+Conversely, you can convert an RGB or RGBA image to grayscale or grayscale
+with alpha. This is intended for conversion of images that really are
+gray (red == green == blue), so the function simply strips out the red
+and blue channels, leaving the green channel in the gray position.
+
+ if (color_type == PNG_COLOR_TYPE_RGB ||
+ color_type == PNG_COLOR_TYPE_RGB_ALPHA)
+ png_set_rgb_to_gray(png_ptr, error_action,
+ float red_weight, float green_weight);
+
+ error_action = 1: silently do the conversion
+ error_action = 2: issue a warning if the original
+ image has any pixel where
+ red != green or red != blue
+ error_action = 3: issue an error and abort the
+ conversion if the original
+ image has any pixel where
+ red != green or red != blue
+
+ red_weight: weight of red component
+ (NULL -> default 54/256)
+ green_weight: weight of green component
+ (NULL -> default 183/256)
+
+If you have set error_action = 1 or 2, you can
+later check whether the image really was gray, after processing
+the image rows, with the png_get_rgb_to_gray_status(png_ptr) function.
+It will return a png_byte that is zero if the image was gray or
+1 if there were any non-gray pixels. bKGD and sBIT data
+will be silently converted to grayscale, using the green channel
+data, regardless of the error_action setting.
+
+With 0.0<=red_weight+green_weight<=1.0,
+the normalized graylevel is computed:
+
+ int rw = red_weight * 256;
+ int gw = green_weight * 256;
+ int bw = 256 - (rw + gw);
+ gray = (rw*red + gw*green + bw*blue)/256;
+
+The default values approximate those recommended in the Charles
+Poynton's Color FAQ, <http://www.inforamp.net/~poynton/>
+Copyright (c) 1998-01-04 Charles Poynton poynton@inforamp.net
+
+ Y = 0.212671 * R + 0.715160 * G + 0.072169 * B
+
+Libpng approximates this with
+
+ Y = 0.211 * R + 0.715 * G + 0.074 * B
+
+which can be expressed with integers as
+
+ Y = (54 * R + 183 * G + 19 * B)/256
+
+The calculation is done in a linear colorspace, if the image gamma
+is known.
+
projects / wxWidgets.git / blobdiff