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

examples/cli/main.cpp

@@ -357,7 +357,8 @@ bool load_images_from_dir(const std::string dir,
             LOG_VERBOSE("load image %zu from '%s'", images.size(), path.c_str());
             int width             = 0;
             int height            = 0;
-            uint8_t* image_buffer = load_image_from_file(path.c_str(), width, height, expected_width, expected_height);
+            int loaded_channel    = 0;
+            uint8_t* image_buffer = load_image_from_file(path.c_str(), width, height, loaded_channel, expected_width, expected_height);
             if (image_buffer == nullptr) {
                 LOG_ERROR("load image from '%s' failed", path.c_str());
                 return false;
@@ -365,7 +366,7 @@ bool load_images_from_dir(const std::string dir,
 
             images.emplace_back(sd_image_t{(uint32_t)width,
                                            (uint32_t)height,
-                                           3,
+                                           (uint32_t)loaded_channel,
                                            image_buffer});
 
             if (max_image_num > 0 && static_cast<int>(images.size()) >= max_image_num) {
@@ -781,7 +782,8 @@ int main(int argc, const char* argv[]) {
     };
 
     if (gen_params.init_image_path.size() > 0) {
-        if (!load_image_and_update_size(gen_params.init_image_path, gen_params.init_image)) {
+        const bool native_init = cli_params.mode == IMG_GEN || cli_params.mode == ADETAILER;
+        if (!load_image_and_update_size(gen_params.init_image_path, gen_params.init_image, true, native_init ? 0 : 3)) {
             return 1;
         }
     }
@@ -795,8 +797,8 @@ int main(int argc, const char* argv[]) {
     if (gen_params.ref_image_paths.size() > 0) {
         gen_params.ref_images.clear();
         for (auto& path : gen_params.ref_image_paths) {
-            SDImageOwner ref_image({0, 0, 3, nullptr});
-            if (!load_image_and_update_size(path, ref_image, false)) {
+            SDImageOwner ref_image({0, 0, 0, nullptr});
+            if (!load_image_and_update_size(path, ref_image, false, 0)) {
                 return 1;
             }
             gen_params.ref_images.push_back(std::move(ref_image));

examples/common/common.cpp

@@ -1848,20 +1848,22 @@ bool decode_base64_image(const std::string& encoded_input,
         return false;
     }
 
-    int decoded_width  = 0;
-    int decoded_height = 0;
-    uint8_t* raw_data  = load_image_from_memory(reinterpret_cast<const char*>(image_bytes.data()),
-                                                static_cast<int>(image_bytes.size()),
-                                                decoded_width,
-                                                decoded_height,
-                                                expected_width,
-                                                expected_height,
-                                                target_channels);
+    int decoded_width    = 0;
+    int decoded_height   = 0;
+    int resolved_channel = target_channels;
+    uint8_t* raw_data    = load_image_from_memory(reinterpret_cast<const char*>(image_bytes.data()),
+                                                  static_cast<int>(image_bytes.size()),
+                                                  decoded_width,
+                                                  decoded_height,
+                                                  resolved_channel,
+                                                  expected_width,
+                                                  expected_height,
+                                                  target_channels);
     if (raw_data == nullptr) {
         return false;
     }
 
-    out_image.reset({(uint32_t)decoded_width, (uint32_t)decoded_height, (uint32_t)target_channels, raw_data});
+    out_image.reset({(uint32_t)decoded_width, (uint32_t)decoded_height, (uint32_t)resolved_channel, raw_data});
     return true;
 }
 
@@ -2215,7 +2217,7 @@ bool SDGenerationParams::from_json_str(
         LOG_ERROR("invalid lora");
         return false;
     }
-    if (!parse_image_json_field(j, "init_image", 3, width, height, init_image)) {
+    if (!parse_image_json_field(j, "init_image", 0, width, height, init_image)) {
         LOG_ERROR("invalid init_image");
         return false;
     }
@@ -2225,7 +2227,7 @@ bool SDGenerationParams::from_json_str(
     }
     if (!parse_image_array_json_field(j,
                                       "ref_images",
-                                      3,
+                                      0,
                                       auto_resize_ref_image ? width : 0,
                                       auto_resize_ref_image ? height : 0,
                                       ref_images)) {

examples/common/media_io.cpp

@@ -261,6 +261,10 @@ uint8_t* decode_webp_image_to_buffer(const uint8_t* data,
     height               = features.height;
     source_channel_count = features.has_alpha ? 4 : 3;
 
+    if (expected_channel == 0) {
+        expected_channel = source_channel_count;
+    }
+
     const size_t pixel_count = static_cast<size_t>(width) * static_cast<size_t>(height);
 
     if (expected_channel == 1) {
@@ -481,7 +485,8 @@ uint8_t* load_image_common(bool from_memory,
                            int& height,
                            int expected_width,
                            int expected_height,
-                           int expected_channel) {
+                           int expected_channel,
+                           int& out_channel) {
     const char* image_path;
     FreeUniquePtr<uint8_t> image_buffer;
     int source_channel_count = 0;
@@ -538,6 +543,32 @@ uint8_t* load_image_common(bool from_memory,
         LOG_ERROR("load image from '%s' failed", image_path);
         return nullptr;
     }
+    if (expected_channel == 0) {
+        expected_channel = source_channel_count == 2 ? 4 : (source_channel_count == 1 ? 3 : source_channel_count);
+        if (expected_channel != source_channel_count) {
+            FreeUniquePtr<uint8_t> promoted((uint8_t*)malloc((size_t)width * height * expected_channel));
+            if (promoted == nullptr) {
+                LOG_ERROR("error: allocate memory for channel promotion, image_path = %s", image_path);
+                return nullptr;
+            }
+            const size_t pixel_count = (size_t)width * (size_t)height;
+            for (size_t i = 0; i < pixel_count; ++i) {
+                if (source_channel_count == 1) {
+                    promoted.get()[i * 3 + 0] = image_buffer.get()[i];
+                    promoted.get()[i * 3 + 1] = image_buffer.get()[i];
+                    promoted.get()[i * 3 + 2] = image_buffer.get()[i];
+                } else {
+                    promoted.get()[i * 4 + 0] = image_buffer.get()[i * 2];
+                    promoted.get()[i * 4 + 1] = image_buffer.get()[i * 2];
+                    promoted.get()[i * 4 + 2] = image_buffer.get()[i * 2];
+                    promoted.get()[i * 4 + 3] = image_buffer.get()[i * 2 + 1];
+                }
+            }
+            image_buffer         = std::move(promoted);
+            source_channel_count = expected_channel;
+        }
+    }
+    // stb reports the source channel count even when it converts the output.
     if (source_channel_count < expected_channel) {
         fprintf(stderr,
                 "the number of channels for the input image must be >= %d,"
@@ -597,7 +628,7 @@ uint8_t* load_image_common(bool from_memory,
         }
         stbir_resize(image_buffer.get(), width, height, 0,
                      resized_image_buffer.get(), expected_width, expected_height, 0, STBIR_TYPE_UINT8,
-                     expected_channel, STBIR_ALPHA_CHANNEL_NONE, 0,
+                     expected_channel, expected_channel == 4 ? 3 : STBIR_ALPHA_CHANNEL_NONE, 0,
                      STBIR_EDGE_CLAMP, STBIR_EDGE_CLAMP,
                      STBIR_FILTER_BOX, STBIR_FILTER_BOX,
                      STBIR_COLORSPACE_SRGB, nullptr);
@@ -605,6 +636,7 @@ uint8_t* load_image_common(bool from_memory,
         height       = expected_height;
         image_buffer = std::move(resized_image_buffer);
     }
+    out_channel = expected_channel;
     return image_buffer.release();
 }
 
@@ -777,10 +809,11 @@ bool write_image_to_file(const std::string& path,
 uint8_t* load_image_from_file(const char* image_path,
                               int& width,
                               int& height,
+                              int& out_channel,
                               int expected_width,
                               int expected_height,
                               int expected_channel) {
-    return load_image_common(false, image_path, 0, width, height, expected_width, expected_height, expected_channel);
+    return load_image_common(false, image_path, 0, width, height, expected_width, expected_height, expected_channel, out_channel);
 }
 
 bool load_sd_image_from_file(sd_image_t* image,
@@ -790,13 +823,14 @@ bool load_sd_image_from_file(sd_image_t* image,
                              int expected_channel) {
     int width;
     int height;
-    image->data = load_image_common(false, image_path, 0, width, height, expected_width, expected_height, expected_channel);
+    int resolved_channel = expected_channel;
+    image->data          = load_image_common(false, image_path, 0, width, height, expected_width, expected_height, expected_channel, resolved_channel);
     if (image->data == nullptr) {
         return false;
     }
     image->width   = width;
     image->height  = height;
-    image->channel = expected_channel;
+    image->channel = resolved_channel;
     return true;
 }
 
@@ -804,10 +838,11 @@ uint8_t* load_image_from_memory(const char* image_bytes,
                                 int len,
                                 int& width,
                                 int& height,
+                                int& out_channel,
                                 int expected_width,
                                 int expected_height,
                                 int expected_channel) {
-    return load_image_common(true, image_bytes, len, width, height, expected_width, expected_height, expected_channel);
+    return load_image_common(true, image_bytes, len, width, height, expected_width, expected_height, expected_channel, out_channel);
 }
 
 static void append_avi_metadata(std::vector<uint8_t>& data, const std::string& parameters) {

examples/common/media_io.h

@@ -32,9 +32,12 @@ bool write_image_to_file(const std::string& path,
                          const std::string& parameters = "",
                          int quality                   = 90);
 
+// expected_channel == 0 preserves native channels (grayscale -> RGB, gray+alpha -> RGBA).
+// out_channel receives the output channel count.
 uint8_t* load_image_from_file(const char* image_path,
                               int& width,
                               int& height,
+                              int& out_channel,
                               int expected_width   = 0,
                               int expected_height  = 0,
                               int expected_channel = 3);
@@ -49,6 +52,7 @@ uint8_t* load_image_from_memory(const char* image_bytes,
                                 int len,
                                 int& width,
                                 int& height,
+                                int& out_channel,
                                 int expected_width   = 0,
                                 int expected_height  = 0,
                                 int expected_channel = 3);

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,