"Bottom Contact Z Distance" bug - Support First Layer Printed Too High

[Miriax]

Description of the bug

The problem is that on my Voron printer profiles, the first layer of supports print 0.52mm above the surface of the part (1,2), at an increased layer height of 0.32mm (3). This leads to the support not sticking to the bottom of the part, which fouls all further support layers that are printed.

What is the expected result? I expect the support to print correctly, like it does with the same part in a Prusa printer profile - 0.4mm above the part (4). Also, on my Voron profile, the support prints in 0.3mm layers for some reason (5).

I have tried all of the layer height and support settings I could find in PrusaSlicer. If I missed something, please let me know.

Bonus bug report: If you set the "Bottom Contact Z Distance" to 0, PrusaSlicer doesn't set corresponding distance to 0mm, it sets it to match the "Top Contact Z Distance" plus the increased support layer (0.32mm).

Playing with the "Bottom Contact Z Distance" in conjunction with the "Top Contact Z Distance," there appears to be a bug in their interaction.
When I set "Top Contact Z Distance" to 0, the "Bottom Contact Z Distance" is overridden to become 0, so the support prints the very next layer after the part surface.

When I set "Top Contact Z Distance" to 0.1, and "Bottom Contact Z Distance" to 0.0, the output Z distance is 0.42mm, and the support prints at the unexpected 0.32mm layer height.

Here is a table of the output Z height that support prints at, as a result of the interaction of these two settings:

	Top Z Contact Distance	0	0.1	0.2	0.3
Bottom Z Contact Distance
0		0	0.42	0.52	0.62
0.1		0	0.42	0.42	0.42
0.2		0	0.52	0.52	0.52
0.3		0	0.62	0.62	0.62
0.4		0	0.72	0.72	0.72

Notice that setting the "Bottom Contact Z Distance" usually has the intended effect, but the increased support layer height of 0.32mm is added. But setting "Bottom Contact Z Distance" to 0 does not have the intended effect - instead it sets the height to the "Top Z Contact Distance" + 0.32mm.

Also, the first layer of support always prints at an increased height of 0.32mm in my Voron profile, which seems undesirable. On a Prusa printer profile, the first layer is actually 0.2mm, with an increased layer height of 0.25mm afterward.

How to Reproduce: Take the project file I supplied and slice it. Adjust the Top and Bottom Contact Z Distances to replicate my results.

Project file & How to reproduce

1

2

3

4

5

Project File: Prusa_Support_Issue.zip

Checklist of files included above

• Project file
• Screenshot

Version of PrusaSlicer

2.9.0

Operating system

Windows 10

Printer model

Voron 2.4 with StealthBurner

[Miriax]

I have been able to make my support printable by setting "Bottom Contact Z Distance" to 0.00001mm. This seems to force PrusaSlicer to use the the near-zero value for "Bottom Contact Z Distance" instead of adding the "Top Contact Z Distance."

	Top Z Contact Distance	0	0.1	0.2	0.3
Bottom Z Contact Distance
0		0	0.42	0.52	0.62
0.00001		0	0.32	0.32	0.32

So in conclusion, I believe there is a bug in the support Z height equations that looks for "Bottom Z Contact Distance" to be 0, and when it is, it substitutes the value "Top Z Contact Distance" into the Z height equations.

[Miriax]

I found this line in GitHub\PrusaSlicer\src\libslic3r\GCode.cpp on Line 366:
double bottom_cd = object.config().support_material_bottom_contact_distance == 0. ? top_cd : object.config().support_material_bottom_contact_distance;

So I am correct, PrusaSlicer's code is inappropriately using 0 as a flag, instead of simply being a zero value. I'm going to submit a pull request to change that flag to -1 and see what happens.

Here's a breakdown for people who don't speak C++:

c++	English
double bottom_cd =	"Create a double-float variable named bottom_cd and make it equal to the result of the following equation:"
object.config().support_material_bottom_contact_distance == 0. ?	"If the config object support_material_bottom_contact_distance is equal to 0..."
top_cd :	"...the result is top_cd. Else..."
object.config().support_material_bottom_contact_distance;	"...the result is the value of the config object support_material_bottom_contact_distance."