Centralize canvas blend gate planning

src/node_canvas.cpp

@@ -1,6 +1,9 @@
 #include "pch.h"
 
+#include <algorithm>
 #include <cstdint>
+#include <memory>
+#include <vector>
 
 #include "app_core/canvas_tool_ui.h"
 #include "app_core/history_ui.h"
@@ -8,6 +11,7 @@
 #include "log.h"
 #include "node_canvas.h"
 #include "node_image_texture.h"
+#include "paint_renderer/compositor.h"
 #include "settings.h"
 #include "renderer_gl/opengl_capabilities.h"
 
@@ -23,6 +27,43 @@ void unbind_texture_2d()
     glBindTexture(pp::renderer::gl::texture_2d_target(), 0);
 }
 
+pp::renderer::RenderDeviceFeatures node_canvas_stroke_composite_features() noexcept
+{
+    return pp::renderer::RenderDeviceFeatures {
+        .framebuffer_fetch = ShaderManager::ext_framebuffer_fetch,
+        .texture_copy = !ShaderManager::ext_framebuffer_fetch,
+    };
+}
+
+bool node_canvas_needs_shader_blend(
+    int width,
+    int height,
+    const std::vector<std::shared_ptr<Layer>>& layers,
+    const Brush* brush) noexcept
+{
+    std::vector<int> layer_blend_modes;
+    layer_blend_modes.reserve(layers.size());
+    for (const auto& layer : layers) {
+        if (!layer) {
+            continue;
+        }
+        layer_blend_modes.push_back(layer->m_blend_mode);
+    }
+
+    const auto plan = pp::paint_renderer::plan_canvas_blend_gate(
+        node_canvas_stroke_composite_features(),
+        pp::paint_renderer::CanvasBlendGateRequest {
+            .extent = pp::renderer::Extent2D {
+                .width = static_cast<std::uint32_t>(std::max(width, 0)),
+                .height = static_cast<std::uint32_t>(std::max(height, 0)),
+            },
+            .layer_blend_modes = layer_blend_modes,
+            .has_stroke_blend_mode = brush != nullptr,
+            .stroke_blend_mode = brush ? brush->m_blend_mode : 0,
+        });
+    return plan ? plan.value().shader_blend : true;
+}
+
 void run_history_undo_if_available()
 {
     const auto plan = pp::app::plan_history_undo(static_cast<int>(ActionManager::I.m_actions.size()));
@@ -252,14 +293,11 @@ void NodeCanvas::draw()
     }
     else
     {
-        // check if any layer use blend, otherwise draw directly on main framebuffer
-        bool use_blend = false;
-        for (size_t i = 0; i < m_canvas->m_layers.size(); i++)
-        {
-            use_blend |= m_canvas->m_layers[i]->m_blend_mode != 0;
-        }
-        if (Canvas::I->m_current_stroke)
-            use_blend |= Canvas::I->m_current_stroke->m_brush->m_blend_mode != 0;
+        const bool use_blend = node_canvas_needs_shader_blend(
+            m_cache_rtt.getWidth(),
+            m_cache_rtt.getHeight(),
+            m_canvas->m_layers,
+            m_canvas->m_current_stroke ? m_canvas->m_current_stroke->m_brush.get() : nullptr);
 
         if (use_blend)
         {