Share retained stroke preview live pass 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. `NodeStrokePreview` dual-pass and
+  main-pass frame-loop execution plus full-frame copy-back now route through
+  shared local helpers; checkerboard/background capture ordering, texture-unit
+  binding, mixer ownership, and final composite semantics remain retained in
+  the preview node.
 - 2026-06-13: DEBT-0036 was narrowed again. `Canvas::stroke_draw` main and
   dual live-pass per-face framebuffer begin/end execution plus pad-face array
   assembly now route through `legacy_canvas_stroke_execution_services.h`;

docs/modernization/roadmap.md

@@ -3103,6 +3103,10 @@ Results:
   assembly now shares one retained utility, while shader activation timing,
   texture/sampler binding, framebuffer ownership, pad execution, and final
   OpenGL draw ordering remain in the legacy Canvas path.
+- `NodeStrokePreview` dual-pass and main-pass frame-loop execution plus
+  full-frame copy-back now share one local retained helper surface, while
+  checkerboard/background capture ordering, texture-unit binding, mixer
+  ownership, and final composite semantics remain in the preview node.
 - `Canvas::stroke_draw` pad-pass destination bind/copy/unbind ordering now
   shares the retained stroke execution helper callback surface, while shader
   setup, pad color selection, framebuffer ownership, and final OpenGL draw

docs/modernization/tasks.md

@@ -509,6 +509,13 @@ Done Checks:
 
 Progress Notes:
 
+- 2026-06-13: `NodeStrokePreview::draw_stroke_immediate()` dual-pass and
+  main-pass frame-loop execution plus full-frame copy-back now route through
+  shared local helpers; checkerboard/background capture ordering, texture-unit
+  binding, mixer ownership, and final composite semantics remain local to the
+  preview node. Next slice should target background/final-copy helpers or
+  pass-level texture binding without reopening the new preview composite
+  helper.
 - 2026-06-13: `Canvas::stroke_draw` main and dual live-pass per-face
   framebuffer begin/end execution plus pad-face array assembly now route
   through `legacy_canvas_stroke_execution_services.h`; shader activation

src/node_stroke_preview.cpp

@@ -187,6 +187,46 @@ void execute_stroke_preview_frames(
     }
 }
 
+void copy_stroke_preview_framebuffer_to_texture(
+    Texture2D& texture,
+    glm::vec2 size,
+    std::uint32_t texture_unit)
+{
+    set_active_texture_unit(texture_unit);
+    texture.bind();
+    copy_framebuffer_to_texture_2d(
+        0,
+        0,
+        0,
+        0,
+        static_cast<int>(size.x),
+        static_cast<int>(size.y));
+}
+
+template <typename ClearTarget, typename ComputeFrames, typename BeforeFrame, typename DrawSample>
+void execute_stroke_preview_live_pass(
+    Texture2D& output_texture,
+    glm::vec2 size,
+    bool copy_stroke_destination,
+    ClearTarget&& clear_target,
+    ComputeFrames&& compute_frames,
+    BeforeFrame&& before_frame,
+    DrawSample&& draw_sample)
+{
+    auto frames = compute_frames();
+    clear_target();
+    execute_stroke_preview_frames(
+        frames,
+        std::forward<BeforeFrame>(before_frame),
+        [&](auto& frame) {
+            draw_sample(frame, output_texture, copy_stroke_destination);
+        });
+    copy_stroke_preview_framebuffer_to_texture(
+        output_texture,
+        size,
+        stroke_preview_composite_slots::kStroke);
+}
+
 }
 
 std::atomic_int NodeStrokePreview::s_instances{ 0 };
@@ -540,32 +580,27 @@ void NodeStrokePreview::draw_stroke_immediate()
 
     if (material.dual_pass.enabled)
     {
-        m_rtt.clear();
         pp::panopainter::setup_legacy_stroke_dual_shader(material.dual_pass.uses_pattern);
         set_active_texture_unit(0U);
         dual_brush->m_tip_texture ?
             dual_brush->m_tip_texture->bind() :
             unbind_texture_2d();
-        auto frames_dual = stroke_draw_compute(m_dual_stroke, zoom);
+        execute_stroke_preview_live_pass(
-        execute_stroke_preview_frames(
+            m_tex_dual,
-            frames_dual,
+            size,
+            copy_stroke_destination,
+            [&] {
+                m_rtt.clear();
+            },
+            [&] {
+                return stroke_draw_compute(m_dual_stroke, zoom);
+            },
             [](auto& frame) {
                 frame.col = { 0, 0, 0, 1 };
             },
-            [&](auto& frame) {
+            [&](auto& frame, Texture2D& blend_texture, bool copy_destination) {
-                /*auto rect =*/ stroke_draw_samples(frame.shapes, m_tex_dual, copy_stroke_destination);
+                /*auto rect =*/ stroke_draw_samples(frame.shapes, blend_texture, copy_destination);
             });
-
-        // copy raw stroke to tex
-        set_active_texture_unit(1U);
-        m_tex_dual.bind();
-        copy_framebuffer_to_texture_2d(
-            0,
-            0,
-            0,
-            0,
-            static_cast<int>(size.x),
-            static_cast<int>(size.y));
     }
 
     // CHEKCERBOARD
@@ -623,10 +658,16 @@ void NodeStrokePreview::draw_stroke_immediate()
         unbind_texture_2d();
     set_active_texture_unit(3U);
     preview_composite_plan.uses_mixer ? m_rtt_mixer.bindTexture() : unbind_texture_2d();
-    auto frames = stroke_draw_compute(m_stroke, zoom);
+    execute_stroke_preview_live_pass(
+        m_tex,
+        size,
+        copy_stroke_destination,
+        [&] {
             m_rtt.clear();
-    execute_stroke_preview_frames(
+        },
-        frames,
+        [&] {
+            return stroke_draw_compute(m_stroke, zoom);
+        },
         [&](auto& frame) {
             if (b->m_tip_mix > 0.f)
             {
@@ -638,23 +679,12 @@ void NodeStrokePreview::draw_stroke_immediate()
                 glm::vec4 { 0, 0, 0, 1 };
             frame.flow = glm::max(frame.flow, m_min_flow);
         },
-        [&](auto& frame) {
+        [&](auto& frame, Texture2D& blend_texture, bool copy_destination) {
-            /*auto rect =*/ stroke_draw_samples(frame.shapes, m_tex, copy_stroke_destination);
+            /*auto rect =*/ stroke_draw_samples(frame.shapes, blend_texture, copy_destination);
         });
     set_active_texture_unit(3U);
     m_rtt_mixer.unbindTexture();
 
-    // copy raw stroke to tex
-    set_active_texture_unit(1U);
-    m_tex.bind();
-    copy_framebuffer_to_texture_2d(
-        0,
-        0,
-        0,
-        0,
-        static_cast<int>(size.x),
-        static_cast<int>(size.y));
-
     // COMPOSITE
 
     execute_stroke_preview_final_composite_pass(