[AlexMelanFromRingo]

The README hopes the community takes on ports to newer versions; this is a port of the mod, its installer, its developer tools and its docs to Minecraft Java Edition 26.3. Whether 26.x should replace 1.21.1 on main or live on its own branch is the maintainers' call; the installer has not been run end to end yet (details below).

This branch builds on #78 (migrate from Yarn to Mojang mappings on 1.21.1) and contains its commit; the review that matters here is the last two commits. It is rebased on main after #77, #61 and #70; #70's new ServerLightSnapshot tests are ported to Mojang names and to 26.3, and the CHANGELOG keeps #61's entry.

Port the renderer to Minecraft 26.3

Minecraft 26.x ships unobfuscated, renders through renderpearl and SDL3, and draws the world through an extract and submit pipeline, so most of the work is a port of APIs rather than names.

• Build: Fabric Loom 1.19 without mappings or a refmap, Fabric Loader 0.20.1, Fabric API 0.165.0+26.3, Java 25 (also in build.yml), access widener in the official namespace.
• Frame and input: the OpenGL backend is forced (GL 4.5 core), since Remix interop imports its image into GL. Presentation hooks GlSurface, the overlay input gate follows SDL events, and the Remix image is blitted into the main target with DSA.
• Terrain: sections are captured from SectionCompiler through RenderSectionRegion, from BlockStateModel parts and the BakedQuad record, whose layer is now per quad. Visibility comes from LevelExtractor, so the 1.21.1 visibility-walk port is removed.
• Entities: RemixSubmitCollector evaluates SubmitNodeCollector submits into the Remix capture in place of vanilla's feature renderers, for entities, block entities, first-person hands and items, and the breaking crack. The Copper Beacon renderer follows the new BlockEntityRenderer API.
• Textures: glyph pages are mirrored from command-encoder writes; animated sprites are forwarded from the animation tick, since 26.x animates them on the GPU. Every CPU atlas copy honours and fills the sprite padding 26.x adds, and the Copper Beacon lens textures are 48 pixels square so they no longer cap the block atlas at one mip level.
• Content: block lights and flame emission for the copper torch and copper lanterns; the Copper Beacon recipe, loot table and item definition use the 26.x data formats.
• Found in game: the atmospheric sky readback sets a tight pack state, because vanilla now leaves GL_PACK_ROW_LENGTH set (about 20 GL errors a second otherwise); the block-entity atlas warm-up targets the chest atlas, since beds are block models in 26.x.

Mixin targets are no longer checked at compile time on 26.x. Every mixin was checked against the 26.3 client JAR at the bytecode level: target methods, @At call sites present in the target method, exact @Shadow, @Accessor and @Invoker descriptors, and injector handler signatures (67 mixins, 148 injection targets).

Move the installer, developer tools and docs to Minecraft 26.3

• Installer: pins Minecraft 26.3, Fabric Loader 0.20.1, Fabric API 0.165.0+26.3 and a new GAME_JAVA_MAJOR of 25. The managed JDK is JDK 25: Adoptium Temurin on x64, and the Microsoft Build of OpenJDK on ARM64, where Adoptium builds no JDK 25, verified against the SHA-256 file Microsoft publishes beside it. The Microsoft launcher's java-runtime-epsilon is probed first. Upgrade moves a Prism instance an earlier release set up for 1.21.1 to the pinned versions and warns that its worlds upgrade when they next open. Fabric installer 1.0.1 still produces the 26.3 profile (checked headless), so it stays pinned.
• Developer tools: require a JDK 25, the only major Prism accepts for 26.3, kept in step with the installer by a test; the diagnostic tools match a game window of any version, not only a 1.x title.
• Zones: a 1.21.1 save's data/rtx_remix_zones.dat, which 26.x's own world upgrade leaves in place, is copied into the current file on first load, and the state lives in the server's save-wide storage so that file is data/rtx-remix-renderer/zones.dat.
• Checks the port had broken: the access widener carries its SPDX header again, fabric.mod.json pins the exact Minecraft version, and the build-configuration test reads the Loom plugin id 26.x uses.
• Docs: player, installer, developer and agent docs name 26.3, Java 25 and the 26.3 classes, events and file locations, and drop the removed visibility-walk switches.

How it was verified

• gradlew.bat build test: 769 tests pass. The suite is smaller than on 1.21.1 because the visibility-walk tests went with the code they tested; new tests cover the atlas padding fill, the lens textures and the 1.21.1 zone migration.
• tools\rtx.bat check --tests: ruff, ty, 1137 Python tests and the public-boundary gate pass.
• installer.ps1 parses in Windows PowerShell 5.1 and PowerShell 7.
• In game on 26.3 through a Prism instance: world load and upgrade of a 1.21.1 world, terrain near and far, block lights, sky by day and night, clouds, water, entities and mobs, particles, held and thrown items, and the Copper Beacon.

Not verified

• The installer has not been run end to end, neither a fresh install nor an upgrade of a 1.21.1 instance, and the ARM64 path has not run on ARM64 hardware.
• The zone migration is covered by unit tests on a real 1.21.1-layout file, not yet by loading such a world in game.
• The redstone torch was redesigned in 26.x with an opaque red shell around its head; it renders as vanilla's geometry, which reads as a glowing cube under path tracing.

🤖 Generated with Claude Code

[sambvfx]

Thanks for this, and for splitting the mappings change out into #78 so each can be reviewed on its own.

To answer your question directly: 26.3 won't replace 1.21.1 on main. If we support more than one version, we don't want two branches that drift apart. Before taking a port, we want to settle how one repository builds and tests more than one game version and loader, so a change doesn't have to be tested by hand in each. This PR is the most complete picture we have of what 26.x needs, including the extract and submit pipeline, SDL3 input, the forced GL backend and Java 25, so it's very useful for that.

The exact set of versions we'll support is still being decided, and it will be chosen based on community interest. If you'd like to weigh in, the RTX Remix Discord has a thread for it: https://discord.com/channels/1028444667789967381/1565060125301936169

Please keep the branch, and feel free to switch it to a draft for now.