ExternBitmap Alpha Blending Issue (9.30 ogl package)

Question

Hi EW Team，

when I use exter bitmap, I found the alpha blending is confusing. 

The result is 

Actually, the white area should be apparent, but it obviously not.  When I change the extern bitmap value to be 0x50505050， it

behaves as correct as the right gray part.

So I 'm wondering that did it use premultied alpha? Cause I have seen you use glBendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA) 

Another question is when I tried it on soft 9.30 package, it is another effect as below(ignore the rotation):

What‘s the reason?

BR

Arnold

 

Answer 1

Hello Arnold,

yes, in case of OpenGL PP, the pixel data is stored as pre-multiplied values. Accordingly, the contents within the Extern bitmap has to be pre-multiplied correctly. Please see the example in the section: Bitmap color format EW_PIXEL_FORMAT_NATIVE variant RGBA8888, especially the usage of the macros EW_PREMULTIPLY_COLOR_CHANNELS, etc.

In case of the SW version, it seems that your PP also uses the pre-multiplied format. In such case if the pixel-values within the bitmap are not pre-multiplied, operations with the pixel values will cause overflows overwriting the adjacent color channels.

Does it help you further?

Best regards

Paul Banach

Comments

Thanks , it helps a lot!