Image-PNG-Simple

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libpng-1.6.17/libpng-manual.txt  view on Meta::CPAN

The default values come from the PNG file cHRM chunk if present; otherwise, the
defaults correspond to the ITU-R recommendation 709, and also the sRGB color
space, as recommended in the Charles Poynton's Colour FAQ,
<http://www.poynton.com/>, in section 9:

   <http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9>

    Y = 0.2126 * R + 0.7152 * G + 0.0722 * B

Previous versions of this document, 1998 through 2002, recommended a slightly
different formula:

    Y = 0.212671 * R + 0.715160 * G + 0.072169 * B

Libpng uses an integer approximation:

    Y = (6968 * R + 23434 * G + 2366 * B)/32768

The calculation is done in a linear colorspace, if the image gamma
can be determined.

libpng-1.6.17/libpng.3  view on Meta::CPAN

The default values come from the PNG file cHRM chunk if present; otherwise, the
defaults correspond to the ITU-R recommendation 709, and also the sRGB color
space, as recommended in the Charles Poynton's Colour FAQ,
<http://www.poynton.com/>, in section 9:

   <http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9>

    Y = 0.2126 * R + 0.7152 * G + 0.0722 * B

Previous versions of this document, 1998 through 2002, recommended a slightly
different formula:

    Y = 0.212671 * R + 0.715160 * G + 0.072169 * B

Libpng uses an integer approximation:

    Y = (6968 * R + 23434 * G + 2366 * B)/32768

The calculation is done in a linear colorspace, if the image gamma
can be determined.

libpng-1.6.17/png.c  view on Meta::CPAN

 * We want log2(value/65535), we have log2(v'/255), where:
 *
 *    value = v' * 256 + v''
 *          = v' * f
 *
 * So f is value/v', which is equal to (256+v''/v') since v' is in the range 128
 * to 255 and v'' is in the range 0 to 255 f will be in the range 256 to less
 * than 258.  The final factor also needs to correct for the fact that our 8-bit
 * value is scaled by 255, whereas the 16-bit values must be scaled by 65535.
 *
 * This gives a final formula using a calculated value 'x' which is value/v' and
 * scaling by 65536 to match the above table:
 *
 *   log2(x/257) * 65536
 *
 * Since these numbers are so close to '1' we can use simple linear
 * interpolation between the two end values 256/257 (result -368.61) and 258/257
 * (result 367.179).  The values used below are scaled by a further 64 to give
 * 16-bit precision in the interpolation:
 *
 * Start (256): -23591

libpng-1.6.17/pngrtran.c  view on Meta::CPAN

         int r, g, b, p;
         sp = row;
         dp = row;
         for (i = 0; i < row_width; i++)
         {
            r = *sp++;
            g = *sp++;
            b = *sp++;

            /* This looks real messy, but the compiler will reduce
             * it down to a reasonable formula.  For example, with
             * 5 bits per color, we get:
             * p = (((r >> 3) & 0x1f) << 10) |
             *    (((g >> 3) & 0x1f) << 5) |
             *    ((b >> 3) & 0x1f);
             */
            p = (((r >> (8 - PNG_QUANTIZE_RED_BITS)) &
                ((1 << PNG_QUANTIZE_RED_BITS) - 1)) <<
                (PNG_QUANTIZE_GREEN_BITS + PNG_QUANTIZE_BLUE_BITS)) |
                (((g >> (8 - PNG_QUANTIZE_GREEN_BITS)) &
                ((1 << PNG_QUANTIZE_GREEN_BITS) - 1)) <<

zlib-1.2.8/adler32.c  view on Meta::CPAN

    z_off64_t len2;
{
    unsigned long sum1;
    unsigned long sum2;
    unsigned rem;

    /* for negative len, return invalid adler32 as a clue for debugging */
    if (len2 < 0)
        return 0xffffffffUL;

    /* the derivation of this formula is left as an exercise for the reader */
    MOD63(len2);                /* assumes len2 >= 0 */
    rem = (unsigned)len2;
    sum1 = adler1 & 0xffff;
    sum2 = rem * sum1;
    MOD(sum2);
    sum1 += (adler2 & 0xffff) + BASE - 1;
    sum2 += ((adler1 >> 16) & 0xffff) + ((adler2 >> 16) & 0xffff) + BASE - rem;
    if (sum1 >= BASE) sum1 -= BASE;
    if (sum1 >= BASE) sum1 -= BASE;
    if (sum2 >= (BASE << 1)) sum2 -= (BASE << 1);