C。读取 BMP 图像像素的问题

Question

我正在尝试使用以下代码读取 BMP 图像的像素。首先，我使用我的函数“readBMPHeader”读取相关图像数据（宽度、高度和标题）。接下来我想使用函数“mapBMP”逐行读取文件的像素。但是，我得到的 bmp 图像“dali”的像素值（您可以在下面找到）不正确，我不知道我做错了什么。你能告诉我我的错在哪里吗？

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
//Macros
#define ALIGNMENT 4
#define HEADER_LOCATION 0x0000
#define DATA_START_LOCATION 0x000A
#define WIDTH_LOCATION 0x0012
#define HEIGHT_LOCATION 0x0016
#define BITS_PER_PIXEL_LOCATION 0x001C


typedef unsigned char byte;



int readBMPHeader(unsigned int* image_header, unsigned int* image_height, unsigned int* image_width,
                  unsigned int* image_start, const char* file_name)
{
  // Here we open the given file for reading in binary mode.
  FILE* image = fopen(file_name, "rb");

  // Here we check whether opening the given file was successful.
  if (image == NULL)
  {
    printf("Invalid File\n");
    return -1;
  }

  // Here we read in the header of the given BM image.
  fseek(image, HEADER_LOCATION, SEEK_SET);
  fread(image_header, 2, 1, image);
  // Here we read in the height of the given BM image.
  fseek(image, HEIGHT_LOCATION, SEEK_SET);
  fread(image_height, 4, 1, image);
  // Here we read in the width of the given BM image.
  fseek(image, WIDTH_LOCATION, SEEK_SET);
  fread(image_width, 4, 1, image);
  fseek(image, DATA_START_LOCATION, SEEK_SET);
  fread(image_start, 4, 1, image);
  fclose(image);

  return 0;

}

void mapBMP(unsigned int height, unsigned int width, char mapped_image[height][width], unsigned int image_start, const char* file_name)
{
  FILE* image = fopen(file_name, "rb");
  // Here we read in the number of bits per pixel in the given BM image.
  unsigned int bits_per_pixel;
  fseek(image, BITS_PER_PIXEL_LOCATION, SEEK_SET);
  fread(&bits_per_pixel, 2, 1, image);
  // Here we compute the number of bytes per pixel in the given BM image.
  unsigned int bytes_per_pixel = ((unsigned int) bits_per_pixel) / 8;

  // Here we compute the stride, i.e. the number of bytes in a row (rounded up by alignment)
  unsigned int stride = (width * bytes_per_pixel) + (ALIGNMENT - 1);
  stride /= ALIGNMENT;
  stride *= ALIGNMENT;
  // Here we read in the pixels of the image, the loops have been shortened for testing purposes.
  byte pixel[bytes_per_pixel];
  for (unsigned int i = height - 1; i > height - 2; i = i - 1) // The proper condition is:(unsigned int i = height - 1; i > 0; i = i - 1)
  {
    fseek(image, i * stride, image_start);

    for (unsigned int j = 0; j < 30; j = j + 1) // The proper condition is: (unsigned int j = 0; j < width; j = j + 1)
    {
      fread(pixel, 3, 1, image);
      printf("pixel = %x %x %x\n", pixel[0], pixel[1], pixel[2]);
    }
  }

  fclose(image);
}

int main()
{


  // Here is the BMP Image test.
  unsigned int image_header;
  unsigned int image_width;
  unsigned int image_height;
  unsigned int image_start;
  char* file_name = "dali.bmp";
  int file_opened = readBMPHeader(&image_header, &image_height, &image_width, &image_start, file_name);

  if (file_opened == -1)
    printf("File could not be opened.");
  else
  {
    // Some prints to see the collected data.
    printf("Size of byte in byte = %llu \n", sizeof(byte));
    printf("header (in hex) = %x \n", image_header);
    printf("height (in pixels) = %u \n", image_height);
    printf("width (in pixels) = %u \n", image_width);
    printf("image_start (in bytes) = %u \n", image_start);

    char mapped_image[image_height][image_width];
    mapBMP(image_height, image_width, mapped_image, image_start, file_name);
    //printTable(image_height, image_width, mapped_image);
  }

  return 0;
}

Answer 1

除了将16位读成32位整数外，图片至少有这个问题

张贴图片显示为压缩文件大小 675K 字节。 @Chris Dodd
读取文件并保存为 24 位/像素生成 1133KB 文件。
OP 的代码无法正确读取发布的文件。

将文件重新保存为未压缩的 24 位/像素（全彩色）并重试。

更好的代码会测试和报告标头的所有十六进制数据，以确保预期的编码。

最好使用固定宽度

uint8_t, uint16_t, uint32_t

类型而不是

char, unsigned

来读取文件。

Ref BMP 文件格式