154 Class Note for IST 250 at PSU
154 Class Note for IST 250 at PSU
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Date Created: 02/06/15
T3L8 Introduction to Graphics on the Web Introduction This lesson is designed for you to gather information about graphics and their use in a web environment When you nish this lesson you should be able to Describe how color is produced on the web Describe how bit depth affects color Describe color calibration Describe how color in browsers is produced Describe the HTML tags that enable color manipulation on a web page Describe the GIF graphic format Describe the JPEG graphic format Describe the PNG file format Describe graphic file optimization techniques Choose appropriate graphics for a given situation Because the colors on the web are displayed through a computer monitor you need to understand some basic color issues Understanding these issues will assist you in Visualizing and creating web pages and sites First you need to understand how color is created on a computer monitor and how browsers and HTML handle color Read the following pages What is Computer Color Bit Depth and Color Color Calibration Color in Browsers Color in HTML Next you need to understand the different graphic formats used to present graphics on the web Read the following pages The GIF Graphic Format The JPEG Graphic Format Other File Formats Finally understanding some of the logistics and proper uses of graphics will help you make decisions about graphic use and placement Read the following pages File Sizes amp Optimization Use of Graphics to Get a Point Across T3L8 Additional Resources Webmonkey Graphics amp Fonts http wwwhotwiredcom webmonke Graphicsifonts tWGraDhicsifonts ltdtheadgt Graphics httpwwwzdnetcomdevheadfiltersqraphics Features and differences of GIF VS JPEG http www5ervtechcom doqu Graphics T3L8 What is Computer Color Computer screenbased color is composed of proj ected light and pixels The use of color adds an entire new dimension to web pages but a designer has to be careful in the selection of colors because of different renderings by different browsers and platforms different video cards and even by individual monitor settings RGB color The color of a pixel one screen dot is made up of three projected colors of light that mix together optically The projected lights form the colors red green and blue and when they are mixed together they create a color system called RGB A digital color image pixel is just a RGB data value Red Green Blue The color is represented by numeric RGB color values in a 24bit number that has three 8bit numbers for each pixel An 8bit byte can contain one of 256 numeric values ranging from 0 to 255 The pixel RGB data value shows how much Red Green and Blue to show 0 none 255all and the three colors and intensity levels are combined for an image pixel Black is an RGB value of 0 0 0 and White is 255 255 255 The composite ofthe three RBG values creates the nal color for that one pixel area Red and Green make Yellow so 255 255 0 means Red and Green each fully saturated 255 is as bright as can be with no Blue at all zero and the resulting color is Yellow Gray has the property of always having equal RGB values So 220 220 220 is a light gray near white and 707070 is a dark gray near black Other colorcoding schemes do exist but are beyond the scope of this discussion Lab lightness a channel and b channel is one HSB hue saturation and brightness is another The schemes Pantone and CMYK are color schemes for printing not displaying colors and are not included in this discussion T3L8 Bit Depth and Color In color bit depth refers to the number of colors in an image or the number of colors a computer is capable of displaying As described in What is Computer Color the RGB color system uses a series of three numbers to represent color The numbers represent the intensity of the Red Green and Blue color components that are projected from the computer screen in each pixel With 24bit color each red green and blue number ranges from 0255 which is 8 bits of storage in the computer That is why the common color system used by designers is called 24bit color 3 colors X 8 bits 24 bits A Bit Depth Table 32 bit 167 million colors 8bit alpha channel for transparency values True color 24bit 167 million colors True color photorealistic graphics possible l6bit 655 thousand colors High resolution 8bit 256 colors 7bit 128 colors 6bit 64 colors 5bit 32 colors 4bit 16 colors 3bit 8 colors 2bit 4 colors lbit 2 colors This has serious implications for web page designers using graphics If a computer has only an 8bit color card it cannot display more than 256 colors at one time The majority of people who create artwork for web sites are viewing the artwork under better conditions than the average end user When people with 256 color systems view your web screens they cannot properly see images in 24bit even if they want to A browser will do it s best to display a 24 bit image on a 16 or 8 bit display but the result is usually less than pleasing Visual Examples of this As a wzb page dz gmx n 15 thus mum um yml rm am139pusgblzwlmyunr shamaudnm 15 pm afwewmg mm mm ambncaxm and wishmm m appmpnam gnphlcs Ifyuucanmtkmw yummldnnce s T3L8 capabilities your safest course is to use 8bit graphics You may have to convert 16 or 24bit graphics to 8bit ones Also the higher quality the graphic the larger its le size usually and the longer it will take to download and display See the File Sizes amp Optimization section for more information T3L8 Color Calibration Different systems have different color cards and monitors and even two identical systems and monitors will display color slightly differently due to calibration differences and gamma the overall brightness of a monitor display settings Thus there will always be color differences and differences in brightness that cannot be controlled by the designer For example Macs are typically brighter than PCs because of differences in native gamma settings An average gamma setting for Mac monitors is 18 gamma while that for an average PC is 25 gamma Web pages can now be viewed over television screens which are calibrated differently from computer monitors The consequences of all this is that graphics that look OK on a PC or Unix monitor will look pale and insipid on a Mac monitor and what looks correct on a Mac will be visibly too dark on a PC or Unix monitor One compromising solution to this problem is to use an intermediate gamma setting of 22 when a graphic is designed this is the stande recommended by the W3C and is the same gamma used for broadcast television Most commercial graphics editing programs allow you to alter the gamma setting That way any GIF or JPEG graphic image that you create should look the same on both platforms and television T3L8 Color in Browsers It is usually safe to assume that if a machine has color capability it has at least 8bit color capability That means the machine39s monitor is capable of displaying 256 colors at one time The question is which 256 colors will be displayed Without some sort of standard set called a palette of 256 colors to call upon when creating 8bit graphics you would have no way of knowing if the palette you used to design your graphic would be the same one used by another user Fortunately early web developers foresaw this potential disaster and a special palette named the Web Palette was designed The Web Palette consists of 216 colors shared by both Macintosh and Windows systems The 2l6color palette has 6 red values 6 green values and 6 blue values and color numbers will all be formed from combinations of hex 00 33 66 99 CC and FF If a machine is set to display 8bit images and a 16 or 24bit image is loaded the browser will automatically dither the image using the Web Palette Dithering means two or more colors from the palette are used placed side by side pixel by pixel to approximate the color The result may or may not be pleasing to the eye If a color background is selected on a 24bit system and then is viewed on a 256color monitor the browser will convert it to one of the 216 colors it will shift the colors to its own palette So it is a good idea to use browsersafe colors when choosing color schemes for backgrounds text or link colors As a web page designer it is thus critical that you nd out if possible what your clientaudience is capable of viewing 8bit 16bit or 24bit color and designincorporate the appropriate graphics If you know you will be using 8bit graphics you should use the Web Palette to design those graphics T3L8 Color in HTML In HTML it is possible to specify colors on a web page that are not part of a graphic For example solid background colors and text colors are speci ed by HTML tags not by a preexisting graphic Information of text color is covered in the section on Fonts To change the color of a background the BODY tag is used See this example httD wwwhidahocom colorcenter cchtml Tags that supp01t this ltBODY BGCOLORquothexadecimalnumberherequotgt ltFONT COLORquothexadecimalnumberherequotgt T3L8 The GIF Graphic Format GIF Graphic Interchange Format is the oldest and possibly most often used graphic format for delivery of graphics to the web It was invented by the UniSys Corporation All visual browsers know how to display GIF graphics A GIF image can contain up to 256 colors and the information about these colors is stored in the image itself Thus for images that can appear well using only 256 colors or less the GIF format is the format of choice Graphics that fall into this category are usually ones with lots of solid colors and ones with little color variation Color cartoons and many technical illustrations fall into this category Color photographs of people or works of visual art are usually poor candidates for the GIF format they have more that 256 variations of color in them Saving these types of images in GIF format may cause a loss of color information people s skin may appear blotchy or a fine work of art may not look colorwise as it does in the real world The use of dithering may reduce this color loss Dithering is a way of simulating a color by using alternating pixels of different colors The color table may not contain all the colors used in the image so to simulate colors not in the color table you can dither the colors Dithering mixes the pixels of the available colors to simulate the missing colors You can choose from three dithering options 0 None Does not dither colors but instead uses the color closest to the missing color This tends to result in sharp transitions between shades of color in the image creating a posterized effect 0 Diffusion Uses an errordiffusion method that produces a less structured dither than the Pattern option 0 Pattern Uses a halftonelike square pattern to simulate any colors not in the color table This option is available only when you are using the Mac OS System palette the Web palette or the Uniform 216 colors palette Experiment with either diffusion dithering or none to determine which works best with your particular image Remember though that dithering increases file size See File Sizes amp Optimization for more information GIF Variations There are two types of GIF GIF87a and GIF89a GIF87a supports transparency and interlacing whereas GIF89a supports transparency interlacing and animation Transparency interlacing and animation can be combined in a single image Most graphics packages will save GIFs in the GIF89a format Interlaced GIFs lO T3L8 Interlaced GIFs are images that start out blocky and appear less and less blocky until they come into full focus Interlacing does not affect the overall size or speed of a GIF Interlacing is supposed to make it possible for your end viewer to get a rough idea of your visuals and to make a decision whether to wait or click onward before the image finishes rendering You should not use interlaced GIFs for essential graphics Transparent GIFs Transparent GlFs are used to create the illusion of irregularly shaped artwork All computermade images end up in rectangularshaped les it s the nature of the medium This usually means you will have an image on a color rectangular background In many cases you don t want viewers to see this background Certain file formats such as GIF can quotmask or hide these regions This quotmasked regionquot appears to be transparent Animated GIFs Animated GIFs are part of the GIF89a specification They are formally called multiblock GIFs because multiple images can be stored as separate blocks within one single GIF document This is very much the same as celbased animation where each frame in the animation is slightly different from the one preceeding it When the GIF document is viewed the multiple images display quickly and in succession and produce a streaming animation Just like other GIF files the number of colors and amount of noise in the frames affect the overall file size If you have a lOOframe animation with each frame totaling 5k your animated GIF will be 500k It simply multiplies in size according to how many frames you create and the file size of the individual frame of artwork The beauty of animated GIFs is that they require no plugins see the Browsers lesson for information on plugins and the authoring tools to create them are often free and easy to learn ll T3L8 The JPEG Graphic Format The second most popular graphics format used to display graphics on web browsers is the JPEG Joint Photographics Experts Group format JPEGs contain 24bit color information as opposed to GIF s 8bit scheme This means JPEGS can contain millions of colors as compared to GIFs 256 color maximum JPEGS are best used for images where 256 colors are just not enough or color accuracy is very important Photographs and color artwork are two examples where JPEG format is often used JPEGs do not yet offer the universal interlacing transparency or animation that GIF images do although these options are being pursued 12 T3L8 Other File Formats Two other graphic le formats currently exist for displaying graphics on web browsers Progressive JPEG and PNG Neither is very popular so only their basics are covered here Progressive J PEG A regular or quotbaselinequot JPEG le is stored as one toptobottom scan of the image Progressive JPEG divides the le into a series of scans The rst scan shows the image at the equivalent of a very low quality setting and therefore it takes very little space Following scans gradually improve the quality much like an interlaced GIF works Each scan adds to the data already provided so that the total storage requirement is about the same as for a baseline JPEG image of the same quality as the nal scan The advantage of progressive JPEG is that if an image is being viewed onthe y as it is transmitted one can see an approximation to the whole image very quickly with gradual improvement of quality as one waits longer this is much nicer than a slow toptobottom display of the image The disadvantage is that each scan takes about the same amount of computation to display as a whole baseline JPEG le would This le format is not supported by all popular browsers PNG Format PNG Portable Network Graphics combines some of best features of GIF and JPEG Like JPEG PNG supports 167 million colors and compresses photographic images to smaller sizes than GIF does PNG is a lossless compression method meaning that no quality loss is incurred when it s applied to images The PNG basic compression is called the ZIP method and is like the quotde atequot method in PKZIP but PNG also incorporates special preprocessing lters that greatly improve the compression ef ciency especially for typical gradient data often found in 24 bit photographic images Unlike GIF or JPEG PNG can be stored at many different bit depths using different storage methods GIF for example can be stored only in 8bit or lower bit depths JPEGs must be stored in 24bit and no lower PNG can be stored in either 8bit or 24bit or 32 bit PNG also has a multitude of different ltering methods PNG was created to be a crossplatform le format and contains information about the characteristics of the authoring platform so that viewing software can automatically compensate and display the image correctly What this means is that Macs and PCs which each utilize different gamma settings can adjust properly for images created in the PNG le format It allows for transparent backgrounds and interlacing and it even improves on those features by allowing for various degrees of transparency and for twodimensional interlacing which transmits a rough overall view of the image faster than the one dimensional scheme used by GIF PNG supports a far superior interlacing scheme than GIF GIF interlacing gives a preview of the image after 1 8th of the image data has been recognized whereas PNG gives a preview after only l64th of the image has loaded 13 T3L8 While PNG has many promising capabilities this le format is not supported by many popular browsers Use it with extreme caution and verify that your clients can indeed View graphics in this format 14 T3L8 File Sizes and Optimization A good rule of thumb to follow with graphics is to limit the total size of all images used on a page to less than 30K This ensures that pages will load quickly even with a slow connection to the internet Nobody likes to wait three minutes for a graphic to load no matter how nice it is This presents the web page designer with a problem how does one go about meeting this rule as often as possible Fortunately there are a number of ways you can use an image maintain its quality and trim your bandwidth all at the same time Both GIF and JPEG images use compression schemes to squeeze graphic data into the smallest space possible GIF File Format GIF format was uses something called the LZW compression algorithm This process looks at the pixels in a row and counts how many are the same color Instead of mapping each individual pixel it converts the pixel information into a mathematical notation dramatically reducing le sizes for graphics that contain large amounts of the same color GIF compression is lossless meaning that the GIF compression algorithm will not cause any unwanted image degradation Because the GIF le format is lossless illustrations with limited colors less than 256 will not lose any quality GIF does signi cantly better on images with only a few distinct colors line drawings and simple cartoons Not only is GIF lossless for such images but it often compresses them more than JPEG can For example large areas of pixels that are all exactly the same color are compressed very ef ciently by GIF JPEG can not squeeze such data as much as GIF does without introducing visible defects One implication of this is that large single color borders are quite cheap in GIF les while they are best avoided in JPEG les The GIF format saves color in a maximum of 8bit mode which means a GIF le can contain a maximum of 256 different colors When you save a GIF le you are asked to select the palette to use when mapping and displaying the colors When creating a GIF le your nal image size and quality are determined by how intelligently you make your indexed conversion choices Indexed conversion is the process of changing the image into a indexed palette which is the table of colors used by GIF images A GIF indexed palette can be 8 bit or less For example if you have a black and white image you can use a 2bit palette to display the two different colors The smaller the palette in terms of bits the smaller the graphic will be in terms of storage space So for GIF images you should try to nd the minimal palette that still displays the graphic how you want it to appear 15 T3L8 Many graphic programs will automatically choose the minimal palette you need You can also experiment with GIF images at GIF Wizard http www qifwizard com or GIF Optimizer http WWW qifoptimizer com One of these graphics is 1124 bytes the cther is 293 bytes Can you tell which is which Amazonmnm could save hundreds of dollars a month by using GIF Wizard to Fix this tune graphic GIF Wizard example If you dither a GIF image its le size will increase If you have to dither a GIF image to maintain quality you should also store the graphic in JPEG format and compare the two le for size display speed and quality It is possible that a JPEG graphic is better choice under these circumstances JPEG File Format JPEG has impressive compression abilities It can easily provide 201 compression of fullcolor data The size ratio is usually more like 41 in GIF However JPEG uses a lossy compression method meaning that the decompressed image is not quite the same as the one with which you started JPEG compresses images by discarding parts of the image information it considers nonessential This lossy compression scheme results in a great degree of compression but also some degradation of the image quality especially around sharp edges JPEG allows adjustments to the amount of information lost This means that the image maker can trade off file size against output image quality you can create very small files with low quality or larger files with higher quality depending on the application that your image is to serve This is a useful feature for creating faster loading pages where perfect image quality may not be essential When saving in JPEG format you can specify the image quality and compression level For the Web there is not much difference in the quality of the image with the different compression levels so you can experiment and save at the lowest quality which is acceptable The more compression the smaller the file s1ze A caveat for working with JPEG is that it loses quality for successive compressions and decompressions Because the JPEG format discards data it is important to save JPEG l6 T3L8 les only once Edit and save your image in a format that does not discard data such as the Photoshop format and save in JPEG format only as a final step If you nd that you must edit the JPEG image and save a JPEG again be sure to use the same value of Quality or Compression EVERY time you save the file Using different values will use different parameters that will aggravate the damage due the lossy compression Unlike the GIF file format JPEGs require both compression and decompression This means that JPEG files need to decompress when they are viewed It takes longer to decode and view a JPEG image than a GIF Although a GIF and a JPEG might be identical sizes or sometimes even when the JPEG is smaller the JPEG may take longer to download or view from a web browser because of the added time required to decompress Thus using JPEG is essentially a timespace tradeoff you give up some time in order to store or transmit an image more cheaply Setting Height and Width HTML Tags Put HEIGHT and WIDTH pixel dimension tags in an image While not speeding the download time of the images the text on a page will load more quickly if the height and width attributes of the ltIMGgt tags These attributes tell the browser how much space the image will take up allowing it to set that area aside download the rest of the page contents and then return to down load the slowerloading graphic file Users then have the chance to readand even move on fromthe page without waiting for the images to download completely The interlacing possible with GIF PNG and Progressive JPEG graphics is another way to save time quickly building up a rough version of the image that becomes quotlivequot immediately If it is an image map readers can click on the graphic and be on their way as soon as they can make out its outline Use of the ALT Tag Include alternate text for each image Use the ltALTquotdescriptionquotgt parameter in your source to specify the text to be seen by people with textonly browsers or those who choose to turn off image display while they39re browsing This could be because they are blind and using a textbased browser or a graphical browser with the image loading feature turned off The text description should be succinct and summarize the content or purpose of the image The alternative text helps visitors understand what is on the page if they are not viewing the graphic The bold text in the following example shows what an ALT attribute looks like in HTML ltIMG SRCquotmylogolargegifquot ALIGNMIDDLE ALTquotTHE LOGO FOR THIS SITEquotgt Use a NULL value for unimportant graphics Some graphical elements may add no content to a page and can be bypassed from viewing by using an ALT attribute with no text By using just two quotation marks with no content you can reduce the amount of distracting text when a page is viewed with graphics capabilities turned off or with a text based browser For example the HTML for a divider bar could be 17 T3L8 ltIMG SRCquotdiViderbargif ALIGNMIDDLE ALTquotquotgt Sighted Visitors will see the divider bar while those Visiting Via a text browser will seehear nothing Conclusion There is no single solution for all images The best way to learn how to balance le size and image quality is to play with your images in PhotoShop or whatever image editing program you use try saVing them in different ways and compare the results By doing this you will start to get a feel for which combinations work for your set of images 18 T3L8 Use of Graphics to Get a Point Across Graphic use falls into two basic categories 0 Aesthetic 0 Instructional Aesthetic graphics are placed on a web page to enhance its appearance They may incorporate a company logo or somehow support the desired quotlook and feelquot of a given site They may be quotattention gettersquot or quoteye candyquot Although important aesthetically they really serve no other purpose Instructional graphics are placed on a web page to convey information You may want to show what a product looks like for example Or you may want to show the steps in a complicated procedure In either case the graphic is designed as a visual gateway to information Instructional graphics can be classi ed as o Representational o Analogical or o Arbitrary Representational graphics share a physical resemblance with the object they portray For example a text description of a given product would best be pictorially represented by a graphic of that product Analogical graphics show one thing that implies another thing These are best used to introduce a new concept to the viewers by using a graphic of something they already understand For example you may show a picture of a book when explaining how hypertext works It is very important that the viewers understand the analogy or the graphic will just cause confusion Arbitrary graphics offer visual clues but do not share any physical resemblance to the concept you are representing Charts and graphs are good examples of this Keep these classi cations in mind when designing graphics for a web page they will help you determine relative importance of a set of graphics and help you determine when to use a graphic to its maximum advantage l9 T3L8 Summary This lesson is designed for you to gather information about graphics and their use in a web environment When you nish this lesson you should be able to Describe how color is produced on the web Describe how bit depth affects color Describe color calibration Describe how color in browsers is produced Describe the HTML tags that enable color manipulation on a web page Describe the GIF graphic format Describe the JPEG graphic format Describe the PNG file format Describe graphic file optimization techniques Choose appropriate graphics for a given situation A short summary of these topics are listed below If you cannot do these things you should review the lesson at least once If you are still having difficulty you should consider other sources of information that compliment this lesson such as textbooks tutors and instructors Color Production Computer screenbased color is composed of proj ected light and pixels The color of a pixel one screen dot is made up of three projected colors of light that mix together optically in a color system named RGB A digital color image pixel is just a RGB data value Red Green Blue An 8bit byte can contain one of 256 numeric values ranging from 0 to 255 thus any color can be represented by a combination of red green and blue values For example gray is represented as 126126126 Bit Depth In color bit depth refers to the number of colors in an image or the number of colors a computer is capable of displaying 8bit color is the most common color scheme If you do not know the capabilities of the user s machine 8bit color is the safest assumption to make 24bit color is millions of colors True photographic quality is possible If you have a 16 or 24bit image that appears on an 8bit system the browser will do it s best to deliver the image correctly through a process named dithering The result may not always be pleasing Color Calibration 20 T3L8 Gamma refers to the overall brightness of a monitor display Macs are typically brighter than PCs because of differences in native gamma settings An average gamma setting for Mac monitors is 18 gamma while that for an average PC is 25 gamma Thus graphics that look OK on a PC or Unix monitor will look pale and insipid on a Mac monitor and what looks correct on a Mac will be visibly too dark on a PC or Unix monitor One compromising solution to this problem is to use an intermediate gamma setting of 22 when a graphic is designed Color in Browsers It is usually safe to assume that if a machine has color capability it has at least 8 bit color capability This means 256 colors can be displayed at one time The question is which 256 colors will be displayed A special palette set of colors named the Web Palette was designed to avoid confusion The Web Palette consists of 216 colors shared by both Macintosh and Windows systems The 2l6color palette has 6 red values 6 green values and 6 blue values and color numbers will all be formed from combinations ofhex 00 33 66 99 CC and FF If you know you will be using 8bit graphics you should use the Web Palette to design those graphics Color in HTML In HTML it is possible to specify colors on a web page that are not part ofa graphic For example solid background colors and text colors are specified by HTML tags not by a preexisting graphic To change the color of a background the BODY tag is used To change the color of the foreground where text is displayed the FONT tag is used The GIF Graphic Format GIF Graphic Interchange Format is the oldest and possibly most often used graphic format for delivery of graphics to the web A GIF image can contain up to 256 colors and the information about these colors is stored in the image itself For images that can appear well using only 256 colors or less the GIF format is the format of choice 21 T3L8 Interlaced GIFs are images that start out blocky and appear less and less blocky until they come into full focus Transparent GlFs are used to remove the background color of the image so images that are not rectangular appear better A round button for example might be a transparent GIF so you don t see the comers of the graphic GIFS can be animated using celbased animation techniques The JPEG Graphic Format JPEGs Joint Photographics Experts Group contain 24bit color information as opposed to GIF s 8bit scheme JPEGS are best used for images where 256 colors are just not enough or color accuracy is very important JPEGs do not yet offer the universal interlacing transparency or animation that GIF images do although these options are being pursued Other File Formats Progressive JPEG divides the le into a series of scans The first scan shows the image at the equivalent of a very low quality setting and therefore it takes very little space Following scans gradually improve the quality much like an interlaced GIF works PNG Portable Network Graphics combines some of best features of GIF and JPEG While a superior graphics technology it is being adopted very slowly if at all File Size and Optimization A good rule of thumb to follow with graphics is to limit the total size of all images used on a page to less than 30K GIF format was uses something called the LZW compression algorithm GIF does significantly better on images with only a few distinct colors line drawings and simple cartoons A GIF indexed palette can be 8 bit or less This means you can save space by discarding the colors not used in a GIF image JPEG allows you to choose the level of compression The more compression the greater the loss of exact detail Use of Graphic to Get a Point Across Graphic use falls into two basic categories Aesthetic and Instructional Aesthetic graphics are placed on a web page to just enhance its appearance Instructional graphics are placed on a web page to convey information 22 T3L8 Instructional graphics can be classi ed as Representational Analogical or Arbitrary Representational graphics share a physical resemblance with the object they portray Analogical graphics show one thing that implies another thing Arbitrary graphics offer Visual clues but do not share any physical resemblance to the concept you are representing Charts and graphs are good examples of this 23
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