Parts of a RenderTexture + Weird shader/blendmode bug

[Vovosunt]

1) I need to reuse parts of a single renderTexture as Textures, so I was wondering how can you do it in SFML?
(or do I actually have to code it in openGL? getTexture(sf::Rect) would be nice )

2) I also found a weird bug... or maybe it's just my GPU
Anyway, I'm basically multiplying the renderTexture over itself and shifting it every time upwards by 1, 2, 4, 8 and so on pixels. For some reason the renderTexture starts "banding" and multiplying itself much further and some weird artifacts appear:

I've tried both shaders and just using BlendMultiply, but the effect perseveres.
But it might be just my GPU... laptop Intel GPUs are shit.

But if anyone knows a better and faster way to "bleed" or smear pixels, I would really appreciate if you'd tell me about it

[Nexus]

To get only a part of a sf::RenderTexture, you probably need to go the long way via sf::Image.

texture.loadFromImage(renderTexture.getTexture().copyToImage(), rect);

But can't you just apply the texture rect when you draw something? Then you don't have to create a new texture or to copy masses of pixels around.

[Vovosunt]

To get only a part of a sf::RenderTexture, you probably need to go the long way via sf::Image.

texture.loadFromImage(renderTexture.getTexture().copyToImage(), rect);

But can't you just apply the texture rect when you draw something? Then you don't have to create a new texture or to copy masses of pixels around.

Actually, I think I can, since I'm using shaders anyway
Thanks!

Now if I only could make this work just a bit faster, damn it XD