Calculate pixel_ratio based on users' display

Author: GeertJohan

example/simpleDemo/main.go

@@ -36,7 +36,6 @@ func main() {
 		gutter.ApplicationICUDataPath(dir + "/icudtl.dat"),
 		gutter.ApplicationWindowDimension(initialApplicationWidth, initialApplicationHeight),
 		gutter.OptionWindowInitializer(setIcon),
-		gutter.OptionPixelRatio(1.2),
 		gutter.OptionVMArguments([]string{"--dart-non-checked-mode", "--observatory-port=50300"}),
 		gutter.OptionAddPluginReceiver(ownPlugin, "plugin_demo"),
 		// Default keyboard is Qwerty, if you want to change it, you can check keyboard.go in gutter package.

flutter/flutter.go

@@ -59,7 +59,6 @@ type EngineOpenGL struct {
 	FPlatfromMessage func(message *PlatformMessage, window unsafe.Pointer) bool
 
 	// Engine arguments
-	PixelRatio  float64
 	AssetsPath  string
 	IcuDataPath string
 }

gutter.go

@@ -2,6 +2,7 @@ package gutter
 
 import (
 	"encoding/json"
+	"fmt"
 	"log"
 	"time"
 	"unsafe"
@@ -10,6 +11,9 @@ import (
 	"github.com/go-gl/glfw/v3.2/glfw"
 )
 
+// dpPerInch defines the amount of display pixels per inch as defined for Flutter.
+const dpPerInch = 160.0
+
 // Run executes a flutter application with the provided options.
 // given limitations this method must be called by the main function directly.
 func Run(options ...Option) (err error) {
@@ -164,17 +168,31 @@ func glfwKey(keyboardLayout KeyboardShortcuts) func(w *glfw.Window, key glfw.Key
 	}
 }
 
-func glfwWindowSizeCallback(window *glfw.Window, width int, height int) {
-
-	index := *(*int)(window.GetUserPointer())
-	flutterOGL := flutter.SelectEngine(index)
+func newGLFWFramebufferSizeCallback(pixelRatio float64, monitorScreenCoordinatesPerInch float64) func(*glfw.Window, int, int) {
+	return func(window *glfw.Window, widthPx int, heightPx int) {
+		index := *(*int)(window.GetUserPointer())
+		flutterOGL := flutter.SelectEngine(index)
+
+		if pixelRatio == 0 {
+			width, _ := window.GetSize()
+			windowPixelsPerScreenCoordinate := float64(widthPx) / float64(width)
+			dpi := windowPixelsPerScreenCoordinate * monitorScreenCoordinatesPerInch
+			pixelRatio = dpi / dpPerInch
+
+			// Limit the ratio to 1 to avoid rendering a smaller UI in standard resolution monitors.
+			if pixelRatio < 1.0 {
+				fmt.Println("calculated pixelRatio limited to a minimum of 1.0")
+				pixelRatio = 1.0
+			}
+		}
 
-	event := flutter.WindowMetricsEvent{
-		Width:      width,
-		Height:     height,
-		PixelRatio: flutterOGL.PixelRatio,
+		event := flutter.WindowMetricsEvent{
+			Width:      widthPx,
+			Height:     heightPx,
+			PixelRatio: pixelRatio,
+		}
+		flutterOGL.EngineSendWindowMetricsEvent(event)
 	}
-	flutterOGL.EngineSendWindowMetricsEvent(event)
 }
 
 func glfwCharCallback(w *glfw.Window, char rune) {
@@ -211,7 +229,6 @@ func runFlutter(window *glfw.Window, c config) *flutter.EngineOpenGL {
 	flutterOGL.FMakeResourceCurrent = func(v unsafe.Pointer) bool {
 		return false
 	}
-	flutterOGL.PixelRatio = c.PixelRatio
 
 	// PlatformMessage
 	flutterOGL.FPlatfromMessage = func(platMessage *flutter.PlatformMessage, window unsafe.Pointer) bool {
@@ -242,8 +259,9 @@ func runFlutter(window *glfw.Window, c config) *flutter.EngineOpenGL {
 		panic("Couldn't launch the FlutterEngine")
 	}
 
+	glfwFramebufferSizeCallback := newGLFWFramebufferSizeCallback(c.PixelRatio, getScreenCoordinatesPerInch())
 	width, height := window.GetFramebufferSize()
-	glfwWindowSizeCallback(window, width, height)
+	glfwFramebufferSizeCallback(window, width, height)
 	var glfwKeyCallback func(w *glfw.Window, key glfw.Key, scancode int, action glfw.Action, mods glfw.ModifierKey)
 
 	if c.KeyboardLayout != nil {
@@ -253,12 +271,32 @@ func runFlutter(window *glfw.Window, c config) *flutter.EngineOpenGL {
 	}
 
 	window.SetKeyCallback(glfwKeyCallback)
-	window.SetFramebufferSizeCallback(glfwWindowSizeCallback)
+	window.SetFramebufferSizeCallback(glfwFramebufferSizeCallback)
 	window.SetMouseButtonCallback(glfwMouseButtonCallback)
 	window.SetCharCallback(glfwCharCallback)
 	return flutterOGL
 }
 
+// getScreenCoordinatesPerInch returns the number of screen coordinates per
+// inch for the main monitor. If the information is unavailable it returns
+// a default value that assumes that a screen coordinate is one dp.
+func getScreenCoordinatesPerInch() float64 {
+	// TODO: multi-monitor support (#74)
+	primaryMonitor := glfw.GetPrimaryMonitor()
+	if primaryMonitor == nil {
+		return dpPerInch
+	}
+	primaryMonitorMode := primaryMonitor.GetVideoMode()
+	if primaryMonitorMode == nil {
+		return dpPerInch
+	}
+	primaryMonitorWidthMM, _ := primaryMonitor.GetPhysicalSize()
+	if primaryMonitorWidthMM == 0 {
+		return dpPerInch
+	}
+	return float64(primaryMonitorMode.Width) / (float64(primaryMonitorWidthMM) / 25.4)
+}
+
 // Update the TextInput with the current state
 func updateEditingState(window *glfw.Window) {
 

option.go

@@ -66,7 +66,9 @@ func OptionWindowInitializer(ini func(*glfw.Window) error) Option {
 	}
 }
 
-// OptionPixelRatio specify the scale factor for the physical screen.
+// OptionPixelRatio forces the the scale factor for the screen.
+// By default, go-flutter will calculate the correct pixel ratio for the user based
+// on their DPI. Setting this option is not adviced.
 func OptionPixelRatio(ratio float64) Option {
 	return func(c *config) {
 		c.PixelRatio = ratio