diff --git a/Architecture.md b/Architecture.md
index 2298da5..a4beffb 100644
--- a/Architecture.md
+++ b/Architecture.md
@@ -179,6 +179,11 @@ src/editor/tile/        TileEditor (header + kind), TileCanvas (every tile of th
 src/editor/statemachine/ StateMachineEditor (header, on the graph canvas)
 src/editor/particles/  ParticlesEditor (graph canvas left of a preview panel, splitter between them),
                        ParticlePreview (the panel: the asset's own particles, wearing its own rows)
+src/editor/shader/     ShaderEditor (graph canvas, preview panel with a slider per property, #168),
+                       ShaderPreview (one sprite mesh wearing the shader through syncShader, own clock),
+                       previewPicture (which selection has a picture, what it shows, slider start values,
+                       the built-in test image)
+src/editor/PreviewSplit.ts  graph left, preview panel right, the splitter between them (Particles, Shader)
 src/editor/list/       ListEditor (a row per item), slot (which list is being edited, and how it is written back)
 src/editor/console/    log (ring buffer, argument formatting, the console.* capture), ConsolePanel (the tail under the tabs)
 api/                   PHP backend: index.php (routing and the sign-in gate),
diff --git a/Development.md b/Development.md
index fcd0490..28cbc54 100644
--- a/Development.md
+++ b/Development.md
@@ -13,7 +13,7 @@ npm run check        # typecheck + lint + format:check + tests, the gate CI runs
 npm run build        # writes dist/ and regenerates index.html
 ./build.sh           # the same under `set -o pipefail`, for callers that pipe the output
 php tools/make-user.php <name> <password>                     # an account; every tool signs in (see Accounts)
-node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1202 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1209 editor checks, screenshots in /tmp/gm-e2e-*.png
 GM_STEPS=20 node tools/e2e.mjs                                # or 1-12, or 6,20 — a subset while working on one step
 ./e2e.sh start [GM_STEPS]                                     # the same run detached, see below
 ./e2e.sh status                                               # live progress, or the last run's result
diff --git a/Shaders.md b/Shaders.md
index 48114db..2a18d2e 100644
--- a/Shaders.md
+++ b/Shaders.md
@@ -17,6 +17,39 @@ and a red frame.
 A shader graph is compiled on every change. A sprite wearing a graph that does not compile is drawn
 plain, as if it wore none.
 
+### The preview
+
+Right of the graph, behind a splitter you can drag like the one in the Particles tab, the tab shows
+one picture wearing the shader. The picture is **whatever you selected last that has one**:
+
+| Selected last | The preview shows |
+|---|---|
+| a placed sprite | that sprite, with its overrides: its picture, its width and height, its colour |
+| a sprite asset | the sprite as the asset describes it |
+| a texture | the whole sheet |
+| a frame (Texture tab) | that frame |
+| nothing of the above yet | a built-in **test image** |
+
+Selecting the shader itself, one of its nodes or anything without a picture (a behavior, a scene)
+keeps the picture that is there. The line below the buttons names it. The picture is looked up again
+on every change, so an edit in the inspector shows at once. Once it is deleted, the test image comes
+back. The test image is 64 pixels square: colour runs from left to right and from top to bottom, a
+checker of 8-pixel cells shows where a UV lands, and the outer 8 pixels fade out so an alpha effect
+has something to work on.
+
+Below the picture there is **a slider for every property** the shader declares. The sliders belong
+to the preview: moving one never writes into the project, not into the sprite and not into the
+Property node. They start at the sprite's own values when the picture is a sprite that already wears
+this shader, otherwise at the property's default. They reach from the property's `min` to its `max`.
+A property without a `max` gets twice its default's distance from the `min`.
+
+A shader with a **Time** node runs on the preview's own clock. `Hold` stops it at the second it shows
+(the button then reads `Run`), `Restart` goes back to 0. Without a Time node both buttons are greyed
+out, because nothing in the picture moves.
+
+The preview draws the picture unsliced, even a nine-sliced frame: it is about what the shader does to
+the pixels, not about how a sprite stretches.
+
 ## The nodes
 
 Every value is either a **number** (a GLSL `float`) or a **colour** (a `vec3`, RGB in 0-1). Ports