Route Canvas erase setup through helper

docs/modernization/debt.md

@@ -18,6 +18,11 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Recent Reductions
 
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::stroke_commit` and
+  `Canvas::draw_merge` retained `CompErase` shader setup now route through
+  `legacy_canvas_stroke_erase_services.h`; Canvas still owns texture binding,
+  mask RTT binding, dirty/layer mutation, framebuffer feedback, and draw
+  execution.
 - 2026-06-13: DEBT-0036 was narrowed again. `NodeCanvas` live temporary-erase
   `CompErase` shader setup now routes through
   `legacy_canvas_stroke_erase_services.h`; the retained node path still owns

docs/modernization/roadmap.md

@@ -3009,6 +3009,10 @@ Results:
   shader setup helper for `CompErase` uniforms, while keeping per-onion-frame
   alpha updates, texture binding, temporary erase texture selection, and draw
   execution in the retained node path.
+- `Canvas::stroke_commit` and `Canvas::draw_merge` retained erase compositing
+  now share the stroke erase shader setup helper, while texture binding, mask
+  RTT binding, dirty/layer mutation, framebuffer feedback, and draw execution
+  remain in retained Canvas code.
 - `Canvas::stroke_draw_mix` now shares the retained stroke composite shader
   helper for mixer-pass `CompDraw` setup, while preserving its caller-specific
   texture slot uniforms. Mixer framebuffer/scissor state, sampler and texture

src/canvas.cpp

@@ -3,6 +3,7 @@
 #include "canvas.h"
 #include "app.h"
 #include "legacy_canvas_draw_merge_services.h"
+#include "legacy_canvas_stroke_erase_services.h"
 #include "legacy_gl_renderbuffer_dispatch.h"
 #include "legacy_canvas_stroke_commit_services.h"
 #include "legacy_canvas_stroke_composite_services.h"
@@ -1124,13 +1125,15 @@ void Canvas::stroke_commit()
                     m_sampler_stencil.bind(4);
                 },
                 .execute_erase_composite = [&](int i) {
-                    ShaderManager::use(kShader::CompErase);
-                    ShaderManager::u_int(kShaderUniform::Tex, 0);
-                    ShaderManager::u_int(kShaderUniform::TexStroke, 1);
-                    ShaderManager::u_int(kShaderUniform::TexMask, 2);
-                    ShaderManager::u_int(kShaderUniform::Mask, m_smask_active);
-                    ShaderManager::u_float(kShaderUniform::Alpha, 1);
-                    ShaderManager::u_mat4(kShaderUniform::MVP, glm::ortho(-.5f, .5f, -.5f, .5f, -1.f, 1.f));
+                    pp::panopainter::setup_legacy_stroke_erase_shader(
+                        pp::panopainter::LegacyStrokeEraseUniforms {
+                            .mvp = glm::ortho(-.5f, .5f, -.5f, .5f, -1.f, 1.f),
+                            .texture_slot = 0,
+                            .stroke_texture_slot = 1,
+                            .mask_texture_slot = 2,
+                            .alpha = 1.0f,
+                            .mask_enabled = m_smask_active,
+                        });
 
                     set_active_texture_unit(0);
                     m_tex2[i].bind();
@@ -1302,15 +1305,17 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
                 m_sampler.bind(1);
                 m_sampler.bind(2);
 
-                ShaderManager::use(kShader::CompErase);
-                ShaderManager::u_int(kShaderUniform::Tex, 0);
-                ShaderManager::u_int(kShaderUniform::TexStroke, 1);
-                ShaderManager::u_int(kShaderUniform::TexMask, 2);
                 //ShaderManager::u_vec2(kShaderUniform::Resolution, zw(m_box) / zoom);
-                ShaderManager::u_float(kShaderUniform::Alpha, m_layers[layer_index]->m_opacity);
                 //ShaderManager::u_int(kShaderUniform::Lock, m_layers[layer_index]->m_alpha_locked);
-                ShaderManager::u_int(kShaderUniform::Mask, m_smask_active);
-                ShaderManager::u_mat4(kShaderUniform::MVP, ortho);
+                pp::panopainter::setup_legacy_stroke_erase_shader(
+                    pp::panopainter::LegacyStrokeEraseUniforms {
+                        .mvp = ortho,
+                        .texture_slot = 0,
+                        .stroke_texture_slot = 1,
+                        .mask_texture_slot = 2,
+                        .alpha = m_layers[layer_index]->m_opacity,
+                        .mask_enabled = m_smask_active,
+                    });
                 set_active_texture_unit(0);
                 m_layers[layer_index]->rtt(plane_index).bindTexture();
                 set_active_texture_unit(1);

src/legacy_canvas_stroke_erase_services.h

@@ -11,12 +11,14 @@ struct LegacyStrokeEraseUniforms {
     int texture_slot = 0;
     int stroke_texture_slot = 1;
     int mask_texture_slot = 2;
+    float alpha = 1.0f;
     bool mask_enabled = false;
 };
 
 struct LegacyStrokeEraseShaderExecution {
     std::function<void(kShader)> use_shader;
     std::function<void(kShaderUniform, int)> set_int;
+    std::function<void(kShaderUniform, float)> set_float;
     std::function<void(kShaderUniform, const glm::mat4&)> set_mat4;
 };
 
@@ -25,6 +27,7 @@ struct LegacyStrokeEraseShaderExecution {
     return {
         .use_shader = [](kShader shader) { ShaderManager::use(shader); },
         .set_int = [](kShaderUniform uniform, int value) { ShaderManager::u_int(uniform, value); },
+        .set_float = [](kShaderUniform uniform, float value) { ShaderManager::u_float(uniform, value); },
         .set_mat4 = [](kShaderUniform uniform, const glm::mat4& value) { ShaderManager::u_mat4(uniform, value); },
     };
 }
@@ -38,6 +41,7 @@ inline void setup_legacy_stroke_erase_shader(
     execution.set_int(kShaderUniform::TexStroke, uniforms.stroke_texture_slot);
     execution.set_int(kShaderUniform::TexMask, uniforms.mask_texture_slot);
     execution.set_int(kShaderUniform::Mask, uniforms.mask_enabled);
+    execution.set_float(kShaderUniform::Alpha, uniforms.alpha);
     execution.set_mat4(kShaderUniform::MVP, uniforms.mvp);
 }