在 C++ 中图像 bmp 中的 Alpha 混合水印 bmp

Question

我在学校有一个任务，要将水印 bmp 图片添加到其他 bmp 图片中。该任务称为 alpha 混合。我必须在用户将在启动时通过程序参数设置的特定坐标插入水印，以及水印混合的 alpha 值。我几乎成功了，但我遇到了小错误。这是代码：

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

#define BMP_SIGNATURE_0         0x42
#define BMP_SIGNATURE_1         0x4D
#define BMP_DEPTH               24
#define BMP_HDR_SIZE            24

int isValidBmp(unsigned char *header)
{
    if (header == NULL)
    return -1;

    if ((header[0] == BMP_SIGNATURE_0) && (header[1] == BMP_SIGNATURE_1))
    {
        if (header[28] != BMP_DEPTH)
            return -1;
        else
            return 0;
    }
    else
        return -1;
}

unsigned long getBmpWidth(unsigned char *header)
{
    unsigned long width;

    if (header == NULL)
        return 0;

    width = ((unsigned long)header[21] << 24) |
        ((unsigned long)header[20] << 16) |
        ((unsigned long)header[19] << 8) |
        (unsigned long)header[18];

    return width;
}

unsigned long getBmpHeight(unsigned char *header)
{
    unsigned long height;

    if (header == NULL)
        return 0;

    height = ((unsigned long)header[25] << 24) |
        ((unsigned long)header[24] << 16) |
        ((unsigned long)header[23] << 8) |
        (unsigned long)header[22];

    return height;
}

int main(int argc, char *argv[])    
{
    FILE *fIn, *fOut, *fWaterMark;
    unsigned char *mIn, *mOut, *mWaterMark;
    unsigned long fsize, zsize;
    unsigned long fwidth, fheight, zwidth, zheight;;

    fIn = fopen("D:\Downloads\image.bmp", "rb");
    if (fIn == NULL)
    {
        printf("ERROR!\n\n");
        return 1;
    }

    fseek(fIn, 0, SEEK_END);
    fsize = ftell(fIn);

    mIn = (unsigned char *)malloc(fsize);
    fseek(fIn, 0, SEEK_SET);
    fread(mIn, sizeof(unsigned char), fsize, fIn);
    fclose(fIn);

    if (isValidBmp(mIn) == -1)
    {
        printf("ERROR!\n\n");
        free(mIn);
        return 1;
    }

    fwidth = getBmpWidth(mIn);
    fheight = getBmpHeight(mIn);

    if ((fwidth == 0) || (fheight == 0))
    {
        printf("ERROR!\n\n");
        free(mIn);
        return 1;
    }

    fWaterMark = fopen("D:\Downloads\watermark.bmp", "rb");
    if (fWaterMark == NULL)
    {
        free(mIn);
        printf("ERROR!\n\n");
        return 1;
    }

    fseek(fWaterMark, 0, SEEK_END);
    zsize = ftell(fWaterMark);

    mWaterMark = (unsigned char *)malloc(zsize);
    fseek(fWaterMark, 0, SEEK_SET);
    fread(mWaterMark, sizeof(unsigned char), zsize, fWaterMark);
    fclose(fWaterMark);

    if (isValidBmp(mWaterMark) == -1)
    {
        printf("ERROR!\n\n");
        free(mIn);
        free(mWaterMark);
        return 1;
    }

    zwidth = getBmpWidth(mWaterMark);
    zheight = getBmpHeight(mWaterMark);

    if ((zwidth == 0) || (zheight == 0))
    {
        printf("ERROR!\n\n");
        free(mIn);
        free(mWaterMark);
        return 1;
    }

    fOut = fopen("D:\Downloads\new_image.bmp", "wb");
    if (fOut == NULL)
    {
        free(mIn);
        free(mWaterMark);
        printf("ERROR!\n\n");
        return 1;
    }

    mOut = (unsigned char *)malloc(fsize);
    fseek(fOut, 0, SEEK_SET);

    double alpha = 0.5;    

    memcpy(mOut, mIn, fsize);
    unsigned int index = BMP_HDR_SIZE;
    unsigned int x = 200, y = 200;
    for (unsigned int i = BMP_HDR_SIZE + x*y; i < x*y + zsize; i++)
    {
        unsigned char v = ((1 - alpha) * mIn[i]) + mWaterMark[index++];
        mOut[i] = v;
    }
    fwrite(mOut, sizeof(unsigned char), fsize, fOut);

    free(mIn);
    free(mOut);
    fclose(fOut);

    return 0;
}

示例 BMP 图片：

Answer 1

问题出在您对二维位图点数组的循环中。将循环更改为双循环。还有：

您没有使用正确的像素范围偏移量。
您需要将宽度乘以 3 才能复制一个像素的所有 3 个分量颜色
您应该使用四舍五入的行长度来确保覆盖所有填充。
您的 y 偏移了图片底部，使用 trueY 使您偏移了顶部。
每个错误都有不同的错误文本有助于了解触发了哪个错误；我把它留作练习，以得出更有用的文字。
我投了你一票，因为我认为这是一个有趣的问题；我必须深入研究 BMP 文件的维基百科页面才能得出最终答案。
您只对源代码使用了 half-alpha，但要重现您在评论中分享的图片，您需要对图片和水印都使用 half-alpha。

另请注意，要重现您在评论中分享的图片，x 必须为 125，y 必须为 100。总而言之，这段代码看起来可行：

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

#define BMP_SIGNATURE_0         0x42
#define BMP_SIGNATURE_1         0x4D
#define BMP_DEPTH               24
#define BMP_HDR_SIZE            24

int isValidBmp(unsigned char *header)
{
    if (header == NULL)
        return -1;

    if ((header[0] == BMP_SIGNATURE_0) && (header[1] == BMP_SIGNATURE_1))
        {
            if (header[28] != BMP_DEPTH)
                return -1;
            else
                return 0;
        }
    else
        return -1;
}

unsigned long getBmpWidth(unsigned char *header)
{
    unsigned long width;

    if (header == NULL)
        return 0;

    width = ((unsigned long)header[21] << 24) |
        ((unsigned long)header[20] << 16) |
        ((unsigned long)header[19] << 8) |
        (unsigned long)header[18];

    return width;
}

unsigned long getBmpHeight(unsigned char *header)
{
    unsigned long height;

    if (header == NULL)
        return 0;

    height = ((unsigned long)header[25] << 24) |
        ((unsigned long)header[24] << 16) |
        ((unsigned long)header[23] << 8) |
        (unsigned long)header[22];

    return height;
}

unsigned long getPixelOffset(unsigned char *header)
{
    unsigned long offset;

    if (header == NULL)
        return 0;

    offset = ((unsigned long)header[13] << 24) |
        ((unsigned long)header[12] << 16) |
        ((unsigned long)header[11] << 8) |
        (unsigned long)header[10];

    return offset;
}

int main(int argc, char *argv[])    
{
    FILE *fIn, *fOut, *fWaterMark;
    unsigned char *mIn, *mOut, *mWaterMark;
    unsigned long fsize, zsize;
    unsigned long fwidth, fheight, zwidth, zheight;
    unsigned long foffset, frow, zoffset, zrow;

    fIn = fopen("srcso.bmp", "rb");
    if (fIn == NULL)
        {
            printf("ERROR 1!\n\n");
            return 1;
        }

    fseek(fIn, 0, SEEK_END);
    fsize = ftell(fIn);

    mIn = (unsigned char *)malloc(fsize);
    fseek(fIn, 0, SEEK_SET);
    fread(mIn, sizeof(unsigned char), fsize, fIn);
    fclose(fIn);

    if (isValidBmp(mIn) == -1)
        {
            printf("ERROR 2!\n\n");
            free(mIn);
            return 1;
        }

    fwidth = getBmpWidth(mIn);
    fheight = getBmpHeight(mIn);
    foffset = getPixelOffset(mIn);
    frow = (BMP_DEPTH * fwidth + 31) / 32 * 4;

    if ((fwidth == 0) || (fheight == 0))
        {
            printf("ERROR 3!\n\n");
            free(mIn);
            return 1;
        }

    fWaterMark = fopen("wmso.bmp", "rb");
    if (fWaterMark == NULL)
        {
            free(mIn);
            printf("ERROR 4!\n\n");
            return 1;
        }

    fseek(fWaterMark, 0, SEEK_END);
    zsize = ftell(fWaterMark);

    mWaterMark = (unsigned char *)malloc(zsize);
    fseek(fWaterMark, 0, SEEK_SET);
    fread(mWaterMark, sizeof(unsigned char), zsize, fWaterMark);
    fclose(fWaterMark);

    if (isValidBmp(mWaterMark) == -1)
        {
            printf("ERROR 5!\n\n");
            free(mIn);
            free(mWaterMark);
            return 1;
        }

    zwidth = getBmpWidth(mWaterMark);
    zheight = getBmpHeight(mWaterMark);
    zoffset = getPixelOffset(mWaterMark);
    zrow = (BMP_DEPTH * zwidth + 31) / 32 * 4;

    if ((zwidth == 0) || (zheight == 0))
        {
            printf("ERROR 6!\n\n");
            free(mIn);
            free(mWaterMark);
            return 1;
        }

    fOut = fopen("new_image.bmp", "wb");
    if (fOut == NULL)
        {
            free(mIn);
            free(mWaterMark);
            printf("ERROR 7!\n\n");
            return 1;
        }

    mOut = (unsigned char *)malloc(fsize);
    fseek(fOut, 0, SEEK_SET);

    double alpha = 0.5;    

    std::copy(mIn, mIn + fsize, mOut);
    ::free(mIn);
    mIn = 0;
    unsigned int index = BMP_HDR_SIZE;
    unsigned int x = 200, y = 200;
    unsigned int trueY = fheight - y - zheight;
    for (unsigned int j = 0; j < zheight; ++j) {
        for (unsigned int i = 0; i < zwidth*3; ++i) {
            const size_t offset = foffset + (j + trueY) * frow + i + x*3;
            unsigned char * const offOut = mOut + offset; 
            unsigned char * const offWM = mWaterMark + zoffset + j * zrow + i;
            *offOut *= 1 - alpha;
            *offWM *= 1 - alpha;
            if ((unsigned int)*offOut + (unsigned int)*offWM < 265)
                *offOut += *offWM;
            else
                *offOut = 255;
        }
    }
    fwrite(mOut, sizeof(unsigned char), fsize, fOut);
    fclose(fOut);

    ::free(mWaterMark);
    ::free(mOut);

    return 0;
}

请注意，如果您的 alpha 小于 0.5，您会得到奇怪的颜色伪影，因为该算法可以 ping 说红色，但让蓝色和绿色保持正常，使蓝色和绿色看起来比它们应有的更亮。从技术上讲，当像素设置发生 else 情况时，它应该通过向其他两个像素添加更多像素来补偿这种影响，从而真正影响其他两个像素。

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