From b98635a8bb874fa8bbcaa86042fc5955f396ffc2 Mon Sep 17 00:00:00 2001
From: omigamedev <omarator@gmail.com>
Date: Sat, 13 Jun 2026 05:30:31 +0200
Subject: [PATCH] Route thumbnail blend setup through helper

---
 docs/modernization/debt.md    |  5 +++++
 docs/modernization/roadmap.md |  4 ++++
 src/canvas.cpp                | 15 +++++++++------
 3 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/docs/modernization/debt.md b/docs/modernization/debt.md
index dbcaf52..ad62c29 100644
--- a/docs/modernization/debt.md
+++ b/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::thumbnail_generate`
+  thumbnail layer `TextureBlend` shader setup now routes through
+  `legacy_canvas_draw_merge_services.h`; thumbnail generation still owns
+  destination feedback copies, sampler/texture binding, grid redraw, readback,
+  and draw execution.
 - 2026-06-13: DEBT-0036 was narrowed again. Desktop VR checkerboard background
   shader setup now routes through `legacy_canvas_draw_merge_services.h`; VR
   still owns render state transitions, per-plane transforms, and draw
diff --git a/docs/modernization/roadmap.md b/docs/modernization/roadmap.md
index 1cf844f..d757caf 100644
--- a/docs/modernization/roadmap.md
+++ b/docs/modernization/roadmap.md
@@ -3024,6 +3024,10 @@ Results:
 - Desktop VR checkerboard background drawing now shares the retained draw-merge
   shader setup helper, while render state transitions, per-plane transforms,
   and draw execution remain in retained VR code.
+- `Canvas::thumbnail_generate` thumbnail layer blending now shares the retained
+  draw-merge shader setup helper for per-layer `TextureBlend` uniforms, while
+  destination feedback copies, sampler/texture binding, grid redraw, readback,
+  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
diff --git a/src/canvas.cpp b/src/canvas.cpp
index 7adac0d..073dd7b 100644
--- a/src/canvas.cpp
+++ b/src/canvas.cpp
@@ -2986,12 +2986,8 @@ Image Canvas::thumbnail_generate(int w, int h)
             apply_canvas_capability(blend_state(), false);
             auto plane_mvp = proj * m_mv * m_plane_transform[i] * glm::translate(glm::vec3(0, 0, -1));
 
-            ShaderManager::use(kShader::TextureBlend);
-            ShaderManager::u_int(kShaderUniform::Tex, 0);
-            ShaderManager::u_mat4(kShaderUniform::MVP, plane_mvp);
             if (copy_layer_destination)
             {
-                ShaderManager::u_int(kShaderUniform::TexBG, 2);
                 set_active_texture_unit(2);
                 blendtex.bind();
                 m_sampler_nearest.bind(2);
@@ -3008,8 +3004,15 @@ Image Canvas::thumbnail_generate(int w, int h)
                     set_active_texture_unit(2);
                     copy_framebuffer_to_texture_2d(0, 0, 0, 0, w, h);
                 }
-                ShaderManager::u_int(kShaderUniform::BlendMode, m_layers[layer_index]->m_blend_mode);
-                ShaderManager::u_float(kShaderUniform::Alpha, m_layers[layer_index]->m_opacity);
+                pp::panopainter::setup_legacy_canvas_draw_merge_texture_blend_shader(
+                    pp::panopainter::LegacyCanvasDrawMergeTextureBlendUniforms {
+                        .mvp = plane_mvp,
+                        .texture_slot = 0,
+                        .destination_texture_slot = 2,
+                        .use_destination_texture = copy_layer_destination,
+                        .blend_mode = m_layers[layer_index]->m_blend_mode,
+                        .alpha = m_layers[layer_index]->m_opacity,
+                    });
                 set_active_texture_unit(0);
                 m_layers[layer_index]->rtt(i).bindTexture();
                 m_face_plane.draw_fill();