volume shows objects from behind the camera

[hsgg]

Versions: Makie v0.23.0, GLMakie v0.12.0

Can be reproduced with a fresh environment, --startup-file=no.

I see the same on MacOS M1, Fedora Intel integrated GPU, and Arch Linux AMD Radeon 7600 XT.

Inside the volume, objects from behind are shown.
Here is a minimal example:

using GLMakie
vol = rand(1000,50,50);
fig, scene, _ = volume(vol)

controls = cameracontrols(scene)

# position the camera at the long end:
controls.eyeposition[] = Vec3f(1300, 25, 25)
controls.lookat[] = Point3f(1200, 25, 25)
update_cam!(scene.scene)

Next, press w to enter the volume. Once inside notice how elements from behind seem to get mirrored across the direction of view, creating an effect where part of the volume is moving past you from behind:

Screencast.From.2025-06-16.21-44-01.mp4

This was previously discussed here.

[ffreyer]

I'm guessing the rays we produce in

Makie.jl/GLMakie/assets/shader/volume.frag

Lines 348 to 364 in f654487

	vec3 eye_unit = vec3(modelinv * vec4(eyeposition, 1));
	vec3 back_position = vec3(modelinv * vec4(frag_vert, 1));
	vec3 dir = normalize(eye_unit - back_position);
	// solve back_position + distance * dir == 1
	// solve back_position + distance * dir == 0
	// to see where it first hits unit cube!
	vec3 solution_1 = (1.0 - back_position) / dir;
	vec3 solution_0 = (0.0 - back_position) / dir;
	float solution = min_bigger_0(solution_1, solution_0);
	vec3 start = back_position + solution * dir;
	// if completely clipped discard this ray tracing attempt
	if (process_clip_planes(start, back_position))
	discard;
	vec3 step_in_dir = (back_position - start) / num_samples;

are only clamped to the volume box, and not also to the view box

[hsgg]

I have managed to produce a fix for this in my fork: hsgg@0b95b5d. This is my first attempt at GLSL programming, and first time contributing to Makie, so feedback is more than welcome.

I have not made a PR, yet, because I have questions:

1. How to/Should I add a unit test?
2. No changelog entry.

[SimonDanisch]

Great! :) Feel free to make a PR, that's easier to review and help you with reference tests etc.