Question

更改png图像的rgb值的推荐方法是什么？现在，我正在使用示例代码来查看libpng的工作方式，它只会打开png图像并写入另一个png图像，但是我想更改此新图像的RGB值，使其变为全蓝色，绿色或红色，无论我选择什么，有没有一种有效的方法？

代码：

#include <stdlib.h>
#include <stdio.h>
#include <png.h>

int width, height;
png_byte color_type;
png_byte bit_depth;
png_bytep *row_pointers;

void read_png_file(char *filename) {
  FILE *fp = fopen(filename, "rb");
  if(!fp) abort();
  png_structp png = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
  if(!png) abort();
  png_infop info = png_create_info_struct(png);
  if(!info) abort();
  if(setjmp(png_jmpbuf(png))) abort();
  png_init_io(png, fp);
  png_read_info(png, info);
  width      = png_get_image_width(png, info);
  height     = png_get_image_height(png, info);
  color_type = png_get_color_type(png, info);
  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);

  row_pointers = (png_bytep*)malloc(sizeof(png_bytep) * height);
  for(int y = 0; y < height; y++) {
    row_pointers[y] = (png_byte*)malloc(png_get_rowbytes(png,info));
  }

  png_read_image(png, row_pointers);

  fclose(fp);
}

void write_png_file(char *filename) {
  int y;

  FILE *fp = fopen(filename, "wb");
  if(!fp) abort();

  png_structp png = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
  if (!png) abort();

  png_infop info = png_create_info_struct(png);
  if (!info) abort();

  if (setjmp(png_jmpbuf(png))) abort();

  png_init_io(png, fp);

  png_set_IHDR(
    png,
    info,
    width, height,
    8,
    PNG_COLOR_TYPE_RGBA,
    PNG_INTERLACE_NONE,
    PNG_COMPRESSION_TYPE_DEFAULT,
    PNG_FILTER_TYPE_DEFAULT
  );

  png_set_expand(png);
  png_write_info(png, info);
  png_write_image(png, row_pointers);
  png_write_end(png, NULL);

  for(int y = 0; y < height; y++) {
    free(row_pointers[y]);
  }
  free(row_pointers);

  fclose(fp);
}

void process_png_file() {
  for(int y = 0; y < height; y++) {
    png_bytep row = row_pointers[y];
    for(int x = 0; x < width; x++) {
      png_bytep px = &(row[x * 4]);
    }
  }
}

int main(int argc, char *argv[]) {
  if(argc != 3) abort();

  read_png_file(argv[1]);
  process_png_file();
  write_png_file(argv[2]);

  return 0;
}

Answer 1

要修改图像，您必须对px数据做一些工作。

例如，如果要将图像设置为红色，则可以这样做

static void process_png(void) {

    /* set wanted color as red */
    png_byte wanted_color[] = {255,0,0};

    for (unsigned int y = 0; y < height; y++) {
        png_bytep row = row_pointers[y];
        for (unsigned int x = 0; x < width; x++) {
            png_bytep px = &(row[x * 4]);

            /* set pixel to wanted color*/
            for(int i = 0; i < 3; ++i)
                px[i] = wanted_color[i];
        }
    }
}

如何使用c从带有libpng的png图像中更改RGB值？

1 个答案: