如何使用c使用libpng从png图像更改rgb值? [英] How to change rgb values from a png image with libpng using c?

问题描述

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，它只打开一个 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，有没有有效的方法来做到这一点?

What is a recommendable way to change the rgb values of a png image? Right now I'm using an example code to see the way libpng works, it only opens a png image and writes another png image, however I would like to change the RGB values of this new image so that it gets to be all blue, green or red, whatever is my choice, is there an efficient way to do it?

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;
}

推荐答案

要修改图片，你必须对 px 数据做一些工作.

To modify the image, you have to do some work on px data.

例如，如果您想将图像设置为红色，您可以这样做

For instance, if you want to set image to RED, you can do this

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];
        }
    }
}

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