TL; DR :将矩形图像裁剪为圆形的最有效方法是什么？
说明/背景:
我正在使用R中的一些代码，这些代码将Spotify艺术家图像显示为圆形，而不是默认的矩形/正方形。我找不到在R中裁切图像的任何程序包或命令，尤其是裁切成圆形，因此我编写了自己的函数circ，该函数读取3维(或4维)RGB(A)数组并将其裁切为使用the parametric equation of a circle圈出以确定每个唯一y的x值。这是我的伪代码:
Given an RGB(A) array:
Find the center of the image, radius = min(x coord, y coord)
Pre-crop the image to a square of dimensions 2r x 2r
For every unique y value:
Determine the x coordinates on the circle
Make pixels outside of the circle transparent
Return the cropped image as an RGBA array
此功能是对上一个功能的巨大改进，前一个功能检查了每个像素的位置以查看它在圆内还是在圆外，但我仍然认为它可以进一步加速。
有没有一种方法可以检查y值的一半而不是全部，并在整个圆上镜像？我可以使用实际的裁剪功能吗？任何帮助都将不胜感激!
编辑以添加一些复制粘贴运行的代码(感谢@lukeA):
我原来的裁剪方法:
circ = function(a){
# First part of the function finds the radius of the circle and crops the image accordingly
xc = floor(dim(a[,,1])[2]/2) # X coordinate of the center
yc = floor(dim(a[,,1])[1]/2) # Y coordinate of the center
r = min(xc, yc) - 1 # Radius is the smaller of the two -1 to avoid reading nonexistent data
ma = array(data = c(a[,,1][(yc-r):(yc+r),(xc-r):(xc+r)], # Read in the cropped image
a[,,2][(yc-r):(yc+r),(xc-r):(xc+r)], # Of dimensions 2r x 2r, centered
a[,,3][(yc-r):(yc+r),(xc-r):(xc+r)], # Around (xc, yc)
rep(1,length(a[,,1][(yc-r):(yc+r),(xc-r):(xc+r)]))), # Add fourth alpha layer
dim = c(length((yc-r):(yc+r)),length((xc-r):(xc+r)),4))
if(yc > xc) yc = xc else if(xc > yc) xc = yc # Re-evaluate your center for the cropped image
xmax = dim(ma[,,1])[2]; ymax = dim(ma[,,1])[1] # Find maximum x and y values
# Second part of the function traces circle by the parametric eqn. and makes outside pixels transparent
for(y in 1:ymax){ # For every y in the cropped image
theta = asin((y - yc) / r) # y = yc + r * sin(theta) by parametric equation for a circle
x = xc + r * cos(theta) # Then we can find the exact x coordinate using the same formula
x = which.min(abs(1:xmax - x)) # Find which x in array is closest to exact coordinate
if(!x - xc == 0 && !xmax - x == 0){ # If you're not at the "corners" of the circle
ma[,,4][y,c(1:(xmax-x), (x+1):xmax)] = 0 # Make pixels on either side of the circle trans.
} else if(!xmax - x == 0) ma[,,4][y,] = 0 # This line makes tops/bottoms transparent
}
return(ma)
}
library(jpeg)
a = readJPEG("http://1.bp.blogspot.com/-KYvXCEvK9T4/Uyv8xyDQnTI/AAAAAAAAHFY/swaAHLS-ql0/s1600/pink-smiley-face-balls-laughing-HD-image-for-faacebook-sharing.jpg")
par(bg = "grey"); plot(1:2, type="n") # Color background to check transparency
rasterImage(circ(a),1,1,2,2)
修改后的版本(感谢@dww):
dwwcirc = function(a){
# First part of the function finds the radius of the circle and crops the image accordingly
xc = floor(dim(a[,,1])[2]/2) # X coordinate of the center
yc = floor(dim(a[,,1])[1]/2) # Y coordinate of the center
r = min(xc, yc) - 1 # Radius is the smaller of the two -1 to avoid reading nonexistent data
ma = array(data = c(a[,,1][(yc-r):(yc+r),(xc-r):(xc+r)], # Read in the cropped image
a[,,2][(yc-r):(yc+r),(xc-r):(xc+r)], # Of dimensions 2r x 2r, centered
a[,,3][(yc-r):(yc+r),(xc-r):(xc+r)], # Around (xc, yc)
rep(1,length(a[,,1][(yc-r):(yc+r),(xc-r):(xc+r)]))), # Add fourth alpha layer
dim = c(length((yc-r):(yc+r)),length((xc-r):(xc+r)),4))
if(yc > xc) yc = xc else if(xc > yc) xc = yc # Re-evaluate your center for the cropped image
xmax = dim(ma[,,1])[2]; ymax = dim(ma[,,1])[1] # Find maximum x and y values
x = rep(1:xmax, ymax) # Vector containing all x values
y = rep(1:ymax, each=xmax) # Value containing all y values
r2 = r^2
ma[,,4][which(( (x-xc)^2 + (y-yc)^2 ) > r2)] = 0
return(ma)
}
library(jpeg)
a = readJPEG("http://1.bp.blogspot.com/-KYvXCEvK9T4/Uyv8xyDQnTI/AAAAAAAAHFY/swaAHLS-ql0/s1600/pink-smiley-face-balls-laughing-HD-image-for-faacebook-sharing.jpg")
par(bg = "grey"); plot(1:2, type="n") # Color background to check transparency
rasterImage(dwwcirc(a),1,1,2,2)
使用magick和plotrix的版本(感谢@lukeA和@hrbrmstr):
library(plotrix)
jpeg(tf <- tempfile(fileext = "jpeg"), 1000, 1000)
par(mar = rep(0,4), yaxs="i", xaxs = "i")
plot(0, type = "n", ylim = c(0, 1), xlim = c(0,1), axes=F, xlab=NA, ylab=NA)
draw.circle(.5,.5,.5,col="black")
dev.off()
library(magick)
img = image_read("http://1.bp.blogspot.com/-KYvXCEvK9T4/Uyv8xyDQnTI/AAAAAAAAHFY/swaAHLS-ql0/s1600/pink-smiley-face-balls-laughing-HD-image-for-faacebook-sharing.jpg")
mask = image_read(tf)
radius = min(c(image_info(img)$width, image_info(img)$height))
mask = image_scale(mask, as.character(radius))
par(bg = "grey"); plot(1:2, type="n")
rasterImage(as.raster(image_composite(image = mask, composite_image = img, operator = "plus")),1,1,2,2)
最佳答案
如果您对数组外的点使用circ，那么对数组执行矢量化的子集分配操作(而不是循环)，则可以提高(x-xc)^2 +(y-yc)^2 > r^2函数的性能。
为此，将函数的第二部分替换为
# Second part of the function traces circle by...
x = rep(1:xmax, ymax)
y = rep(1:ymax, each=xmax)
r2 = r^2
ma[,,4][which(( (x-xc)^2 + (y-yc)^2 ) > r2)] <- 0
return(ma)
关于r - 裁剪图像以圆形的最有效方法(以R为单位)？，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/40066806/