Automatically fetch cursor width/height ratio

image.go

@@ -3,6 +3,8 @@ package main
 import (
 	"image"
 	"os"
+	"syscall"
+	"unsafe"
 )
 
 // load an image stored in the given path
@@ -16,6 +18,16 @@ func load(filename string) (image.Image, error) {
 	return img, err
 }
 
+// canvasSize returns the terminal columns, rows, and cursor aspect ratio
+func canvasSize() (int, int, float64) {
+	var size [4]uint16
+	if _, _, err := syscall.Syscall6(syscall.SYS_IOCTL, uintptr(os.Stdout.Fd()), uintptr(syscall.TIOCGWINSZ), uintptr(unsafe.Pointer(&size)), 0, 0, 0); err != 0 {
+		panic(err)
+	}
+	rows, cols, width, height := size[0], size[1], size[2], size[3]
+	return int(cols), int(rows), float64(height/rows) / float64(width/cols)
+}
+
 // scales calculates the image scale to fit within the terminal width/height
 func scale(imgW, imgH, termW, termH int, whratio float64) float64 {
 	hr := float64(imgH) / (float64(termH) * whratio)

main.go

@@ -12,26 +12,21 @@ import (
 	"github.com/nsf/termbox-go"
 )
 
-var whratio float64 // The terminal's cursor width/height ratio
-
-func init() {
-	flag.Float64Var(&whratio, "r", 2.35, "Cursor width/height ratio in your terminal")
-}
-
 func draw(img image.Image) {
-	w, h := termbox.Size()
+	// Get terminal size and cursor width/height ratio
+	width, height, whratio := canvasSize()
 
 	bounds := img.Bounds()
 	imgW, imgH := bounds.Dx(), bounds.Dy()
 
-	imgScale := scale(imgW, imgH, w, h, whratio)
+	imgScale := scale(imgW, imgH, width, height, whratio)
 
 	// Resize canvas to fit scaled image
-	w, h = int(float64(imgW)/imgScale), int(float64(imgH)/(imgScale*whratio))
+	width, height = int(float64(imgW)/imgScale), int(float64(imgH)/(imgScale*whratio))
 
 	termbox.Clear(termbox.ColorDefault, termbox.ColorDefault)
-	for y := 0; y < h; y++ {
-		for x := 0; x < w; x++ {
+	for y := 0; y < height; y++ {
+		for x := 0; x < width; x++ {
 			// Calculate average color for the corresponding image rectangle
 			// fitting in this cell. We use a half-block trick, wherein the
 			// lower half of the cell displays the character ▄, effectively