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[devel] Documented the pngvalid gamma error summary with additional comments

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and print statements.
This commit is contained in:
Glenn Randers-Pehrson 2010-12-07 14:40:33 -06:00
parent 845b74e6d4
commit f1cf90238b
3 changed files with 24 additions and 2 deletions
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2
ANNOUNCE
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@ -460,6 +460,8 @@ Version 1.5.0beta56 [December 7, 2010]
Included documentation of changes in 1.5.0 from 1.4.x in libpng-manual.txt
Version 1.5.0beta57 [December 7, 2010]
Documented the pngvalid gamma error summary with additional comments and
print statements.
Send comments/corrections/commendations to png-mng-implement at lists.sf.net:
(subscription required; visit

2
CHANGES
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@ -3098,6 +3098,8 @@ Version 1.5.0beta56 [December 7, 2010]
Included documentation of changes in 1.5.0 from 1.4.x in libpng-manual.txt
Version 1.5.0beta57 [December 7, 2010]
Documented the pngvalid gamma error summary with additional comments and
print statements.
Send comments/corrections/commendations to png-mng-implement at lists.sf.net
(subscription required; visit

22
pngvalid.c
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@ -3463,7 +3463,19 @@ perform_gamma_test(png_modifier *pm, int speed, int summary)
if (summary)
{
printf("Gamma correction error summary (output value error):\n");
printf("Gamma correction error summary\n\n");
printf("The printed value is the maximum error in the pixel values\n");
printf("calculated by the libpng gamma correction code. The error\n");
printf("is calculated as the difference between the output pixel\n");
printf("value (always an integer) and the ideal value from the\n");
printf("libpng specification (typically not an integer).\n\n");
printf("Expect this value to be less than .5 for 8 bit formats,\n");
printf("less than 1 for formats with fewer than 8 bits and a small\n");
printf("number (typically less than 5) for the 16 bit formats.\n");
printf("For performance reasons the value for 16 bit formats\n");
printf("increases when the image file includes an sBIT chunk.\n\n");
printf(" 2 bit gray: %.5f\n", pm->error_gray_2);
printf(" 4 bit gray: %.5f\n", pm->error_gray_4);
printf(" 8 bit gray: %.5f\n", pm->error_gray_8);
@ -3556,7 +3568,13 @@ int main(int argc, PNG_CONST char **argv)
pm.sbitlow = 8U; /* because libpng doesn't do sBIT below 8! */
pm.use_input_precision_16to8 = 1U; /* Because of the way libpng does it */
/* Some default values (set the behavior for 'make check' here) */
/* Some default values (set the behavior for 'make check' here).
* These values simply control the maximum error permitted in the gamma
* transformations. The practial limits for human perception are described
* below (the setting for maxpc16), however for 8 bit encodings it isn't
* possible to meet the accepted capabilities of human vision - i.e. 8 bit
* images can never be good enough (regardless of encoding.)
*/
pm.maxout8 = .1; /* Arithmetic error in *encoded* value */
pm.maxabs8 = .00005; /* 1/20000 */
pm.maxpc8 = .499; /* I.e. .499% fractional error */