Filter imagmagick：根据r/g/b条件有选择地填充像素？

有没有一种方法可以基于使用像素的R、G或B值的算术表达式选择性地替换颜色

示例：假设我有一个RGB图像foobar.png，我想更改红色通道为的所有像素，您可以使用

假设我创建一个测试图像

convert -size 400x400 gradient:red-blue input.png

将任何像素替换为红色值<100，假设最大值为8位255，可表示为

convert input.png -fx 'r < (100/255) ? #FFFFFF : u' output.png

你可以用

假设我创建一个测试图像

convert -size 400x400 gradient:red-blue input.png

将任何像素替换为红色值<100，假设最大值为8位255，可表示为

convert input.png -fx 'r < (100/255) ? #FFFFFF : u' output.png

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这与emcconville的优秀解决方案类似，但略有不同。这是Unix语法

#1 compute the 100 out of 255 threshold in percent
#2 read the input
#3 clone the input and make it completely white
#4 clone the input and separate the red channel, threshold and negate so that the white part represents values less than 100 out of 255
#5 use the threshold image as a mask in a composite to select between the original and the white images
#6 write the output

thresh=`convert xc: -format "%[fx:100*100/255]" info:`
convert image.png \
\( -clone 0 -fill white -colorize 100 \) \
\( -clone 0 -channel r -separate +channel -threshold $thresh% -negate \) \
-compose over -composite \
result.png

---

这与emcconville的优秀解决方案相似，但略有不同。这是Unix语法

#1 compute the 100 out of 255 threshold in percent
#2 read the input
#3 clone the input and make it completely white
#4 clone the input and separate the red channel, threshold and negate so that the white part represents values less than 100 out of 255
#5 use the threshold image as a mask in a composite to select between the original and the white images
#6 write the output

thresh=`convert xc: -format "%[fx:100*100/255]" info:`
convert image.png \
\( -clone 0 -fill white -colorize 100 \) \
\( -clone 0 -channel r -separate +channel -threshold $thresh% -negate \) \
-compose over -composite \
result.png