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1 Pngrewrite is command-line utility that reduces the unnecessarily large
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2 palettes that some programs write into PNG files. It also optimizes
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3 transparency data, and reduces the bits-per-pixel if possible. Handy for
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4 post-processing PNG files before putting them on a web site.
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5
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6 Pngrewrite will:
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7
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8 * Remove any unused palette entries, and write a palette that is only as large
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9 as needed.
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10 * Remove (collapse) any duplicate palette entries.
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11 * Convert non-palette image to palette images, provided they contain no more
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12 than 256 different colors.
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13 * Move any colors with transparency to the beginning of the palette, and write
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14 a tRNS chunk that is a small as possible.
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15 * Reduce the bit-depth (bits per pixel) as much as possible.
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16 * Write images as grayscale when possible, if that is compatible with the goal
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17 of using the minimum possible bit depth.
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18
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19 Under no circumstances does pngrewrite change the actual pixel colors, or
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20 background color, or transparency of the image. If it ever does, that's a bug.
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21
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22 --WARNING--
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23
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24 pngrewrite removes most extra (ancillary) information from the PNG file, such
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25 as text comments. Although this does make the file size smaller, the removed
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26 information may sometimes be important.
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27
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28 The only ancillary chunks that are NOT removed are:
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29
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30 * gAMA - Image gamma setting
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31 * sRGB - srgb color space indicator
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32 * tIME - creation time
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33 * pHYs - physical pixel size
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34 * bKGD and tRNS - Background color and transparency are maintained. The
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35 actual chunk may be modified according to the new color structure.
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36
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37 If the original image was interlaced, the new one will also be interlaced.
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38
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39 Pngrewrite will not work at all on images that have more than 256 colors.
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40 Colors with the same RGB values but a different level of transparency count as
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41 different colors. The background color counts as an extra color if it does not
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42 occur in the image.
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43
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44 It will also not work at all on images that have a color depth of 16 bits,
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45 since they cannot have a palette.
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46
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47 -----------------
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48
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49 This is a very inefficient program. It is (relatively) slow, and may use a lot
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50 of memory. To be specific, it uses about 5 bytes per pixel, no matter what the
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51 bit depth of the image is.
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52
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53 This program is (hopefully) reasonably portable, and should compile without too
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54 much effort on most C compilers. It requires the libpng and zlib libraries.
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55
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56 The pngrewrite code is structured as a library that could be used in other
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57 applications, but I have not documented the interface.
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58
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59 -----------------
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60
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61 How to use:
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62
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63 From a command-line, run
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64
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65 pngrewrite.exe <input-file.png> <output-file.png>
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66
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67 To read from standard-input, or write to standard-output, use "-" for the
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68 filename.
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