[BUG] - wide seams along intersections and edges with dual depth peeling

[mqnc]

Describe the bug.
There are large seams and gaps where surfaces intersect or meet when using dual depth peeling. These only start to appear at a certain zoom level, you have to zoom in a little bit. I think they are too large and too regular-looking to be explained by some z fighting or depth resolution issues.

Tell us how to reproduce the bug.

#include <easy3d/util/initializer.h>
#include <easy3d/viewer/viewer.h>
#include <easy3d/renderer/drawable_triangles.h>
#include <easy3d/renderer/dual_depth_peeling.h>

using namespace easy3d;

class TransparencyViewer: public easy3d::Viewer {
public:
	explicit TransparencyViewer(const std::string& title = ""): Viewer(title) {
		transparency_ = new DualDepthPeeling(camera());
	}
	~TransparencyViewer() override {
		delete transparency_;
	}

protected:
	void draw() const override {
		std::vector<TrianglesDrawable*> surfaces;
		for (auto d: drawables_) {
			surfaces.push_back(dynamic_cast<TrianglesDrawable*>(d.get()));
		}
		transparency_->draw(surfaces);
	}

private:
	easy3d::DualDepthPeeling* transparency_;
};


std::shared_ptr<TrianglesDrawable> make_cube(const vec3& offset) {
	std::vector<vec3> vertices = {
		{-1, -1, -1}, {1, -1, -1}, {1, 1, -1}, {-1, 1, -1},
		{-1, -1, 1}, {1, -1, 1}, {1, 1, 1}, {-1, 1, 1}
	};
	for (auto& v: vertices) v += offset;

	std::vector<unsigned int> indices = {
		0, 1, 2, 0, 2, 3,
		1, 5, 6, 1, 6, 2,
		5, 4, 7, 5, 7, 6,
		4, 0, 3, 4, 3, 7,
		3, 2, 6, 3, 6, 7,
		4, 5, 1, 4, 1, 0
	};

	auto d = std::make_shared<TrianglesDrawable>("cube");
	d->update_vertex_buffer(vertices);
	d->update_element_buffer(indices);

	return d;
}

int main(int argc, char** argv) {
	initialize();

	TransparencyViewer viewer("Two Transparent Cubes");

	viewer.add_drawable(make_cube(vec3(0, 0, 0)));
	viewer.add_drawable(make_cube(vec3(0.5f, 0.5f, 0.5f)));

	viewer.run();
}

Development/Running environment:

• OS: kubuntu 24.04
• Compiler G++ 13.3.0
• Release

[LiangliangNan]

The issue comes from here:

Easy3D/resources/shaders/transparency/dual_depth_peeling_peel_rendering.frag

Line 129 in 8cf25f4

normal = normalize(cross(dFdx(DataIn.position), dFdy(DataIn.position)));

The fragment normals are computed on the fly (so it avoids computing face normals and uploading them to the GPU), and it seems these normals are not quite accurate.

To resolve the artifacts near sharp corners, compute the face normals in the CPU and upload them to the GPU.
This requires duplicating the vertices (a vertex might be shared by multiple adjacent faces), so it will consume more GPU memory.