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Efficient Alpha Blending

asked 2013-07-22 13:02:53 -0500

Adi gravatar image

I want to write the fastest alpha blending possible with OpenCV.
Alpha blending means taking 2 (usually color) images, say A and B and an additional single channel image W, the alpha channel, in my case a CV_8UC1 (but a general solution should probably support CV_32FC1 too) and perform a weighted convex sum of the two images, such that the alpha channel is scaled to between [0,1]. This allows creating transparency effects in the blending.

My current solution transforms the singl channel image intoa 3 channel image, alpha3C and the does the following:

cv::Mat blended = A.mul(alpha3C, 1.0/255) + B.mul(cv::Scalar::all(255)-alpha3C, 1.0/255);

This seems quite inefficient due to the following issues:

  1. The alpha channel must be converted to 3 channels which takes up both run-time and extra (x4) memory. This is because mul() cannot handle images of 2 different channel numbers.
  2. The scaling is done twice, and subtraction requires a temporary image for storage.

Of course, I can write my own loop over all the pixels, but I was wondering if it is possible to extend MatExpr to support such usage, which isn't so uncommon.

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answered 2014-04-02 12:39:28 -0500

B. Bogart gravatar image

If you have opencv_gpu, you could try cv::gpu::alphaComp(), which is likely quite efficient!

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Unfortunately, I need it on multiple mobile platforms with no (NVIDIA) GPU.

Adi gravatar imageAdi ( 2014-04-03 07:07:22 -0500 )edit

@Adi ok. Do you have to render in opencv, or could you render in opengl? I have been using alpha channels in my application, and when I was doing the rendering in opencv, I was doing so with a loop over all pixels... Now I'm rendering in opengl (openframeworks), but applying the alpha channel is not efficient either: http://answers.opencv.org/question/31136/fastest-way-to-apply-alpha-channel-to-image/ OpenCV does need some better alpha handling...

B. Bogart gravatar imageB. Bogart ( 2014-04-04 11:32:57 -0500 )edit

@B.Bogart: Can you share your OpenGL code?

Adi gravatar imageAdi ( 2014-04-07 08:48:22 -0500 )edit

@Adi is my code useful in being in openframeworks?

B. Bogart gravatar imageB. Bogart ( 2014-04-07 10:53:58 -0500 )edit

I don't know.

Adi gravatar imageAdi ( 2014-04-08 05:43:08 -0500 )edit
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Asked: 2013-07-22 13:02:53 -0500

Seen: 814 times

Last updated: Apr 02 '14