Basic optimization for ShadeVertsLinearColorGradientKeepAlpha() - especially for debug overhead - since it's used massively by some of our experiments.

imgui_draw.cpp

@@ -1599,13 +1599,19 @@ void ImGui::ShadeVertsLinearColorGradientKeepAlpha(ImDrawList* draw_list, int ve
     float gradient_inv_length2 = 1.0f / ImLengthSqr(gradient_extent);
     ImDrawVert* vert_start = draw_list->VtxBuffer.Data + vert_start_idx;
     ImDrawVert* vert_end = draw_list->VtxBuffer.Data + vert_end_idx;
+    const int col0_r = (int)(col0 >> IM_COL32_R_SHIFT) & 0xFF;
+    const int col0_g = (int)(col0 >> IM_COL32_G_SHIFT) & 0xFF;
+    const int col0_b = (int)(col0 >> IM_COL32_B_SHIFT) & 0xFF;
+    const int col_delta_r = ((int)(col1 >> IM_COL32_R_SHIFT) & 0xFF) - col0_r;
+    const int col_delta_g = ((int)(col1 >> IM_COL32_G_SHIFT) & 0xFF) - col0_g;
+    const int col_delta_b = ((int)(col1 >> IM_COL32_B_SHIFT) & 0xFF) - col0_b;
     for (ImDrawVert* vert = vert_start; vert < vert_end; vert++)
     {
         float d = ImDot(vert->pos - gradient_p0, gradient_extent);
         float t = ImClamp(d * gradient_inv_length2, 0.0f, 1.0f);
-        int r = ImLerp((int)(col0 >> IM_COL32_R_SHIFT) & 0xFF, (int)(col1 >> IM_COL32_R_SHIFT) & 0xFF, t);
-        int g = ImLerp((int)(col0 >> IM_COL32_G_SHIFT) & 0xFF, (int)(col1 >> IM_COL32_G_SHIFT) & 0xFF, t);
-        int b = ImLerp((int)(col0 >> IM_COL32_B_SHIFT) & 0xFF, (int)(col1 >> IM_COL32_B_SHIFT) & 0xFF, t);
+        int r = (int)(col0_r + col_delta_r * t);
+        int g = (int)(col0_g + col_delta_g * t);
+        int b = (int)(col0_b + col_delta_b * t);
         vert->col = (r << IM_COL32_R_SHIFT) | (g << IM_COL32_G_SHIFT) | (b << IM_COL32_B_SHIFT) | (vert->col & IM_COL32_A_MASK);
     }
 }