Implement image resizing and grayscale conversion

cmd/convert.go

@@ -0,0 +1,97 @@
+/*
+Copyright © 2022 NAME HERE <EMAIL ADDRESS>
+
+*/
+package cmd
+
+import (
+	"fmt"
+	"image"
+	"image/jpeg"
+	"image/png"
+	"log"
+	"os"
+
+	"github.com/spf13/cobra"
+	"golang.org/x/image/draw"
+)
+
+// convertCmd represents the convert command
+var convertCmd = &cobra.Command{
+	Use:   "convert",
+	Short: "A brief description of your command",
+	Long: `A longer description that spans multiple lines and likely contains examples
+and usage of using your command. For example:
+
+Cobra is a CLI library for Go that empowers applications.
+This application is a tool to generate the needed files
+to quickly create a Cobra application.`,
+	Run: func(cmd *cobra.Command, args []string) {
+		width, _ := cmd.Flags().GetInt("width")
+		height, _ := cmd.Flags().GetInt("height")
+
+		convertImage(width, height)
+	},
+}
+
+func init() {
+	rootCmd.AddCommand(convertCmd)
+
+	convertCmd.Flags().Int("width", 540, "Target width")
+	convertCmd.Flags().Int("height", 960, "Target height")
+}
+
+// Returns a new Rectangle that is resized and centered in `dst`
+func fitRectInto(src *image.Rectangle, dst *image.Rectangle) image.Rectangle {
+	var targetWidth int
+    var targetHeight int
+	var scale float64
+
+	srcRatio := float64(src.Max.X) / float64(src.Max.Y)
+	dstRatio := float64(dst.Max.X) / float64(dst.Max.Y)
+
+    if srcRatio < dstRatio {
+		// center horizontally, scale vertically
+		scale = float64(dst.Max.Y) / float64(src.Max.Y)
+	} else {
+		// center vertically, scale horizontally
+		scale = float64(dst.Max.X) / float64(src.Max.X)
+	}
+
+	targetWidth = int(float64(src.Max.X) * scale)
+	targetHeight = int(float64(src.Max.Y) * scale)
+
+	targetX := (dst.Max.X - targetWidth) / 2
+	targetY := (dst.Max.Y - targetHeight) / 2
+
+	return image.Rect(targetX, targetY, targetWidth + targetX, targetHeight + targetY)
+}
+
+func convertImage(width int, height int) {
+	// for now, we're reading a predefined input file
+	input, err := os.Open("cat.jpg")
+	if err != nil {
+		log.Fatalf("Error opening file: %v", err)
+	}
+	defer input.Close()
+
+	// … and writing to a predefined output file
+	output, err := os.OpenFile("cat_resized.png", os.O_RDWR|os.O_CREATE, 0644)
+	if err != nil {
+		log.Fatalf("Error writing output file: %v", err)
+	}
+	defer output.Close()
+
+	src, _ := jpeg.Decode(input)
+	dst := image.NewGray16(image.Rect(0, 0, width, height))
+
+	srcBounds := src.Bounds()
+	targetRect := fitRectInto(&srcBounds, &dst.Rect)
+
+	fmt.Printf("targetRect: %v", targetRect)
+
+	draw.CatmullRom.Scale(dst, targetRect, src, src.Bounds(), draw.Over, nil)
+
+	// we're taking care of a more efficient output encoding later
+	png.Encode(output, dst)
+}