From bccf70b29ac3024e85ee00263afdb76fe9892d8c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Benoi=CC=82t=20Rouleau?= <benoit.rouleau@icloud.com>
Date: Sun, 14 Jun 2026 01:43:37 -0400
Subject: [PATCH] android: populate dpi_scale from DisplayMetrics.density via
 JNI
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Upstream miniquad leaves dpi_scale at its 1.0 default on Android. As a
result, macroquad's screen_width() / mouse_position() (which divide by
dpi_scale to return density-independent units) return physical-pixel
values instead of dp — so the same code that lays out UI correctly on
macOS Retina has to special-case Android.

This change queries the activity's Resources.getDisplayMetrics().density
via JNI when init_with_activity sets up NativeDisplayData and again on
every SurfaceChanged. With dpi_scale set, callers get consistent
dp units across iOS / Android / macOS / Windows.

query_display_density() is fail-soft — every nullable JNI lookup falls
back to 1.0, matching upstream's behaviour for apps that don't yet
expect a non-1 dpi_scale on Android.
---
 src/native/android.rs | 54 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 54 insertions(+)

diff --git a/src/native/android.rs b/src/native/android.rs
index 3fae9bb0..9542f221 100644
--- a/src/native/android.rs
+++ b/src/native/android.rs
@@ -190,9 +190,11 @@ impl MainThreadState {
             },
             Message::SurfaceChanged { width, height } => {
                 {
+                    let density = unsafe { query_display_density() };
                     let mut d = crate::native_display().lock().unwrap();
                     d.screen_width = width as _;
                     d.screen_height = height as _;
+                    d.dpi_scale = density;
                 }
                 self.event_handler.resize_event(width as _, height as _);
             }
@@ -323,6 +325,56 @@ pub unsafe fn attach_jni_env() -> *mut ndk_sys::JNIEnv {
     env
 }
 
+/// `Resources.getDisplayMetrics().density` — 1.0 = mdpi, 2.0 = xhdpi,
+/// etc. Returns 1.0 on any JNI hiccup.
+unsafe fn query_display_density() -> f32 {
+    if VM.is_null() || ACTIVITY.is_null() {
+        return 1.0;
+    }
+    let env = attach_jni_env();
+    if env.is_null() {
+        return 1.0;
+    }
+
+    let resources = ndk_utils::call_object_method!(
+        env,
+        ACTIVITY,
+        "getResources",
+        "()Landroid/content/res/Resources;"
+    );
+    if resources.is_null() {
+        return 1.0;
+    }
+
+    let metrics = ndk_utils::call_object_method!(
+        env,
+        resources,
+        "getDisplayMetrics",
+        "()Landroid/util/DisplayMetrics;"
+    );
+    if metrics.is_null() {
+        return 1.0;
+    }
+
+    let get_object_class = (**env).GetObjectClass.unwrap();
+    let get_field_id = (**env).GetFieldID.unwrap();
+    let get_float_field = (**env).GetFloatField.unwrap();
+
+    let metrics_class = get_object_class(env, metrics);
+    let density_field = get_field_id(
+        env,
+        metrics_class,
+        b"density\0".as_ptr() as _,
+        b"F\0".as_ptr() as _,
+    );
+    if density_field.is_null() {
+        return 1.0;
+    }
+
+    let density = get_float_field(env, metrics, density_field);
+    if density > 0.0 { density } else { 1.0 }
+}
+
 pub struct AndroidClipboard {}
 impl AndroidClipboard {
     pub fn new() -> AndroidClipboard {
@@ -454,9 +506,11 @@ where
 
         let clipboard = Box::new(AndroidClipboard::new());
         let tx_fn = Box::new(move |req| tx.send(Message::Request(req)).unwrap());
+        let density = query_display_density();
         crate::set_or_replace_display(NativeDisplayData {
             high_dpi: conf.high_dpi,
             blocking_event_loop: conf.platform.blocking_event_loop,
+            dpi_scale: density,
             ..NativeDisplayData::new(screen_width as _, screen_height as _, tx_fn, clipboard)
         });