位图将 16 bpp 转换为 8 bpp

Question

我正在寻找将位图从 16 bpp 转换为 8，我的灵感来自我发现将 24 bpp 转换为 8 bpp 的代码，我认为我的文件的写入是正确的（见图）但文件是仍然无效！

我认为数据是以小端存储的，但我不确定所以我认为我的问题在于提取 RGB 颜色，即使它不应该使文件无效

#include <stdio.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <inttypes.h>

#pragma pack(push, 2)
typedef struct {
    int16_t bfType;
    int32_t bfSize;
    int16_t bfReserved1;
    int16_t bfReserved2;
    int32_t bfOffBits;
} BITMAPFILEHEADER;

typedef struct {
    int32_t biSize;
    int32_t biWidth;
    int32_t biHeight;
    int16_t biPlanes;
    int16_t biBitCount;
    int32_t biCompression;
    int32_t biSizeImage;
    int32_t biXPelsPerMeter;
    int32_t biYPelsPerMeter;
    int32_t biClrUsed;
    int32_t biClrImportant;
} BITMAPINFOHEADER;
#pragma pack(pop)

int main()
{
    FILE* fin = fopen("SCR.bmp", "rb");
    FILE* fout = fopen("fout.bmp", "wb");
    if(!fin) { printf("fin error\n"); goto error; }
    if(!fout) { printf("fout error\n"); goto error; }

    BITMAPFILEHEADER bf;
    BITMAPINFOHEADER bi;
    fread(&bf, sizeof bf, 1, fin);
    fread(&bi, sizeof bi, 1, fin);

    if(sizeof bf != 14) { printf("Wrong pragma pack\n"); goto error; }
    if(sizeof bi != 40) { printf("Wrong pragma pack\n"); goto error; }
    if(bf.bfType != 0x4D42) { printf("not bmp, or not LE system\n"); goto error; }
    //if(bi.biSize != 40) { printf("Can't handle this bitmap format\n"); goto error; }
    //if(bi.biBitCount != 24) { printf("not 24-bit\n"); goto error; }

    int height = bi.biHeight;
    if(height < 0) height = -height;

    //width in bytes:
    int src_wb = ((bi.biWidth * 16 + 31) / 32) * 4;
    printf(" src wb =%d et width =%d et %d",src_wb,bi.biWidth,bi.biHeight);
    int dst_wb = ((bi.biWidth * 8 + 31) / 32) * 4;
    int src_size = src_wb * height;
    int dst_size = dst_wb * height;

    //allocate for source and destination
    uint8_t *src = malloc(src_size);
    uint8_t *dst = malloc(dst_size);

    //read pixels
    fread(src, 1, src_size, fin);

    //make gray scale color-table
    uint8_t clr[1024] = { 0 };
    for(int i = 0; i < 256; i++)
        clr[i * 4 + 0] = clr[i * 4 + 1] = clr[i * 4 + 2] = (uint8_t)i;

    for(int y = height - 1; y >= 0; y--)
    {
        for(int x = 0; x < bi.biWidth; x++)
        {
            uint8_t pt1 = src[y * src_wb + x * 2 + 0]; // first byte of pixel (LSB because little endian)
            uint8_t pt2 = src[y * src_wb + x * 2 + 1]; // second byte of pixel (MSB)
          //uint8_t grn = src[y * src_wb + x * 3 + 2];
            uint8_t red = pt2>>3;// 5 bit
            uint8_t grn = (uint8_t)((pt1 & 0xE)<<5)| (uint8_t)((pt2 &7)>>5) ;// 6 bit
            uint8_t blu = (pt1 & 0x11); // 5 bit
            uint8_t gry = (uint8_t)(.33 * red + .34 * grn + .33 * blu);
            dst[y * dst_wb + x] = gry; //this will match the index in color-table
        }
    }

    //modify bitmap headers
    bf.bfSize = 54 + 1024 + dst_size;
    bf.bfOffBits =54 +1024 ;
    bi.biBitCount = 8;
    bi.biSizeImage = dst_size;
    bi.biClrUsed=256;
    bi.biCompression=0;

    fwrite(&bf, sizeof bf, 1, fout);
    fwrite(&bi, sizeof bi, 1, fout);
    fwrite(clr, 1, 1024, fout);
    fwrite(dst, 1, dst_size, fout);
    free(src);
    free(dst);
error:
    fclose(fout);
    fclose(fin);
    return 0;
}