\/color\/a_chroma.html'>play with a three-point gamut<\/a> in real time.\n\n<\/p><\/li>\n\n<\/ol>\n\n<p>\n\nThere is more to the design of a color space than the location of its four\ncolor points. Another important consideration is how it represents\nbrightness. Human perception of brightness is not linear; if you double the\nluminance of a particular light, for example, yes, it is perceived as\nbrighter, but still much dimmer than twice as bright.\n\n<\/p><p>\n\nThe non-linearity of our perception of brightness can be seen in the\n&#8220;18% gray card&#8221; that photographers use when metering exposure\nand white balance. Have you ever wondered how it got its name? It appears\n<b>50%<\/b> as bright as a white sheet of paper, but achieves that reduction\nin brightness by reflecting only <b>18%<\/b> of the light that hits it. It\ncould well be called a &#8220;50% white&#8221; card or a &#8220;half bright&#8221; card; I suppose\nthe &#8220;18% gray&#8221; name was chosen because how much light it reflects is a\nreference to a quantitatively measurable characteristic, as opposed to a\nsubjective characteristic of human perception like brightness.\n\n<\/p><p>\n\nAnyway, because of this non-linear nature of brightness perception, color\nspaces are more efficient with their finite amount of encoding space if\nthey encode brightness in a non-linear fashion as well. This non-linear\ncomponent is the color space's <a\nhref='http:\/\/en.wikipedia.org\/wiki\/Gamma_correction'>gamma<\/a>.\n\n<\/p>\n<p><b>Continued on the Next Page<\/b><\/p>\n<p>\n\nIf you're really interested in the design tradeoffs that go into a color\nspace, you'll want to continue with the next page: <a\nhref='\/blog\/photo-tech\/color-spaces-page6\/'>Page 6:\nColor-Space Design Tradeoffs<\/a>.\n\n<\/p><p>\n\nHowever, if you'd like to skip that, jump right to <a href='\/blog\/photo-tech\/color-spaces-page7\/'>Page 7: Recommendations and Links<\/a>.\n\n\n<\/p>\n\n\n\n\n\n\n\n","protected":false},"excerpt":{"rendered":"<p> When talking about color spaces, it seems to be obligatory to show a chromaticity diagram, so here is a common CIE xy chromaticity diagram: <\/p> <p> This kind of diagram is often seen, but not so often understood, so here are some notes: <\/p> <p>The shark-fin shaped blob is meant to represent the chromaticity that the human eye can see. \"Chromaticity\" is color without regard to brightness; each point on the graph represents a specific spectrum (relative combination of visible wavelengths of light). Such light doesn't become a \"color\" until perceived by the eye. <\/p><p> Keep in mind that [...]","protected":false},"author":1,"featured_media":0,"parent":251,"menu_order":5,"comment_status":"open","ping_status":"open","template":"","meta":{"footnotes":""},"_links":{"self":[{"href":"https:\/\/regex.info\/blog\/wp-json\/wp\/v2\/pages\/256"}],"collection":[{"href":"https:\/\/regex.info\/blog\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/regex.info\/blog\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/regex.info\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/regex.info\/blog\/wp-json\/wp\/v2\/comments?post=256"}],"version-history":[{"count":0,"href":"https:\/\/regex.info\/blog\/wp-json\/wp\/v2\/pages\/256\/revisions"}],"up":[{"embeddable":true,"href":"https:\/\/regex.info\/blog\/wp-json\/wp\/v2\/pages\/251"}],"wp:attachment":[{"href":"https:\/\/regex.info\/blog\/wp-json\/wp\/v2\/media?parent=256"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}