Created attachment 3304 [details]
My stuff to verify the problem

Hello,
This problem concerns at least SDL_RENDERER_ACCELERATED. I didn't made tests with Software Renderers.

The test is done on various Linux platforms but it certainly concerns SDL2 in general and since the begining. It's not a new problem as I found games development forums already talking about it in 2015 (Westnoth)!

After having read many forums relating this problem about SDL2 and SDL2_gpu uses in game development, maybe I still didn't fully understand how to use blend modes and maybe when to call these functions exactly but I also think there is a real problem in SDL2 about transparency management.

The problem is that one:

I'm trying to RenderCopy an alpha image in a transparent or semi-transparent texture and then RenderCopy this result in a background.
The report is that original pixels are lost during RenderCopy on even fully transparent texture and the more transparent intermediate texture you add, the more pixel you loose whereas, as we are talking about transparent texture, no alteration should be perceived on the result.

Here is a reference image which illustrates what I'm expecting for:
![reference](https://i.imgur.com/dCsknVp.png)

So I wrote a test (main.c attached in a zip) which is supposed the produce the same result and [here](https://i.imgur.com/l8HRg68.png) is what I get. 

I explain:

In the 1st square on the left, I render the white disk directly on the background. The result is already a bit more transparent than expected.

In the 2nd square, the white disk is rendered on a solid black texture and then the result is rendered in the background. The result is as expected.

In the 3rd square, the white disk is rendered in a half transparent texture and the result is rendered in the background. You can see that the white disk is more transparent than in the 1st square which is not normal.

In the 4th square, the white disk is rendered in a fully transparent texture and the result is rendered in the background. You can see that the white disk is more transparent than in the 1st square which is not normal. the transparent intermediate texture shouldn't alter the white disk in any way.

In the 5th disk, I render the white disk in a fully transparent texture. Then I render this result in another fully transparent texture, then this result in the background. The white disk is again more transparent than in the 4th square, showing that transparency filters out pixels in some way!
 
This behavior is a very very big problem and I really hope you could quickly take care about it.

I attached my test source code and the images used to experiment.

Kind Regards,