[Up-477] Optimization example - remove unused resources (#229)

* added example for remove unused resources optimizer

* changed some comments

* updated README content and unipdf version

* updated README

compress/README.md

@@ -14,11 +14,14 @@ type Options struct {
 	CleanFonts                      bool
 	SubsetFonts                     bool
 	CleanContentstream              bool
+	CleanUnusedResources            bool
 }
 ```
 
+From the available filters listed above, all of them except `ImageQuality` and `ImageUpperPPI` enable lossless compression.
 
 ## Examples
 
 - [pdf_optimize.go](pdf_optimize.go) compresses a PDF file with some typical options.
 - [pdf_font_subsetting.go](pdf_font_subsetting.go) illustrates how to reduce a PDF file size by subsetting all fonts used in the document using `SubsetFonts` Optimizer option.
+- [pdf_remove_unused_resources.go](pdf_remove_unused_resources.go) reduces file size by removing unused resources such as Images, Xforms, fonts and external graphics state dictionaries.

compress/pdf_optimize.go

@@ -71,6 +71,7 @@ func main() {
 		UseObjectStreams:                true,
 		ImageQuality:                    80,
 		ImageUpperPPI:                   100,
+		CleanUnusedResources:            true,
 	}))
 
 	// Create output file.

compress/pdf_remove_unused_resources.go

@@ -0,0 +1,88 @@
+/*
+ * This example shows how to use remove unused resources optimization example.
+ * The compression accomplished using this filter is lossless.
+ *
+ * Run as: go run pdf_remove_unused_resources.go <input.pdf> <output.pdf>
+ */
+
+package main
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"time"
+
+	"github.com/unidoc/unipdf/v3/common/license"
+	"github.com/unidoc/unipdf/v3/model"
+	"github.com/unidoc/unipdf/v3/model/optimize"
+)
+
+func init() {
+	// Make sure to load your metered License API key prior to using the library.
+	// If you need a key, you can sign up and create a free one at https://cloud.unidoc.io
+	err := license.SetMeteredKey(os.Getenv(`UNIDOC_LICENSE_API_KEY`))
+	if err != nil {
+		panic(err)
+	}
+}
+
+func main() {
+	args := os.Args
+	if len(args) < 3 {
+		fmt.Printf("Usage: %s <INPUT_PDF_PATH> <OUTPUT_PDF_PATH>\n", os.Args[0])
+		return
+	}
+	inputPath := args[1]
+	outputPath := args[2]
+
+	// Initialize starting time
+	start := time.Now()
+
+	inputFile, err := os.Open(inputPath)
+	if err != nil {
+		log.Fatalf("Failed to read file: %v", err)
+	}
+	defer inputFile.Close()
+
+	pdfReader, err := model.NewPdfReader(inputFile)
+	if err != nil {
+		log.Fatalf("Failed to create reader: %v", err)
+	}
+
+	pdfWriter, err := pdfReader.ToWriter(nil)
+	if err != nil {
+		fmt.Printf("Failed to get writer from reader %v", err)
+	}
+
+	// Set optimizer
+	pdfWriter.SetOptimizer(optimize.New(optimize.Options{
+		CleanUnusedResources: true,
+	}))
+
+	// Write document to file.
+	if err := pdfWriter.WriteToFile(outputPath); err != nil {
+		log.Fatalf("Failed to write to file %v", err)
+
+	}
+
+	// Get processing time.
+	duration := float64(time.Since(start)) / float64(time.Millisecond)
+
+	// Get input file stat.
+	inputFileInfo, err := os.Stat(inputPath)
+	if err != nil {
+		log.Fatalf("Failed to get input file stat: %v\n", err)
+	}
+
+	// Get output file stat
+	outputFileInfo, err := os.Stat(outputPath)
+	if err != nil {
+		log.Fatalf("Failed to get output file path: %v\n", err)
+	}
+
+	// Print information
+	fmt.Printf("Input file size = %d bytes \n",inputFileInfo.Size())
+	fmt.Printf("Optimized file Size = %d bytes\n", outputFileInfo.Size())
+	fmt.Printf("Time taken to process %s = %.2fms\n", inputPath, duration)
+}