Strange behaviour with setTextureRect flip

[hagel]

I found a strange bug while working on my game today.
I had to flip my sprites for different directions. I used to use setScale with (-1,1) and it worked great, but I had to manage positions etc. I then found out about negative width on the setTextureRect function and started using it.

Here's the problem (SFML 2.3).
When drawing a flipped sprite ( SPR.setTextureRect(sf::IntRect(width, 0, -width, height); ) and setting this sprite's position to a half pixel (0.5f, 1.5f ... X.5f) does not use the correct rectangle. It actually offsets it one pixel to the right, but ONLY at half pixel positions. I assume this is some kind of rounding error.

But why would the sprite's position have any impact on the texture rectangle?
I temporarily fixed it by casting my sprite's x position to int. You can't draw to half pixels anyway.

[Mr_Blame]

Do you call sf::Texture::setSmooth

[hagel]

Yeah, it's set to false.

[eXpl0it3r]

If you want pixel accuracy, then don't draw on half-pixel positions. When something should be placed between pixels, OpenGL's rasterizer has to decide what it wants to show and chances are it will pick something that doesn't fit what you want.
I don't fully know how the rasterizer works, but since you have one big texture and try to cut out a part, it's not too far fetched, that when OpenGL decides what to use, it might think of the big texture as being drawn and then just picks the part you want. Which in combination with the half-pixel can lead to that the displayed texture is actually one pixel off.

[Hapax]

SPR.setTextureRect(sf::IntRect(width, 0, -width, height);

Unsure of if this is just a typo but why do you use width for the horizontal position and zero for the vertical position?
It might just be a typo since there's also a bracket missing.

[Laurent]

Unsure of if this is just a typo but why do you use width for the horizontal position and zero for the vertical position?

That's typically how you flip a rectangle on X axis. Left becomes right, and right becomes left.

[Hapax]

My mistake. The values are indeed the expected ones.
I read it as if the height was negative too.