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IDProjectCategoryView StatusLast Update
0004516The Dark ModCodingpublic20.12.2018 05:01
Reporterstgatilov Assigned Tostgatilov  
Status resolvedResolutionfixed 
Product VersionSVN 
Target VersionTDM 2.06Fixed in VersionTDM 2.06 
Summary0004516: Unstable visportals clipping
DescriptionThe original code for clipping visportals contains instability. It may cause the shape of clipping plane to change randomly. As a result, some items may become invisible (e.g. scissored out).
Steps To ReproduceI have noticed a visual glitch in "Inn Business" mission, in the tavern room (you get into there if you enter through the "front door"), when looking into the inn and stairs.

The glitch is visible on the attached image "unstable_visportals_setup.png".
The image "tavern_no_glitch.png" is taken from the same position without looking glass, but it does NOT have this issue.

TagsNo tags attached.


related to 0004127 resolvedstgatilov DrawSurf scissor rect invalid 
related to 0004829 resolved Frobbing a just-closed door just after closing causes a problem 




30.04.2017 15:51




30.04.2017 16:09

administrator   ~0008815

In order to get through a sequence of portals, the following things are done:
1. The set of frustum planes is taken as initial "portal planes".
2. The portal polygon (i.e. "winding") is considered.
3. It is clipped against all the portal planes, obtaining smaller polygon (winding).
4. New set of portal planes is created from the clipped winding: each pair of subsequent generates a new plane (passing through eye).
5. Go to step 2 with new portal.

The problem here is that both step 3 (clipping) and step 4 (generating planes) have no tolerance (aka "epsilon") in the checks.
In other words, after clipping the winding can contain arbitrarily close adjacent points. When a portal plane is generated passing through such pair of close points, it is not checked properly if these two vectors are colinear or almost colinear.

So the problem happens like this. Imagine that a portal plane goes through a vertex of the winding (or very close to it). Then the clipping routine produces two equal or almost equal vertices. When we generate a plane going through them, its normal is very unstable: its direction depends on luck and behavior of floating point errors and can be arbitrary. It can for instance cut the real winding into halves, thus incorrectly reducing the view visible through the portals. This is then taken into account when screen rectangle is calculated: it becomes less than it must be. As a result, scissor test culls out everything outside this erratic rectangle.


30.04.2017 16:27

administrator   ~0008816

In order to fix the issue, we need to check whether the normal of new portal plane is stably defined. Which is equivalent to having angle between two adjacent points of the winding not too small. If the angle is very small, we should simply drop the portal plane (dropping planes is always OK).

Currently I added a check if the angle is less than 1.19e-4. If it is greater, than the plane's normal should have less than 1e-3 relative error after calculation, which should be OK. The angle threshold corresponds to about a half of arcminute, which is usually less than a pixel on player's screen.


30.04.2017 16:32

administrator   ~0008817

One important note is that such a issue can happen only when the visportals in a sequence are not in general position in the player's view.

For instance, if you have many visportals sharing a common edge (or almost sharing), then such a problem is very likely to happen. Other case is when an edge of visportal lies on another visportal, with its endpoints being on the edges.
If any vertex of any visportal in a sequence does NOT concide with another portal, then the issue is very unlikely to happen.


30.04.2017 16:48


tavern_no_glitch.png (999,499 bytes)


01.05.2017 01:15

developer   ~0008818

Fixed per stgatilov

Rev 6840
Binary 14784

(Confirmed by me.)

Issue History

Date Modified Username Field Change
30.04.2017 15:51 stgatilov New Issue
30.04.2017 15:51 stgatilov Status new => assigned
30.04.2017 15:51 stgatilov Assigned To => stgatilov
30.04.2017 15:51 stgatilov File Added: unstable_visportals_setup.png
30.04.2017 16:09 stgatilov Note Added: 0008815
30.04.2017 16:27 stgatilov Note Added: 0008816
30.04.2017 16:32 stgatilov Note Added: 0008817
30.04.2017 16:48 stgatilov File Added: tavern_no_glitch.png
30.04.2017 16:50 stgatilov Steps to Reproduce Updated View Revisions
01.05.2017 01:15 nbohr1more Note Added: 0008818
01.05.2017 01:16 nbohr1more Status assigned => resolved
01.05.2017 01:16 nbohr1more Resolution open => fixed
01.05.2017 01:16 nbohr1more OS => ALL
01.05.2017 01:16 nbohr1more Fixed in Version => TDM 2.06
15.09.2017 02:47 nbohr1more Relationship added related to 0004127
20.12.2018 05:01 nbohr1more Relationship added related to 0004829