PNG 元数据格式规范
PNG 本机格式逐块编码 PNG 文件的完整内容，但包含实际图像数据的 IDAT 块除外。 PNG 1.2 规范中未定义的块存储在 UnknownChunks 元素下。请注意，应用程序软件有责任实施 PNG 阅读器、编写器和编辑器的规则（例如，关于不安全复制块的规则）在 PNG 规范中描述。图像 I/O 库本身并不是规范意义上的 PNG 阅读器、编写器或编辑器。相反，它是一种可用于构建 PNG 阅读器、编写器和编辑器的工具。
图像的实际宽度、高度、位深度和颜色类型将覆盖通过元数据传递给编写器的任何值，但有一个例外。如果图像有 IndexColorModel ，将检查颜色表条目以确定它们是否形成统一的灰度渐变。如果是这样，图像通常将使用灰度而不是调色板颜色进行编码。但是，如果元数据中设置的颜色类型是“调色板”，则将使用调色板颜色。
如果在对图像进行编码时未提供元数据，则会从正在编码的图像中初始化标头，并且不包含可选块。
PNG规格 中描述了标准块的语义。请注意，对于每种颜色类型可能出现的块有一些限制。尤其，
PLTE块可能不会出现在Gray或GrayAlpha图像中hIST和tRNS块需要一个先验的PLTE块tRNS块可能不会出现在GrayAlpha和RGBA图像中
bKGD、sBIT 和 tRNS 块的子节点必须与图像的颜色类型匹配。
某些块可能有意义地在 PNG 文件中多次出现，尤其是与文本相关的块。为了简化元数据格式，这些块的多个实例被合并在一个父节点下（例如tEXt 节点），它可能有零个或多个子节点（例如, tEXtEntry 个节点）。如果没有孩子，则不会写入任何块。同样，未知块存储为单个 UnknownChunks 节点的子节点。
不可能在写入块时控制块的顺序，也无法确定正在读取的文件中块的顺序。
<!DOCTYPE "javax_imageio_png_1.0" [
<!ELEMENT "javax_imageio_png_1.0" (IHDR?, PLTE?, bKGD?, cHRM?,
gAMA?, hIST?, iCCP?, iTXt?, pHYS?, sBIT?, sPLT?, sRGB?, tEXt?,
tIME?, tRNS?, zTXt?, UnknownChunks?)>
<!ELEMENT "IHDR" EMPTY>
<!-- The IHDR chunk, containing the header -->
<!ATTLIST "IHDR" "width" #CDATA #REQUIRED>
<!-- The width of the image in pixels -->
<!-- Data type: Integer -->
<!-- Min value: 1 (inclusive) -->
<!-- Max value: 2147483647 (inclusive) -->
<!ATTLIST "IHDR" "height" #CDATA #REQUIRED>
<!-- The height of the image in pixels -->
<!-- Data type: Integer -->
<!-- Min value: 1 (inclusive) -->
<!-- Max value: 2147483647 (inclusive) -->
<!ATTLIST "IHDR" "bitDepth" ("1" | "2" | "4" | "8" | "16")
#REQUIRED>
<!-- The bit depth of the image samples -->
<!ATTLIST "IHDR" "colorType" ("Grayscale" | "RGB" | "Palette" |
"GrayAlpha" | "RGBAlpha") #REQUIRED>
<!-- The color type of the image -->
<!ATTLIST "IHDR" "compressionMethod" ("deflate") #REQUIRED>
<!-- The compression used for image data, always "deflate" -->
<!ATTLIST "IHDR" "filterMethod" ("adaptive") #REQUIRED>
<!-- The filtering method used for compression, always "adaptive" -->
<!ATTLIST "IHDR" "interlaceMethod" ("none" | "adam7") #REQUIRED>
<!-- The interlacing method, "none" or "adam7" -->
<!ELEMENT "PLTE" (PLTEEntry)*>
<!-- The PLTE chunk, containing the palette -->
<!-- Min children: 1 -->
<!-- Max children: 256 -->
<!ELEMENT "PLTEEntry" EMPTY>
<!-- A palette entry -->
<!ATTLIST "PLTEEntry" "index" #CDATA #REQUIRED>
<!-- The index of a palette entry -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ATTLIST "PLTEEntry" "red" #CDATA #REQUIRED>
<!-- The red value of a palette entry -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ATTLIST "PLTEEntry" "green" #CDATA #REQUIRED>
<!-- The green value of a palette entry -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ATTLIST "PLTEEntry" "blue" #CDATA #REQUIRED>
<!-- The blue value of a palette entry -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ELEMENT "bKGD" (bKGD_Grayscale | bKGD_RGB | bKGD_Palette)>
<!-- The bKGD chunk, containing the background color -->
<!ELEMENT "bKGD_Grayscale" EMPTY>
<!-- A grayscale background color, for Gray and GrayAlpha images -->
<!ATTLIST "bKGD_Grayscale" "gray" #CDATA #REQUIRED>
<!-- A gray value to be used as a background -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 65535 (inclusive) -->
<!ELEMENT "bKGD_RGB" EMPTY>
<!-- An RGB background color, for RGB and RGBAlpha images -->
<!ATTLIST "bKGD_RGB" "red" #CDATA #REQUIRED>
<!-- A red value to be used as a background -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 65535 (inclusive) -->
<!ATTLIST "bKGD_RGB" "green" #CDATA #REQUIRED>
<!-- A green value to be used as a background -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 65535 (inclusive) -->
<!ATTLIST "bKGD_RGB" "blue" #CDATA #REQUIRED>
<!-- A blue value to be used as a background -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 65535 (inclusive) -->
<!ELEMENT "bKGD_Palette" EMPTY>
<!-- A background palette index -->
<!ATTLIST "bKGD_Palette" "index" #CDATA #REQUIRED>
<!-- A palette index to be used as a background -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ELEMENT "cHRM" EMPTY>
<!-- The cHRM chunk, containing color calibration -->
<!ATTLIST "cHRM" "whitePointX" #CDATA #REQUIRED>
<!-- The CIE x coordinate of the white point, multiplied by 1e5 -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 65535 (inclusive) -->
<!ATTLIST "cHRM" "whitePointY" #CDATA #REQUIRED>
<!-- The CIE y coordinate of the white point, multiplied by 1e5 -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 65535 (inclusive) -->
<!ATTLIST "cHRM" "redX" #CDATA #REQUIRED>
<!-- The CIE x coordinate of the red primary, multiplied by 1e5 -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 65535 (inclusive) -->
<!ATTLIST "cHRM" "redY" #CDATA #REQUIRED>
<!-- The CIE y coordinate of the red primary, multiplied by 1e5 -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 65535 (inclusive) -->
<!ATTLIST "cHRM" "greenX" #CDATA #REQUIRED>
<!-- The CIE x coordinate of the green primary, multiplied by 1e5 -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 65535 (inclusive) -->
<!ATTLIST "cHRM" "greenY" #CDATA #REQUIRED>
<!-- The CIE y coordinate of the green primary, multiplied by 1e5 -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 65535 (inclusive) -->
<!ATTLIST "cHRM" "blueX" #CDATA #REQUIRED>
<!-- The CIE x coordinate of the blue primary, multiplied by 1e5 -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 65535 (inclusive) -->
<!ATTLIST "cHRM" "blueY" #CDATA #REQUIRED>
<!-- The CIE y coordinate of the blue primary, multiplied by 1e5 -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 65535 (inclusive) -->
<!ELEMENT "gAMA" EMPTY>
<!-- The gAMA chunk, containing the image gamma -->
<!ATTLIST "gAMA" "value" #CDATA #REQUIRED>
<!-- The image gamma, multiplied by 1e5 -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 2147483647 (inclusive) -->
<!ELEMENT "hIST" (hISTEntry)*>
<!-- The hIST chunk, containing histogram information -->
<!-- Min children: 1 -->
<!-- Max children: 256 -->
<!ELEMENT "hISTEntry" EMPTY>
<!-- A histogram entry -->
<!ATTLIST "hISTEntry" "index" #CDATA #REQUIRED>
<!-- The palette index of this histogram entry -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ATTLIST "hISTEntry" "value" #CDATA #REQUIRED>
<!-- The frequency of this histogram entry -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 65535 (inclusive) -->
<!ELEMENT "iCCP" EMPTY>
<!-- The iCCP chunk, containing an ICC color profile -->
<!-- User object: array of byte -->
<!-- Min length: 0 -->
<!-- Max length: 2147483647 -->
<!ATTLIST "iCCP" "profileName" #CDATA #REQUIRED>
<!-- The name of this ICC profile -->
<!-- Data type: String -->
<!ATTLIST "iCCP" "compressionMethod" ("deflate") #REQUIRED>
<!-- The compression method used to store this ICC profile -->
<!ELEMENT "iTXt" (iTXtEntry)*>
<!-- The iTXt chunk, containing internationalized text -->
<!-- Min children: 1 -->
<!-- Max children: 2147483647 -->
<!ELEMENT "iTXtEntry" EMPTY>
<!-- A localized text entry -->
<!ATTLIST "iTXtEntry" "keyword" #CDATA #REQUIRED>
<!-- The keyword -->
<!-- Data type: String -->
<!ATTLIST "iTXtEntry" "compressionFlag" ("TRUE" | "FALSE")
#REQUIRED>
<!ATTLIST "iTXtEntry" "compressionMethod" #CDATA #REQUIRED>
<!-- The compression method used to store this iTXt entry -->
<!-- Data type: String -->
<!ATTLIST "iTXtEntry" "languageTag" #CDATA #REQUIRED>
<!-- The ISO tag describing the language this iTXt entry -->
<!-- Data type: String -->
<!ATTLIST "iTXtEntry" "translatedKeyword" #CDATA #REQUIRED>
<!-- The translated keyword for iTXt entry -->
<!-- Data type: String -->
<!ATTLIST "iTXtEntry" "text" #CDATA #REQUIRED>
<!-- The localized text -->
<!-- Data type: String -->
<!ELEMENT "pHYS" EMPTY>
<!-- The pHYS chunk, containing the pixel size and aspect ratio -->
<!ATTLIST "pHYS" "pixelsPerUnitXAxis" #CDATA #REQUIRED>
<!-- The number of horizontal pixels per unit, multiplied by 1e5 -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 2147483647 (inclusive) -->
<!ATTLIST "pHYS" "pixelsPerUnitYAxis" #CDATA #REQUIRED>
<!-- The number of vertical pixels per unit, multiplied by 1e5 -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 2147483647 (inclusive) -->
<!ATTLIST "pHYS" "unitSpecifier" ("unknown" | "meter") #REQUIRED>
<!-- The unit specifier for this chunk (i.e., meters) -->
<!ELEMENT "sBIT" (sBIT_Grayscale | sBIT_GrayAlpha | sBIT_RGB |
sBIT_RGBAlpha | sBIT_Palette)>
<!-- The sBIT chunk, containing significant bit information -->
<!ELEMENT "sBIT_Grayscale" EMPTY>
<!-- Significant bit information for gray samples -->
<!ATTLIST "sBIT_Grayscale" "gray" #CDATA #REQUIRED>
<!-- The number of significant bits of the gray samples -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ELEMENT "sBIT_GrayAlpha" EMPTY>
<!-- Significant bit information for gray and alpha samples -->
<!ATTLIST "sBIT_GrayAlpha" "gray" #CDATA #REQUIRED>
<!-- The number of significant bits of the gray samples -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ATTLIST "sBIT_GrayAlpha" "alpha" #CDATA #REQUIRED>
<!-- The number of significant bits of the alpha samples -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ELEMENT "sBIT_RGB" EMPTY>
<!-- Significant bit information for RGB samples -->
<!ATTLIST "sBIT_RGB" "red" #CDATA #REQUIRED>
<!-- The number of significant bits of the red samples -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ATTLIST "sBIT_RGB" "green" #CDATA #REQUIRED>
<!-- The number of significant bits of the green samples -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ATTLIST "sBIT_RGB" "blue" #CDATA #REQUIRED>
<!-- The number of significant bits of the blue samples -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ELEMENT "sBIT_RGBAlpha" EMPTY>
<!-- Significant bit information for RGBA samples -->
<!ATTLIST "sBIT_RGBAlpha" "red" #CDATA #REQUIRED>
<!-- The number of significant bits of the red samples -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ATTLIST "sBIT_RGBAlpha" "green" #CDATA #REQUIRED>
<!-- The number of significant bits of the green samples -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ATTLIST "sBIT_RGBAlpha" "blue" #CDATA #REQUIRED>
<!-- The number of significant bits of the blue samples -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ATTLIST "sBIT_RGBAlpha" "alpha" #CDATA #REQUIRED>
<!-- The number of significant bits of the alpha samples -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ELEMENT "sBIT_Palette" EMPTY>
<!-- Significant bit information for RGB palette entries -->
<!ATTLIST "sBIT_Palette" "red" #CDATA #REQUIRED>
<!-- The number of significant bits of the red palette entries -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ATTLIST "sBIT_Palette" "green" #CDATA #REQUIRED>
<!-- The number of significant bits of the green palette entries -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ATTLIST "sBIT_Palette" "blue" #CDATA #REQUIRED>
<!-- The number of significant bits of the blue palette entries -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ELEMENT "sPLT" (sPLTEntry)*>
<!-- The sPLT chunk, containing a suggested palette -->
<!-- Min children: 1 -->
<!-- Max children: 256 -->
<!ELEMENT "sPLTEntry" EMPTY>
<!-- A suggested palette entry -->
<!ATTLIST "sPLTEntry" "index" #CDATA #REQUIRED>
<!-- The index of a suggested palette entry -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ATTLIST "sPLTEntry" "red" #CDATA #REQUIRED>
<!-- The red value of a suggested palette entry -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ATTLIST "sPLTEntry" "green" #CDATA #REQUIRED>
<!-- The green value of a suggested palette entry -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ATTLIST "sPLTEntry" "blue" #CDATA #REQUIRED>
<!-- The blue value of a suggested palette entry -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ATTLIST "sPLTEntry" "alpha" #CDATA #REQUIRED>
<!-- The blue value of a suggested palette entry -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ELEMENT "sRGB" EMPTY>
<!-- The sRGB chunk, containing rendering intent information -->
<!ATTLIST "sRGB" "renderingIntent" ("Perceptual" |
"Relative colorimetric" | "Saturation" |
"Absolute colorimetric") #REQUIRED>
<!-- The rendering intent -->
<!ELEMENT "tEXt" (tEXtEntry)*>
<!-- The tEXt chunk, containing text -->
<!-- Min children: 1 -->
<!-- Max children: 2147483647 -->
<!ELEMENT "tEXtEntry" EMPTY>
<!-- A text entry -->
<!ATTLIST "tEXtEntry" "keyword" #CDATA #REQUIRED>
<!-- The keyword -->
<!-- Data type: String -->
<!ATTLIST "tEXtEntry" "value" #CDATA #REQUIRED>
<!-- The text -->
<!-- Data type: String -->
<!ELEMENT "tIME" EMPTY>
<!-- The tIME chunk, containing the image modification time -->
<!ATTLIST "tIME" "year" #CDATA #REQUIRED>
<!-- The year when the image was last modified -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 65535 (inclusive) -->
<!ATTLIST "tIME" "month" #CDATA #REQUIRED>
<!-- The month when the image was last modified, 1 = January -->
<!-- Data type: Integer -->
<!-- Min value: 1 (inclusive) -->
<!-- Max value: 12 (inclusive) -->
<!ATTLIST "tIME" "day" #CDATA #REQUIRED>
<!-- The day of the month when the image was last modified -->
<!-- Data type: Integer -->
<!-- Min value: 1 (inclusive) -->
<!-- Max value: 31 (inclusive) -->
<!ATTLIST "tIME" "hour" #CDATA #REQUIRED>
<!-- The hour when the image was last modified -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 23 (inclusive) -->
<!ATTLIST "tIME" "minute" #CDATA #REQUIRED>
<!-- The minute when the image was last modified -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 59 (inclusive) -->
<!ATTLIST "tIME" "second" #CDATA #REQUIRED>
<!-- The second when the image was last modified, 60 = leap second -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 60 (inclusive) -->
<!ELEMENT "tRNS" (tRNS_Grayscale | tRNS_RGB | tRNS_Palette)>
<!-- The tRNS chunk, containing transparency information -->
<!ELEMENT "tRNS_Grayscale" EMPTY>
<!-- A grayscale value that should be considered transparent -->
<!ATTLIST "tRNS_Grayscale" "gray" #CDATA #REQUIRED>
<!-- The gray value to be considered transparent -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 65535 (inclusive) -->
<!ELEMENT "tRNS_RGB" EMPTY>
<!-- An RGB value that should be considered transparent -->
<!ATTLIST "tRNS_RGB" "red" #CDATA #REQUIRED>
<!-- The red value to be considered transparent -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 65535 (inclusive) -->
<!ATTLIST "tRNS_RGB" "green" #CDATA #REQUIRED>
<!-- The green value to be considered transparent -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 65535 (inclusive) -->
<!ATTLIST "tRNS_RGB" "blue" #CDATA #REQUIRED>
<!-- The blure value to be considered transparent -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 65535 (inclusive) -->
<!ELEMENT "tRNS_Palette" EMPTY>
<!-- A palette index that should be considered transparent -->
<!ATTLIST "tRNS_Palette" "index" #CDATA #REQUIRED>
<!-- A palette index to be considered transparent -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ATTLIST "tRNS_Palette" "alpha" #CDATA #REQUIRED>
<!-- The transparency associated with the palette entry -->
<!-- Data type: Integer -->
<!-- Min value: 0 (inclusive) -->
<!-- Max value: 255 (inclusive) -->
<!ELEMENT "zTXt" (zTXtEntry)*>
<!-- The zTXt chunk, containing compressed text -->
<!-- Min children: 1 -->
<!-- Max children: 2147483647 -->
<!ELEMENT "zTXtEntry" EMPTY>
<!-- A compressed text entry -->
<!-- User object: array of byte -->
<!-- Min length: 0 -->
<!-- Max length: 2147483647 -->
<!ATTLIST "zTXtEntry" "keyword" #CDATA #REQUIRED>
<!-- The keyword -->
<!-- Data type: String -->
<!ATTLIST "zTXtEntry" "compressionMethod" ("deflate")
#REQUIRED>
<!-- The compressed text -->
<!ELEMENT "UnknownChunks" (UnknownChunk)*>
<!-- A set of unknown chunks -->
<!-- Min children: 1 -->
<!-- Max children: 2147483647 -->
<!ELEMENT "UnknownChunk" EMPTY>
<!-- Unknown chunk data stored as a byte array -->
<!-- User object: array of byte -->
<!-- Min length: 0 -->
<!-- Max length: 2147483647 -->
<!ATTLIST "UnknownChunk" "type" #CDATA #REQUIRED>
<!-- The 4-character type of the unknown chunk -->
<!-- Data type: String -->
]>