mtmd: fix LFM2 image tiling threshold (#27057)

* mtmd: fix LFM2 image tiling threshold

* refactor testing

* fix

* fix on windows

---------

Co-authored-by: Xuan Son Nguyen <son@huggingface.co>

tools/mtmd/CMakeLists.txt

@@ -90,6 +90,9 @@ if (BUILD_SHARED_LIBS)
     set_target_properties     (mtmd PROPERTIES POSITION_INDEPENDENT_CODE ON)
     target_compile_definitions(mtmd PRIVATE LLAMA_BUILD)
     target_compile_definitions(mtmd PUBLIC  LLAMA_SHARED)
+
+    # export all symbols so that internal components can be tested by test-mtmd-impl
+    set_target_properties     (mtmd PROPERTIES WINDOWS_EXPORT_ALL_SYMBOLS ON)
 endif()
 
 set(MTMD_PUBLIC_HEADERS

tools/mtmd/clip-impl.h

@@ -858,6 +858,9 @@ static std::ifstream open_ifstream_binary(const std::string & fname) {
 }
 #endif
 
+// in test-mtmd-impl, we include woth common.h and this file, and these functions are duplicated
+// this is a quick fix to avoid compilation errors
+#ifndef DIRECTORY_SEPARATOR
 static std::string string_format(const char * fmt, ...) {
     va_list ap;
     va_list ap2;
@@ -915,6 +918,7 @@ inline bool string_ends_with(std::string_view str, std::string_view suffix) {
     return str.size() >= suffix.size() &&
            str.compare(str.size() - suffix.size(), suffix.size(), suffix) == 0;
 }
+#endif
 
 //
 // gguf utils

tools/mtmd/mtmd-image.cpp

@@ -1013,14 +1013,31 @@ mtmd_image_preproc_out mtmd_image_preprocessor_lfm2::preprocess(const clip_image
     return output;
 }
 
+bool mtmd_image_preprocessor_lfm2::should_tile(
+        const clip_hparams & hparams,
+        const clip_image_size & original_size) {
+    const int align_size = hparams.patch_size * hparams.n_merge;
+
+    const auto round_by_factor = [align_size](float x) {
+        // see https://github.com/ggml-org/llama.cpp/pull/27057#discussion_r3796264887
+        return static_cast<int>(std::nearbyint(static_cast<double>(x) / align_size)) * align_size;
+    };
+
+    const int h_bar = std::max(hparams.patch_size, round_by_factor(original_size.height));
+    const int w_bar = std::max(hparams.patch_size, round_by_factor(original_size.width));
+
+    return static_cast<double>(h_bar) * static_cast<double>(w_bar) >
+           static_cast<double>(hparams.image_max_pixels) * max_pixels_tolerance;
+}
+
 mtmd_image_preprocessor_llava_uhd::slice_instructions mtmd_image_preprocessor_lfm2::get_slice_instructions(const clip_image_size & original_size) {
     mtmd_image_preprocessor_llava_uhd::slice_instructions inst;
     const int align_size = hparams.patch_size * hparams.n_merge;
     inst.overview_size = img_tool::calc_size_preserved_ratio(
                             original_size,
                             { align_size, hparams.image_min_pixels, hparams.image_max_pixels, 0 });
-    // tile if either dimension exceeds tile_size with tolerance
-    const bool needs_tiling = original_size.width > tile_size * max_pixels_tolerance || original_size.height > tile_size * max_pixels_tolerance;
+
+    const bool needs_tiling = should_tile(hparams, original_size);
 
     if (!needs_tiling) {
         inst.refined_size = clip_image_size{0, 0};

tools/mtmd/mtmd-image.h

@@ -148,6 +148,8 @@ struct mtmd_image_preprocessor_lfm2 : mtmd_image_preprocessor_llava_uhd {
     mtmd_image_preproc_out preprocess(const clip_image_u8 & img) override;
     slice_instructions get_slice_instructions(const clip_image_size & original_size) override;
 
+    static bool should_tile(const clip_hparams & hparams, const clip_image_size & original_size);
+
 private:
     clip_image_size find_closest_aspect_ratio(
             float aspect_ratio,