# A Note on Transparency In rgl: 3D Visualization Using OpenGL

if (!requireNamespace("rmarkdown") || !rmarkdown::pandoc_available("1.14")) {
warning(call. = FALSE, "These vignettes assume rmarkdown and pandoc version 1.14.  These were not found. Older versions will not work.")
knitr::knit_exit()
}

## Example

The classic example of an impossible to sort scene involves three triangles arranged cyclicly so each one is behind one and in front of one of the others (based on https://paroj.github.io/gltut/Positioning/Tut05%20Overlap%20and%20Depth%20Buffering.html).

theta <- 2*pi*c(0:2, 4:6, 8:10)/12
x <- cos(theta)
y <- sin(theta)
z <- rep(c(0,0,1), 3)
xyz <- cbind(x, y, z)
xyz <- xyz[c(1,2,6, 4,5,9, 7,8,3),]
open3d()
par3d(userMatrix = M)
triangles3d(xyz, col = rep(c("red", "green", "blue"), each = 3))


To see the effect of the calculations above, consider the following four displays.

open3d()
par3d(userMatrix = M)
layout3d(matrix(1:9, ncol = 3, byrow=TRUE),
widths = c(1,2,2), heights = c(1, 3,3),
sharedMouse = TRUE)
text3d(0,0,0, " ")
next3d()
next3d()
next3d()
text3d(0,0,0, "alpha = 0.7")
next3d()
triangles3d(xyz, col = rep(c("red", "green", "blue"), each = 3), alpha = 0.7, depth_mask = TRUE)
next3d()
triangles3d(xyz, col = rep(c("red", "green", "blue"), each = 3), alpha = 0.7, depth_mask = FALSE)
next3d()
text3d(0,0,0, "alpha = 0.3")
next3d()
triangles3d(xyz, col = rep(c("red", "green", "blue"), each = 3), alpha = 0.3, depth_mask = TRUE)
next3d()
triangles3d(xyz, col = rep(c("red", "green", "blue"), each = 3), alpha = 0.3, depth_mask = FALSE)


As you rotate the figures, you can see imperfections in rendering. On the right, the last drawn appears to be on top, while on the left, the first drawn appears more opaque than it should.

In the figure below, the three triangles each have different transparency, and each use the recommended setting:

open3d()
par3d(userMatrix = M)
triangles3d(xyz[1:3,], col = "red", alpha = 0.3, depth_mask = FALSE)
triangles3d(xyz[4:6,], col = "green", alpha = 0.7, depth_mask = TRUE)
triangles3d(xyz[7:9,], col = "blue", depth_mask = TRUE)


In this figure, all three triangles are the same colour, only lighting affects the display:

open3d()
par3d(userMatrix = M)
layout3d(matrix(1:9, ncol = 3, byrow=TRUE),
widths = c(1,2,2), heights = c(1, 3,3),
sharedMouse = TRUE)
text3d(0,0,0, " ")
next3d()
next3d()
next3d()
text3d(0,0,0, "alpha = 0.7")
next3d()
triangles3d(xyz, col = "red", alpha = 0.7, depth_mask = TRUE)
next3d()
triangles3d(xyz, col = "red", alpha = 0.7, depth_mask = FALSE)
next3d()
text3d(0,0,0, "alpha = 0.3")
next3d()
triangles3d(xyz, col = "red", alpha = 0.3, depth_mask = TRUE)
next3d()
triangles3d(xyz, col = "red", alpha = 0.3, depth_mask = FALSE)


Here depth_mask = FALSE seems to be the right choice in both cases.

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rgl documentation built on March 5, 2021, 1:07 a.m.