Shader graph: Clamp is the Output's default edge, and the sheet's own mode is a constant of its own (#81)

The default of the Edge param and the address mode the shared sheet physically
carries were the same constant; changing the value would have turned every
sentinel with it and given each shader sprite a full copy of its texture.
SHEET_EDGE ('mirror', what TextureLibrary uploads) is now separate from
DEFAULT_EDGE ('clamp'). A graph whose Sample reads nothing but its own pixel
draws with the plain sheet whatever the Edge says, so the common shader costs
no second copy.

Without a shader the quad's coordinates never leave 0..1, and mirror-repeat
reads like clamp within a texel - so clamp is what a sprite shows anyway.

src/core/shader/compile.ts

@@ -10,7 +10,9 @@ import {
   floatLiteral,
   isShaderNodeDef,
   SHADER_OUTPUT,
+  SHADER_SAMPLE,
   SHADER_TIME,
+  SHEET_EDGE,
   STRAIGHT_FN,
   TEXEL_ALPHA,
   TEXEL_COLOR,
@@ -29,7 +31,10 @@ export interface CompiledShader {
   uniforms: ShaderUniform[];
   /** A Time node feeds the Output: the fragment takes `uTime`, and a sprite wearing it redraws every tick. */
   time: boolean;
-  /** How the picture continues past its border – the texture variant the sprite's mesh draws with. */
+  /**
+   * The texture variant the sprite's mesh draws with – the Output's Edge where the graph samples
+   * past the border, and the plain sheet where it cannot.
+   */
   edge: EdgeMode;
 }
 
@@ -177,5 +182,14 @@ export function compileShader(graph: Graph): ShaderBuild {
     '}',
     '',
   ].join('\n');
-  return { compiled: { fragment, uniforms, time, edge: edgeOf(output.params) }, problems, reason: '' };
+  // The edge only shows where a Sample reads somewhere other than its own pixel. A graph that never
+  // shifts a UV stays on the plain sheet, so the common shader costs no second copy of the picture.
+  const shifted = graph.nodes.some(
+    (n) =>
+      n.type === SHADER_SAMPLE &&
+      emitted.has(n.id) &&
+      (edgesInto(graph, n.id, 'u').length > 0 || edgesInto(graph, n.id, 'v').length > 0),
+  );
+  const edge = shifted ? edgeOf(output.params) : SHEET_EDGE;
+  return { compiled: { fragment, uniforms, time, edge }, problems, reason: '' };
 }

src/core/shader/nodes.ts

@@ -56,16 +56,22 @@ export const STRAIGHT_FN = 'vec3 straight(vec4 c) { return c.a > 0.0 ? c.rgb / c
 
 /**
  * How the picture continues past its edge when a `Sample texture` reads outside 0..1 – a setting
- * of the Output node, because the sampler state is one per draw. Mirror is what every picture has
- * anyway (`TextureLibrary`); repeat is for tiles.
+ * of the Output node, because the sampler state is one per draw.
  */
 export type EdgeMode = 'repeat' | 'mirror' | 'clamp';
 export const EDGE_PARAM = 'edge';
-export const DEFAULT_EDGE: EdgeMode = 'mirror';
+/**
+ * The address mode the shared sheet carries anyway (`TextureLibrary`): within a texel it reads
+ * like clamp, and no other mode can have it without a cut of its own. Asking for it costs nothing,
+ * so it is what a sprite without a shader draws with.
+ */
+export const SHEET_EDGE: EdgeMode = 'mirror';
+/** Clamp: the border texel goes on, which is what a sprite without a shader shows (#81). */
+export const DEFAULT_EDGE: EdgeMode = 'clamp';
 const EDGE_OPTIONS: { value: EdgeMode; label: string }[] = [
+  { value: 'clamp', label: 'Clamp' },
   { value: 'mirror', label: 'Mirror' },
   { value: 'repeat', label: 'Repeat' },
-  { value: 'clamp', label: 'Clamp' },
 ];
 
 export function edgeOf(params: PropertyBag): EdgeMode {