Transparent hexagon-masked collage using PHP GD

help you fix your problem This can be done by first setting its alpha value to 0 of the regions that you want to make them fully transparent (255 for others), and then save it to PNG.
To set the alpha value of pixel-(x,y), it can be done:

image.at<cv::Vec4b>(y, x)[3] = 0;

cv::cvtColor(image, image, CV_BGR2BGRA);

cv::Mat mask; // 0 for transparent regions, 255 otherwise (serve as the alpha channel)

std::vector<cv::Mat> channels;
cv::split(image, channels);

channels.push_back(mask);
cv::Mat result;
cv::merge(channels, result);

To fix the issue you can do I implemented custom ShowCase for activity , I solve this

 Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
        paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));
        mCanvas.drawBitmap(cBitmap,mXPosition,mYPosition,paint);
       // mShape.draw(canvas, mEraser, mXPosition, mYPosition, mShapePadding);
        // Draw the bitmap on our views  canvas.
        canvas.drawBitmap(mBitmap, 0, 0, null);

it fixes the issue Hexagon with rounded corners are complex shapes to create and I usually recommend using SVG for creating them. The need for a transparent background makes it even more better suited for SVG. With SVG you can get better control over the shape, its curves etc and you don't have to add a lot of extra (unnecessary) elements to your markup also.
All that is needed for creating this shape with SVG is to use a single path element along with a few L (line) and A (arc) commands. The L (line) command basically draws a line from point 1 to point 2 whereas the A (arc) command draws an arc of the specified radius (the first two values immediately following the A command).

svg {
  height: 200px;
  width: 240px;
}
path {
  stroke: #777;
  fill: none;
}

body {
  background: black;
}

<svg viewBox='0 0 120 100'>
  <path d='M38,2 
           L82,2 
           A12,12 0 0,1 94,10 
           L112,44 
           A12,12 0 0,1 112,56
           L94,90       
           A12,12 0 0,1 82,98
           L38,98
           A12,12 0 0,1 26,90
           L8,56
           A12,12 0 0,1 8,44
           L26,10
           A12,12 0 0,1 38,2' />
</svg>

.roundHex {
  position: relative;
  margin: 0 auto;
  background: transparent;
  border-radius: 10px;
  height: 300px;
  width: 180px;
  box-sizing: border-box;
  transition: all 1s;
  border: 10px solid transparent;
  border-top-color: black;
  border-bottom-color: black;
}
.roundHex:before,
.roundHex:after {
  content: "";
  border: inherit;
  position: absolute;
  top: -10px;
  left: -10px;
  background: inherit;
  border-radius: inherit;
  height: 100%;
  width: 100%;
}
.roundHex:before {
  transform: rotate(60deg);
}
.roundHex:after {
  transform: rotate(-60deg);
}

<div class="roundHex"></div>

fixed the issue. Will look into that further You can modify the background-color of the pseudo element with gradient to color only half of it:

.hexagon {
  position: relative;
  width: 290px;
  height: 173.21px;
  margin: 86.60px 0;
  border-left: solid 5px #333333;
  border-right: solid 5px #333333;
  background-color: rgb(102, 204, 34, 0.7);
}

.hexagon:before,
.hexagon:after {
  box-sizing: border-box;
  content: "";
  position: absolute;
  z-index: 1;
  width: 212.13px;
  height: 212.13px;
  -webkit-transform: scaleY(0.5774) rotate(-45deg);
  -ms-transform: scaleY(0.5774) rotate(-45deg);
  transform: scaleY(0.5774) rotate(-45deg);
  left: 38.9340px;
}

.hexagon:before {
  top: -106.0660px;
  border-top: solid 7.0711px #333333;
  border-right: solid 7.0711px #333333;
  background:linear-gradient(to bottom left,rgb(102, 204, 34, 0.7) 48%,transparent 48%);
}

.hexagon:after {
  bottom: -106.0660px;
  border-bottom: solid 7.0711px #333333;
  border-left: solid 7.0711px #333333;
  background:linear-gradient(to top right,rgb(102, 204, 34, 0.7) 48%,transparent 49%);
}

<div class="hexagon">
  <span style="font-size: 11px">1</span>
  <span>2     </span>
</div>

body {
 margin:0;
 background:linear-gradient(to right, blue 10%,pink);
}

.box {
  background: url('data:image/svg+xml;utf8,<svg version="1.1" xmlns="http://www.w3.org/2000/svg" width="174" height="200" viewBox="-5 -5 180 220"><path fill="rgb(102, 204, 34, 0.5)" stroke="black" stroke-width="3" d="M86.60254037844386 0L173.20508075688772 50L173.20508075688772 150L86.60254037844386 200L0 150L0 50Z"></path></svg>');
  height: 200px;
  width: 180px;
  background-repeat: no-repeat;
  background-position: center;
  display:inline-block;
  line-height:150px;
  text-align:center;
}
svg {
 display:inline-block;
 vertical-align:top;
}

<div class="box">
  SVG as background
</div>
<svg version="1.1" xmlns="http://www.w3.org/2000/svg" width="174" height="200" viewBox="-5 -5 180 220"><path fill="rgb(102, 204, 34, 0.5)" stroke="black" stroke-width="3" d="M86.60254037844386 0L173.20508075688772 50L173.20508075688772 150L86.60254037844386 200L0 150L0 50Z"></path>
<text x=20 y=80 >Normal SVG</text>
</svg>

body {
 background:linear-gradient(to right,pink, blue);
}
.hexagon {
  position: relative;
  width: 290px;
  height: 290px;
  margin: 50px;
  background:
  linear-gradient(to bottom left,transparent calc(50% - 3px),#000 calc(50% - 3px))0 100%/50% 25% no-repeat,
  linear-gradient(to bottom right,transparent calc(50% - 3px),#000 calc(50% - 3px))100% 100%/50% 25% no-repeat,
  linear-gradient(to top right,transparent calc(50% - 3px),#000 calc(50% - 3px))100% 0/50% 25% no-repeat,
  linear-gradient(to top left,transparent calc(50% - 3px),#000 calc(50% - 3px))0 0/50% 25% no-repeat,
  linear-gradient(#000,#000)0 0/3px 100% no-repeat,
  linear-gradient(#000,#000)100% 0/3px 100% no-repeat,
  rgba(102, 204, 34, 0.5);
  -webkit-clip-path: polygon(50% 0%, 100% 25%, 100% 75%, 50% 100%, 0% 75%, 0% 25%);
clip-path: polygon(50% 0%, 100% 25%, 100% 75%, 50% 100%, 0% 75%, 0% 25%);
}

<div class="hexagon">
</div>