# How to use opacity in drawKeypoints or drawMatches?

**URL:** <https://forum.opencv.org/t/how-to-use-opacity-in-drawkeypoints-or-drawmatches/20792>\
**Category:** Python\
**Tags:** features2d, imgproc\
**Created:** [April 10, 2025, 4:35pm UTC](https://forum.opencv.org/t/how-to-use-opacity-in-drawkeypoints-or-drawmatches/20792 "2025-04-10T16:35:47Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![dustinfreeman](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/dustinfreeman/32/12355_2.png) [@dustinfreeman](https://forum.opencv.org/u/dustinfreeman)\
**Post date:** [April 10, 2025, 4:35pm UTC](https://forum.opencv.org/t/how-to-use-opacity-in-drawkeypoints-or-drawmatches/20792/1 "2025-04-10T16:35:47Z")

</div>

I’m using OpenCV 4.10 in python, but this should generalize to anyone using drawKeypoints or drawMatches. I want to draw the keypoints with less than 100% opacity over the image. I have tried:  
`img3 = cv2.drawKeypoints(img1,kp1,img3,color=(0,200,0, 128))`  
`img3 = cv2.drawKeypoints(img1,kp1,img3,color=(0,200,0, 0.5))`  
I even threw in:  
`img3 = cv2.drawKeypoints(img1,kp1,img3,color=(0,200,0), alpha=0.5)`

But none of the above work. Is this not possible?

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<div class="post-metadata">

**Author:** ![crackwitz](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/crackwitz/32/14_2.png) [@crackwitz](https://forum.opencv.org/u/crackwitz)\
**Post date:** [April 10, 2025, 8:51pm UTC](https://forum.opencv.org/t/how-to-use-opacity-in-drawkeypoints-or-drawmatches/20792/2 "2025-04-10T20:51:08Z")

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an alpha channel on the color won’t do what you want it to do. it’s just a fourth color channel to all the drawing functions. the drawing functions in opencv are _ancient_ and have no concept of transparency.

the LINE\_AA mode lines kinda blend into the base, but also don’t take any alpha values into account, _and_ the implementations and results are a disaster (thickness not correct in at least two different ways).

neither `drawKeypoints` nor `drawMatches` have any `alpha` parameter. why should that work? documentation:

- [OpenCV: Drawing Function of Keypoints and Matches](https://docs.opencv.org/4.x/d4/d5d/group __features2d__ draw.html#ga5d2bafe8c1c45289bc3403a40fb88920) (one signature)
- [OpenCV: Drawing Function of Keypoints and Matches](https://docs.opencv.org/4.x/d4/d5d/group __features2d__ draw.html#gaf92cd1c6e9400e4753ce393d2fdc06b0) (three signatuers)

if you want alpha blending, you’ll have to do that manually. draw onto a blank canvas, draw calls fully opaque, then blend that onto the real destination, with `addWeighted()`

opencv has no functions to actually alpha-blend with per-pixel alpha. to achieve that, you have to take the alpha channel and apply the blending equation to the involved matrices/arrays, making sure to keep the value ranges and data types in mind.

```python
# alpha being an array with values ranged 0.0 .. 1.0
# if typed uint8, then products will need enough bits to be 16 bit, plus a sign, so everything must be int32

# simpler, two multiplications, no worries about underflow
composite = alpha * overlay + (1-alpha) * base

# one multiplication, but mind the subtraction will underflow an uint8
# so .astype() to int16 first (int16, not uint16) or do everything with floats
composite = base + alpha * (overlay - base)

```