sensor.c module question

I’m trying to understand the sensor.c module code and I have a question about the DCMI_DMAConvCpltUser() function…I hope you can help me to get on the right track.
As the comment says:

// This function is called back after each line transfer is complete,
// with a pointer to the line buffer that was used. At this point the
// DMA transfers the next line to the other half of the line buffer.
void DCMI_DMAConvCpltUser(uint32_t addr)

So, if “addr” (and consequently “src” below) contains the pointer to the line buffer, why when the memory copy is performed, the “src” is offset by MAIN_FB()->x?

uint8_t *src = (uint8_t*) addr;
                case PIXFORMAT_GRAYSCALE:
                    dst += (offset - MAIN_FB()->y) * MAIN_FB()->w;
                    if (sensor.gs_bpp == 1) {
                        // 1BPP GRAYSCALE.
                        src += MAIN_FB()->x;
                        unaligned_memcpy(dst, src, MAIN_FB()->w);

Thanks in advance,

x,y are the window offset set in sensor_set_windowing.

oh yes…I got it now.