[Octave-bug-tracker] [bug #39535] FaceAlpha property not working for OpenGL backend

[Markus Mützel]

Update of bug #39535 (project octave):

                  Status:               Confirmed => Patch Submitted        
        Operating System:               GNU/Linux => Any                    

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Follow-up Comment #5:

Attached is a patch that adds some very primitive support for alpha on faces
of patch objects. It does not include any sorting primitives. However, it
might give better results than the current implementation (which hides patches
with alpha != 0) in very simple cases as the ones from bug #50520:

graphics_toolkit("qt")
figure
rectangle("position", [0.2 0.2 0.5 0.4], "facecolor", "g", "FaceAlpha",.3)
rectangle("position", [0.3 0.1 0.3 0.6], "facecolor", "y", "FaceAlpha",.3)
axis([0 1 0 1])


or bug #46538:

T =[
1 2 4
2 3 5
4 5 7
5 6 8
5 4 2
6 5 3
8 7 5
9 8 6];

P = [
0.00000 0.00000 0.00000 0.50000 0.50000 0.50000 1.00000 1.00000 1.00000
0.00000 0.50000 1.00000 0.00000 0.50000 1.00000 0.00000 0.50000 1.00000];

x = P(1,:); y = P(2,:);
z = 1 - (x-0.5).^2 - (y-0.5).^2;
i=1:length(T);

a1 = [ x(T(i,1)) ; y(T(i,1)) ; z(T(i,1)) ];
a2 = [ x(T(i,2)) ; y(T(i,2)) ; z(T(i,2)) ];
a3 = [ x(T(i,3)) ; y(T(i,3)) ; z(T(i,3)) ];

s = ( a1(1:2,i) + a2(1:2,i) + a3(1:2,i) )./3;

r1 = a3(:,i) - a1(:,i);
r2 = a2(:,i) - a1(:,i);
n = cross(r1,r2);

dx = n(1,:)./n(3,:);
%dy = n(2,:)./n(3,:);

patch('Faces',T,'Vertices',P','FaceVertexCData',dx','FaceColor','flat','FaceAlpha',.8);


or the one in comment #0.

Since no sorting primitives are included, it fails with more complex layouts
as in:

[x,y,z] = meshgrid (-2:0.2:2, -2:0.2:2, -2:0.2:2);
val = x.^2 + y.^2 + z.^2;
fv1 = isosurface (x, y, z, val, .6);
patch (fv1, "FaceAlpha", .3, "FaceColor", "r", "EdgeColor", "none");
fv2 = isosurface (x, y, z, val, 1);
patch (fv2, "FaceAlpha", .3, "FaceColor", "g", "EdgeColor", "none");


It also only adds support for double valued "FaceAlpha". "flat" or "interp"
skip rendering the faces (like without the patch).

Even if this is by far no complete implementation of transparency, this very
limited support might be helpful in some use cases.

(file #39977)
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Additional Item Attachment:

File name: bug39535_patch_facealpha.patch Size:2 KB


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  <http://savannah.gnu.org/bugs/?39535>

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