Glass Pane Vertical Bar — Cause, Fix and Rationale - song682/CatFrame-Compat GitHub Wiki

Status: fixed on the template side, built into CatFrameCompat 0.0.4 Scope: minecraft:glass_pane rendered through the CatFrame VMM pipeline with RPMCP CTM active

1. Symptom

A full-height, roughly 2 px wide vertical strip of the glass texture appears on a run of glass panes; it disappears when the pane is isolated and it is not the pane's top/bottom plate (glass_pane_top, i.e. the #edge texture) — it is made of the edge pixels of the #pane (texture/blocks/glass.png) artwork. The same scene rendered by a mod that uses the classic pane renderer (GlassPaneRenderer / PaneRenderHelper) does not show it.

2. Current state (what the fix is)

CatFrameCompat now ships a resource pack containing two templates that shadow CatFrame core's templates of the same name inside the mod jar:

src/main/resources/assets/minecraft/models/block/template_glass_pane_side.json

{ "from": [ 7, 0, 0 ], "to": [ 9, 16, 7 ],
  "faces": {
    "down":  { "uv": [  7, 9,  9, 16 ], "texture": "#edge", "cullface": "down" },
    "up":    { "uv": [  7, 0,  9,  7 ], "texture": "#edge", "cullface": "up" },
    "north": { "uv": [  7, 0,  9, 16 ], "texture": "#edge", "cullface": "north" },
    "west":  { "uv": [  0, 0,  7, 16 ], "texture": "#pane" },
    "east":  { "uv": [  9, 0, 16, 16 ], "texture": "#pane" } } }

src/main/resources/assets/minecraft/models/block/template_glass_pane_side_alt.json

{ "from": [ 7, 0, 9 ], "to": [ 9, 16, 16 ],
  "faces": {
    "down":  { "uv": [  7, 9,  9, 16 ], "texture": "#edge", "cullface": "down" },
    "up":    { "uv": [  7, 0,  9,  7 ], "texture": "#edge", "cullface": "up" },
    "south": { "uv": [  7, 0,  9, 16 ], "texture": "#edge", "cullface": "south" },
    "west":  { "uv": [  9, 0, 16, 16 ], "texture": "#pane" },
    "east":  { "uv": [  0, 0,  7, 16 ], "texture": "#pane" } } }

Diff against CatFrame core (tmps/CatFrame-0.8.3-deobf-sources/assets/minecraft/models/block/):

file	face	CatFrame core	CatFrameCompat
`..._side.json`	west	`[16, 0, 9, 16]`	`[0, 0, 7, 16]`
`..._side.json`	down	`[7, 0, 9, 7]`	`[7, 9, 9, 16]`
`..._side.json`	up / down	no cullface	`cullface up` / `cullface down`
`..._side_alt.json`	west	`[7, 0, 0, 16]`	`[9, 0, 16, 16]`
`..._side_alt.json`	down	`[7, 0, 9, 7]`	`[7, 9, 9, 16]`
`..._side_alt.json`	up / down	no cullface	`cullface up` / `cullface down`

Provenance: the west-UV and down-UV values come from the community "glass pane fix" pack (credit: Made By WantedRobot); the up/down cullface entries were added locally afterwards and are cosmetic only (they are not part of the mechanism described below).

No Java code is involved in the fix.

3. Root cause

The pane's flat faces are sampled with the wrong orientation relative to the texture the CTM engine hands us. Two conventions exist and they are mirror images of each other.

3.1 What CatFrame's built-in templates do

The templates (ambientocclusion: false, cuboid [7,0,0]–[9,16,7]) are the modern-vanilla originals, which use local / mirrored arm UVs. With CatFrame's baking convention (JsonModelBake.assignUVForFace: for WEST, s = iz, hence u0 = uv[0] sits at the cuboid's min-z corner and u1 = uv[2] at max-z) this means, on the west-side flat of a north–south run:

side north arm, z ∈ [0, 7/16] → u runs 16 → 9 (i.e. decreases along +z)
centre 2 px (noside, rotated y270) → u runs 9 → 7
side_alt south arm, z ∈ [9/16, 1] → u runs 7 → 0

So the tile's right edge lands on the north junction and its left edge on the next junction: the artwork is laid down mirrored.

3.2 What the CTM icon expects: RPMCP `PaneRenderHelper`

tmps/Right-Proper-MCPatcher-master/.../internal/modules/ctm/PaneRenderHelper.java is the renderer the CTM icon set was authored for. Its constants and emitters state the convention (PaneRenderHelper.png is that class' own Javadoc figure):

PANE_BELOW_UV = 7.0, PANE_ABOVE_UV = 9.0 (L48-49) — the post/cross boundaries
block edges map to u = 0 / 16 (minX/minZ/maxX/maxZ)
renderXNeg(x, zMin, zMax, uMin, uMax, ...) (L437-442) puts uMin at zMin and uMax at zMax, i.e. u increases along the arm
the arm segments are emitted as uMin→uBelow (block edge → post) and uAbove→uMax (post → far block edge) (L409-435)
sides facing the other way use a mirrored icon range via SideIconData.update(icon, negU) (L833-841)

3.3 Point-by-point comparison

sample span	`PaneRenderHelper`	CatFrame core	CatFrameCompat
Z-run, north arm west face	`u` 0 → 7 (`renderXNegMinBelow`)	`[16,0,9,16]` → 16 → 9	`[0,0,7,16]` → 0 → 7
Z-run, south arm west face	`u` 9 → 16 (`renderXNegAboveMax`)	`[7,0,0,16]` → 7 → 0	`[9,0,16,16]` → 9 → 16
X-run, east arm (`side` y90, world north face)	`u` 7 → 0 (`renderZNegAboveMax`)	9 → 16	7 → 0
X-run, west arm (`side_alt` y90, world north face)	`u` 16 → 9 (`renderZNegMinBelow`)	0 → 7	16 → 9
centre 2 px (noside)	7 → 9 (`renderXNegBelowAbove`)	9 → 7	still 9 → 7 (residual)
down face (v axis)	7 / 9 (`TopIconData.vBelow/vAbove`)	`[7,0,9,7]`	`[7,9,9,16]`

Every value the fix introduces equals the PaneRenderHelper requirement; every value CatFrame core shipped is its mirror. That is the entire fix.

3.4 Why a mirrored sampling is visible at all

The icon written into RenderContext#iconOverride is not a free-standing, freely tileable texture. It is one of 47 composed tiles produced by CompactCTMDecoder: 5 base images, each split into 4 quadrants, give 20 quarter-tiles a…t (the CompactCTM.png figure — ab / cd, ef / gh, ij / kl, mn / op, qr / st), and a 47-entry LUT assembles one 16×16 tile per connection state.

Only 5 of the 47 LUT entries are single-source (abcd, efgh, ijkl, mnop, qrst); the other 42 mix quadrants from different base images — entry 1 is ancp, i.e. left half from image 0 and right half from image 3. Therefore the u direction along the arm is semantic: sampling it backwards draws the other half's content, including the border/edge column that belongs at the arm's outer end, across a span whose neighbours are connected. That is the bar.

4. The multipart / rotation factor

The pane is a multipart block and its arms are rotated by the blockstate, which is why one template-side fix is enough to cover all four orientations:

blockstates/glass_pane.json applies arms as north→side, south→side_alt, east→side y90, west→side_alt y90; the nosides as y0 / y90 / y270.
JsonModelBake.applyYRotation (L345-377) documents and implements "keep UVs unchanged": it rotates vertex positions, faceNormal and cullface, then re-winds the quad, so UV rides along rigidly with the geometry.
Consequence, re-derived to confirm it: on an east–west run the east arm (side y90) covers x ∈ [9/16, 1] and the fixed UV yields u 7 → 0, equal to renderZNegAboveMax; the west arm (side_alt y90) yields u 16 → 9, equal to renderZNegMinBelow. Both orientations match, so the fix does not trade one axis for the other.
Residual: the pieces that actually get rotated are the nosides (the centre 2 px). They still carry CatFrame's mirrored orientation, so the panel is not 100 % aligned to PaneRenderHelper — currently invisible because the composed icon's centre columns carry no artwork, but it is the first suspect if a seam ever appears exactly in the middle of a pane.

5. Why this approach was chosen

A simple, basic, containable change was preferred over an engine-side one:

Java untouched — no engine-side re-normalisation of pane side-face UVs, therefore none of the copy-on-write hazard that comes with mutating BakedQuads sourced from a shared bake cache.
The existing iconOverride plumbing (RpmcpRenderExtension) already delivers the right icon per face (NORTH→ZNeg, SOUTH→ZPos, WEST→XNeg, EAST→XPos); only the template's sampling convention had to be brought in line with the icon's composer.
The earlier CTM research document had already reached the same conclusion for route 1 ("usable immediately, no development needed") and parked route 2 (engine-side linear re-normalisation of q.up/q.vp) as P3.
Rejected alternative: patch apply/CtmRenderExtension to rescale pane side-face UVs at render time — more capable, more risk, and unnecessary while the built-in template covers it.

6. Verification

Controlled-variable experiments, single variable = the arm west UV.

group	setup	result
A	west face reverted to CatFrame core values (`[16,0,9,16]` / `[7,0,0,16]`), cullface and down unchanged	the bar comes back ⇒ the west UV is the cause
B	all external resource packs disabled, only the built-in CatFrameCompat 0.0.4 jar	correct ⇒ the mod jar's `builtin_packs/glass_pane_fix/assets/minecraft/models/block` does override CatFrame core's same-named templates

Build: gradlew build succeeds and the 0.0.4 jars contain both templates.

7. A correction to earlier reasoning (do not repeat)

An earlier derivation concluded that the two UV variants were "mathematically equivalent — the tile's two edge columns just swap left/right at the junction" and therefore that the west UV could not be the culprit. That was wrong; experiment A refutes it. The error was modelling the CTM icon as a symmetric, freely tileable texture, which §3.4 shows it is not.

8. Open items

.gitignore: the bare minecraft pattern is gone (# minecraft, then .minecraft, eclipse, run), so src/main/resources/assets/minecraft/** is trackable without git add -f. If a root-level run directory named minecraft/ ever reappears, prefer the rooted pattern /minecraft/ rather than restoring the unrooted one.
Optional pixel-level dump: compose the LUT entry for the observed connection state out of the 5 base images and print its columns, to close out "which column lands where" exactly.
Corner/edge variants (ANCP vs MNOP) remain an acknowledged limitation (README): CatFrame's generic neighbour detection does not use canPaneConnectToBlock, so some corner states pick a different composite. Related to, but distinct from, the direction issue fixed here.

9. Anchors

CatFrame core templates: tmps/CatFrame-0.8.3-deobf-sources/assets/minecraft/models/block/template_glass_pane_{post,side,side_alt,noside,noside_alt}.json and .../blockstates/glass_pane.json
Baking: .../decok/dfcdvadstf/catframe/model/core/baking/JsonModelBake.java — assignUVForFace (WEST: s = iz), applyYRotation (L345-377), recalculateWinding (L516-552)
RPMCP: .../modules/ctm/PaneRenderHelper.java (L45-49 constants, L92-106 mask, L138-166 icon query, L409-551 quad emitters, L824-862 icon data) and .../modules/ctm/CompactCTMDecoder.java (L56-104 LUT + composition)
CatFrameCompat: src/main/java/decok/dfcdvadstf/catframe/compact/mcpatcher/RpmcpRenderExtension.java