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.24.Comment Extension.a.Description.The Comment Extension contains textual information whichis not part of the actual graphics in the GIF Data Stream.It is suitablefor including comments about the graphics, credits, descriptions or anyother type of non-control and non-graphic data.The Comment Extensionmay be ignored by the decoder, or it may be saved for later processing;under no circumstances should a Comment Extension disrupt or interferewith the processing of the Data Stream.This block is OPTIONAL; any number of them may appear in the Data Stream.b.Required Version.89a.18c.Syntax.7 6 5 4 3 2 1 0 Field Name Type+---------------+0 | | Extension Introducer Byte+---------------+1 | | Comment Label Byte+---------------++===============+| |N | | Comment Data Data Sub-blocks| |+===============++---------------+0 | | Block Terminator Byte+---------------+i) Extension Introducer - Identifies the beginning of an extensionblock.This field contains the fixed value 0x21.ii) Comment Label - Identifies the block as a Comment Extension.This field contains the fixed value 0xFE.iii) Comment Data - Sequence of sub-blocks, each of size at most255 bytes and at least 1 byte, with the size in a byte precedingthe data.The end of the sequence is marked by the BlockTerminator.iv) Block Terminator - This zero-length data block marks the end ofthe Comment Extension.d.Extensions and Scope.This block does not have scope.This blockcannot be modified by any extension.e.Recommendations.i) Data - This block is intended for humans.It should containtext using the 7-bit ASCII character set.This block shouldnot be used to store control information for custom processing.ii) Position - This block may appear at any point in the DataStream at which a block can begin; however, it is recommended thatComment Extensions do not interfere with Control or Data blocks;they should be located at the beginning or at the end of the DataStream to the extent possible.25.Plain Text Extension.a.Description.The Plain Text Extension contains textual data and theparameters necessary to render that data as a graphic, in a simple form.The textual data will be encoded with the 7-bit printable ASCIIcharacters.Text data are rendered using a grid of character cells19defined by the parameters in the block fields.Each character is renderedin an individual cell.The textual data in this block is to be renderedas mono-spaced characters, one character per cell, with a best fittingfont and size.For further information, see the section onRecommendations below.The data characters are taken sequentially fromthe data portion of the block and rendered within a cell, starting withthe upper left cell in the grid and proceeding from left to right andfrom top to bottom.Text data is rendered until the end of data isreached or the character grid is filled.The Character Grid contains anintegral number of cells; in the case that the cell dimensions do notallow for an integral number, fractional cells must be discarded; anencoder must be careful to specify the grid dimensions accurately so thatthis does not happen.This block requires a Global Color Table to beavailable; the colors used by this block reference the Global Color Tablein the Stream if there is one, or the Global Color Table from a previousStream, if one was saved.This block is a graphic rendering block,therefore it may be modified by a Graphic Control Extension.This blockis OPTIONAL; any number of them may appear in the Data Stream.b.Required Version.89a
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