Alpha blend watermark bmp in image bmp in c++ - c++

Question

I have a task at school to add watermark bmp image into some other bmp image. The task is called alpha blending. I have to insert watermark at specific coordinates which user will set through program parameters on start, as well as alpha value for watermark blending. I am almost succeed, but I am getting small error. Here is the code:
#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;
}
Sample BMP image:

Answer 1

The problem is in your loop over a 2-dimensional array of bitmap points. Change your loop to be a double loop. Also:
You are not using the correct offset for the pixel range.
You need to multiply width times 3 to copy all 3 component colors of a pixels
You should use the rounded off row length to make sure you cover any padding.
Your y is offset from the bottom of the picture, use trueY to have your offset from the top.
It helps to have a different error text for each error to know which error is triggering; I've left it as an exercise to come up with more helpful text.
I voted you up because I think this is an interesting question; I had to dig into the Wikipedia page for BMP files to come up with the final answer.
You were only using half-alpha on the source, but to reproduce that image you shared in your comments, you need to take half-alpha for both the image and the watermark.
Note also to reproduce the image you shared in the comments, x must be 125 and y must be 100. All that said, this code looks like it works:
#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;
}
Note, the if you have an alpha less than .5, you will get strange color artifacts because the algorithm could ping say red but leave blue and green normal making the blue and green seem brighter than they should be. Technically, when the else case happens for the pixel setting, it should really affect the other two pixels by adding more to them to compensate for this effect.