Example of Using LibWebP

The following example shows how to convert a PNG image to a WebP image.

1. Create the main.cpp file and write the following sample code into it:

   #include <stdio.h>
   #include <stdlib.h>
   #include <png.h>
   #include <webp/encode.h>
   
   unsigned char* decodePNGtoRGBA(const char* filename, int* width, int* height) {
       FILE *fp = fopen(filename, "rb");
       if (!fp) {
           perror("Failed to open the PNG file");
           return NULL;
       }
   
       png_structp png = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
       if (!png) return NULL;
   
       png_infop info = png_create_info_struct(png);
       if (!info) {
           png_destroy_read_struct(&png, NULL, NULL);
           return NULL;
       }
   
       if (setjmp(png_jmpbuf(png))) {
           png_destroy_read_struct(&png, &info, NULL);
           fclose(fp);
           return NULL;
       }
   
       png_init_io(png, fp);
       png_read_info(png, info);
   
       *width = png_get_image_width(png, info);
       *height = png_get_image_height(png, info);
       png_byte color_type = png_get_color_type(png, info);
       png_byte bit_depth  = png_get_bit_depth(png, info);
   
       if (bit_depth == 16) png_set_strip_16(png);
       if (color_type == PNG_COLOR_TYPE_PALETTE) png_set_palette_to_rgb(png);
       if (color_type == PNG_COLOR_TYPE_GRAY && bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png);
       if (png_get_valid(png, info, PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png);
       if (color_type == PNG_COLOR_TYPE_RGB || color_type == PNG_COLOR_TYPE_GRAY || color_type == PNG_COLOR_TYPE_PALETTE)
           png_set_filler(png, 0xFF, PNG_FILLER_AFTER);
       if (color_type == PNG_COLOR_TYPE_GRAY || color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
           png_set_gray_to_rgb(png);
   
       png_read_update_info(png, info);
   
       unsigned char* rgba_data = (unsigned char*)malloc((*width) * (*height) * 4);
       png_bytep* row_pointers = (png_bytep*)malloc(sizeof(png_bytep) * (*height));
       for (int y = 0; y < *height; y++) {
           row_pointers[y] = rgba_data + (y * (*width) * 4);
       }
   
       png_read_image(png, row_pointers);
   
       fclose(fp);
       free(row_pointers);
       png_destroy_read_struct(&png, &info, NULL);
   
       return rgba_data;
   }
   
   int encodeRGBAtoWebP(unsigned char* rgba_data, int width, int height, const char* output_path) {
       uint8_t* output;
       size_t output_size;
   
       output_size = WebPEncodeRGBA(rgba_data, width, height, width * 4, 80.0, &output);
       
       if (output_size == 0) {
           fprintf(stderr, "Failed to encode WebP\n");
           return -1;
       }
   
       FILE* out_file = fopen(output_path, "wb");
       if (!out_file) {
           perror("Failed to create the output file");
           WebPFree(output);
           return -1;
       }
   
       fwrite(output, output_size, 1, out_file);
       fclose(out_file);
   
       printf("WebP saved to: %s (%zu bytes)\n", output_path, output_size);
   
       WebPFree(output);
       return 0;
   }
   
   int main() {
       const char* input_png = "./input .png";    // Default input
       const char* output_webp = "./output.webp"; // Default output
   
       int width, height;
       unsigned char* rgba_buffer = NULL;
   
       printf("Reading PNG: %s ...\n", input_png);
       rgba_buffer = decodePNGtoRGBA(input_png, &width, &height);
   
       if (rgba_buffer) {
           printf("Decoding successful: %dx%d\n", width, height);
           
           printf("Converting and saving as WebP: %s ...\n", output_webp);
           if (encodeRGBAtoWebP(rgba_buffer, width, height, output_webp) != 0) {
               fprintf(stderr, "WebP conversion failed\n");
           }
   
           // Free the memory allocated by decodePNGtoRGBA.
           free(rgba_buffer);
       } else {
           fprintf(stderr, "Failed to read PNG. Check whether the file exists.\n");
           return 1;
       }
   
       printf("Conversion task completed.\n");
       return 0;
   }

2. Perform the compilation.

   g++ -std=c++17 -I/opt/webpInstall/include -I/opt/webpInstall/include/GraphicsMagick -I/opt/webpInstall/include/webp -L/opt/webpInstall/lib -L/opt/webpInstall/lib64 -lpng -lwebp -Wl,-rpath,/opt/webpInstall/lib64  main.cpp -o main && ./main

Expected result: The output.webp file is generated.