Centering Background Image Horizontally for Responsive Layout

Question

I have two background images stacked on top of each other. Based on the results of an ajax call, the top image is slid up to reveal the image behind it. I have the following layout:

<div id="bg_before"><img class="bg_image"></div>
<div id="bg_after"><img class="bg_image"></div>

The image uploaded will vary based on what a user chooses. The CSS is as follows:

#bg_before, #bg_after {
    position: absolute;
    top: 0;
    left: 0;
    overflow: hidden;
    z-index: -10;
    width: 100%;
    height: 100%;
}

.bg_image {
    background-size: cover;
    overflow: hidden;
}

The image centers properly on a monitor (we recommend uploading an image 1920x1080). However, on a tablet or mobile I'd like the 1920x1080 image to fit centered, cropping the left and right of the image and keeping the height at 100%. I cannot seem to get this to work. I've tried transform, margin-left:-50% and background-position: center; to no avail.

Example at: http://cbotriage.bid/boards/appeal.php?item_id=569578

EDIT: I'm trying to shift a background image to the left so that the center of the background image lands in the center of the container div "bg_before" which is 100% width and 100% height of the display. Currently the background image is top left of the "bg_before" div no matter the resolution of the window.

Essentially I'm trying to make the background image center no matter the device (landscape/portrait).

Answer 1

Hello M Charles please use this css... like i did it

#bg_before, #bg_after {
    position: absolute;
    top: 0;
    left: 0;
    overflow: hidden;
    z-index: -10;
    width: 100%;
    height: 100%;
}
.bg_image {
    background-size: cover;
    overflow: hidden;
 }
#bg_before .bg_image {
    object-fit: cover;
    object-position: center;
    width: 100%;
    height: 100%;
}

<div id="bg_before"><img class="bg_image"></div>
<div id="bg_after"><img class="bg_image"></div>