fix: add alpha channel input for Qwen Image 2.1 and relative docs (#2021)

examples/server/api.md

@@ -735,12 +735,15 @@ Any image field accepts:
 
 Channel expectations:
 
-- `init_image`: 3 channels
-- `ref_images[]`: 3 channels
+- `init_image`: native channels (3 or 4); alpha is preserved and applied per model
+- `ref_images[]`: native channels (3 or 4); alpha is preserved and applied per model
 - `control_image`: 3 channels
 - `ip_adapter_image`: 3 channels
 - `mask_image`: 1 channel
 
+Models that support RGBA (e.g. Qwen-Image 2.1) use the alpha channel of `init_image`
+and `ref_images[]`. RGB-only models drop it, so sending RGBA is safe for every model.
+
 If omitted or null:
 
 - single-image fields map to an empty `sd_image_t`

examples/server/routes_openai.cpp

@@ -158,19 +158,20 @@ static bool build_openai_edit_request(const httplib::Request& req,
     request.gen_params.batch_count = n;
 
     for (auto& bytes : images_bytes) {
-        int img_w           = 0;
-        int img_h           = 0;
-        uint8_t* raw_pixels = load_image_from_memory(
-            reinterpret_cast<const char*>(bytes.data()),
-            static_cast<int>(bytes.size()),
-            img_w, img_h,
-            0, 0, 3);
+        int img_w            = 0;
+        int img_h            = 0;
+        int resolved_channel = 0;
+        uint8_t* raw_pixels  = load_image_from_memory(
+             reinterpret_cast<const char*>(bytes.data()),
+             static_cast<int>(bytes.size()),
+             img_w, img_h, resolved_channel,
+             0, 0, 0);
         if (raw_pixels == nullptr) {
             continue;
         }
 
         const bool is_first_ref_image = request.gen_params.ref_images.empty();
-        SDImageOwner image_owner({(uint32_t)img_w, (uint32_t)img_h, 3, raw_pixels});
+        SDImageOwner image_owner({(uint32_t)img_w, (uint32_t)img_h, (uint32_t)resolved_channel, raw_pixels});
         request.gen_params.set_width_and_height_if_unset(image_owner.get().width, image_owner.get().height);
 
         if (is_first_ref_image) {
@@ -181,15 +182,16 @@ static bool build_openai_edit_request(const httplib::Request& req,
                 init_h = request.gen_params.height;
             }
 
-            int init_img_w       = 0;
-            int init_img_h       = 0;
-            uint8_t* init_pixels = load_image_from_memory(
-                reinterpret_cast<const char*>(bytes.data()),
-                static_cast<int>(bytes.size()),
-                init_img_w, init_img_h,
-                init_w, init_h, 3);
+            int init_img_w            = 0;
+            int init_img_h            = 0;
+            int init_resolved_channel = 0;
+            uint8_t* init_pixels     = load_image_from_memory(
+                 reinterpret_cast<const char*>(bytes.data()),
+                 static_cast<int>(bytes.size()),
+                 init_img_w, init_img_h, init_resolved_channel,
+                 init_w, init_h, 0);
             if (init_pixels != nullptr) {
-                request.gen_params.init_image.reset({(uint32_t)init_img_w, (uint32_t)init_img_h, 3, init_pixels});
+                request.gen_params.init_image.reset({(uint32_t)init_img_w, (uint32_t)init_img_h, (uint32_t)init_resolved_channel, init_pixels});
             }
         }
 
@@ -203,13 +205,14 @@ static bool build_openai_edit_request(const httplib::Request& req,
             expected_width  = request.gen_params.width;
             expected_height = request.gen_params.height;
         }
-        int mask_w = 0;
-        int mask_h = 0;
+        int mask_w       = 0;
+        int mask_h       = 0;
+        int mask_channel = 0;
 
         uint8_t* mask_raw = load_image_from_memory(
             reinterpret_cast<const char*>(mask_bytes.data()),
             static_cast<int>(mask_bytes.size()),
-            mask_w, mask_h,
+            mask_w, mask_h, mask_channel,
             expected_width, expected_height, 1);
         request.gen_params.mask_image.reset({(uint32_t)mask_w, (uint32_t)mask_h, 1, mask_raw});
         const sd_image_t& mask_image = request.gen_params.mask_image.get();

examples/server/routes_sdapi.cpp

@@ -199,7 +199,7 @@ static bool build_sdapi_img_gen_request(const json& j,
 
         if (j.contains("init_images") && j["init_images"].is_array() && !j["init_images"].empty()) {
             if (decode_base64_image(j["init_images"][0].get<std::string>(),
-                                    3,
+                                    0,
                                     expected_width,
                                     expected_height,
                                     request.gen_params.init_image)) {
@@ -243,7 +243,7 @@ static bool build_sdapi_img_gen_request(const json& j,
             }
             SDImageOwner image_owner;
             if (decode_base64_image(extra_image.get<std::string>(),
-                                    3,
+                                    0,
                                     request.gen_params.auto_resize_ref_image && request.gen_params.width_and_height_are_set()
                                         ? request.gen_params.width
                                         : 0,