Ground Overlay with Transparency - google-maps-api-3

Question

I have an online map that contains a ground-overlay image, and would like to make the image semi-transparent so that the base map will show through. Is it possible to add a transparency value to this overlay?
http://www.tpwd.state.tx.us/fishboat/fish/recreational/lakes/ingulf1c.phtml
Here's the code for the ground overlay:
var imageBounds = new google.maps.LatLngBounds(
new google.maps.LatLng(25.71438633861514, -98.33555959121725),
new google.maps.LatLng(30.40813339247205, -93.57953893270167));
function initialize() {
var map = new google.maps.Map(document.getElementById("map"), {
center: new google.maps.LatLng(28.0041,-96.3618),
zoom: 7,
mapTypeId: 'roadmap'
});
overlay = new google.maps.GroundOverlay(
'/fishboat/fish/recreational/lakes/images/statemaps/gulfregion.gif',
imageBounds);
overlay.setMap(map);

Answer 1

There is a setOpacity method of the GroundOverlay (works for me with values between 0 and 1.0):
var overlay = null;
function initialize() {
var map = new google.maps.Map(document.getElementById("map"), {
center: new google.maps.LatLng(28.0041,-96.3618),
zoom: 7,
mapTypeId: 'roadmap'
});
overlay = new google.maps.GroundOverlay('http://www.tpwd.state.tx.us/fishboat/fish/recreational/lakes/images/statemaps/gulfregion.gif',
imageBounds);
overlay.setMap(map);
}
google.maps.event.addDomListener(window, 'load', initialize);
function setOpacity() {
var opacityStr = document.getElementById('opacity').value;
var opacity = parseFloat(opacityStr);
overlay.setOpacity(opacity);
}
<input type="text" id="opacity" value="0.2"></input>
<input type="button" value="setOpacity" onclick="setOpacity();"></input>
working example - proof of concept fiddle
code snippet:
var overlay = null;
function initMap() {
var map = new google.maps.Map(document.getElementById("map"), {
center: new google.maps.LatLng(28.0041, -96.3618),
zoom: 7,
mapTypeId: 'roadmap'
});
var imageBounds = new google.maps.LatLngBounds(
new google.maps.LatLng(25.71438633861514, -98.33555959121725),
new google.maps.LatLng(30.40813339247205, -93.57953893270167));
overlay = new google.maps.GroundOverlay(
'http://www.tpwd.state.tx.us/fishboat/fish/recreational/lakes/images/statemaps/gulfregion.gif',
imageBounds);
overlay.setMap(map);
map.fitBounds(imageBounds);
}
function setOpacity() {
var opacityStr = document.getElementById('opacity').value;
var opacity = parseFloat(opacityStr);
overlay.setOpacity(opacity);
}
html,
body {
width: 100%;
height: 100%;
margin: 0px;
padding: 0px;
}
#map {
width: 100%;
height: 90%;
}
<div id="map"></div>
<input type="text" id="opacity" value="0.2" />
<input type="button" value="setOpacity" onclick="setOpacity();" />
<!-- Replace the value of the key parameter with your own API key. -->
<script async defer src="https://maps.googleapis.com/maps/api/js?key=AIzaSyCkUOdZ5y7hMm0yrcCQoCvLwzdM6M8s5qk&callback=initMap"></script>