how does this code work?

hi, is someone could explain to me the following part of code. It comes from the book "teach yourself DirectX7 in 24 hours". It displays a street (made up of 3 bitmap image A, B and C). the goal of this algorithm is to find out the part of each image which will be displayed. thanks for any help you can give. // loop through the layers for (int i=0;i.parallax; // set position of first object long obj_pos=layers[i].start_x+ (screen_pos-layers[i].start_x) /layers[i].width; // loop through objects until off right side of screen while (obj_pos=screen_pos) { // yes, calculate the offset to the left edge of the screen int screen_x=obj_pos-screen_pos; // clip the object if necessary rct.left=0; rct.top=0; rct.right=layers[i].width; rct.bottom=layers[i].height; // test for clipping if (screen_x<0) { rct.left-=screen_x; screen_x=0; } else if (screen_x+rct.right>640) { rct.right=640-screen_x; } if (rct.bottom+layers[i].start_y>480) { rct.bottom=480-layers[i].start_y; } // draw the object to the screen lpDDSBack->BltFast(screen_x,layers[i].start_y,layers[i].surf,&rct,layers[i].blit_flags); } // increment to next object position obj_pos+=layers[i].interval; } }