How to flip texture horizontally in easier way?

[Andrey01]

I am trying to render a cube that has on each its face a mirror. I add the cube with scene->addCubeSceneNode(). But I need to flip the rendered texture at 180 degrees horizontally to keep symmetry relatively to the mirror plane. I know I can access its meshbuffer, go through indices of each vertex in the indices list (meshbuffer->getIndices()) and get the texture coordinates of the each vertex with the corresponding index. But I assume to flip the texture coordinates, I need to know in which sequence the indices are saved inside. I need to know in which order the node renders its vertices, i.e. each its face in which direction (clockwise/counter-clockwise) and since which vertex it starts rendering. Is there easier way to do that than going through indices of its vertices?

[CuteAlien]

Flipping uv's definitely one option. As long as you want to flip them all you can just do something like 1-u for each uv-coordinate.
You can check the Irrlicht source how cubes are created - not that complicated. Also as mentioned in another thread a few days ago (viewtopic.php?f=1&t=52811) there's a another cube-type with independent sides now in Irrlicht trunk. Which might give you more options/ideas to handle it.

Or maybe try scaling cube by -1 in one direction. I think you have to also flip front/backbuffer culling for this to work.

And the probably worst solution - mirror the image itself (coincidentally I just wrote code for that last month in a project, might end up in Irrlicht soon as well):

Code: Select all

void mirrorImage(irr::video::IImage* image, bool xAxis, bool yAxis)
{
	using namespace irr;
	if ( !image )
		return;

	if ( !xAxis && !yAxis)
		return;

	const core::dimension2du dim(image->getDimension());
	if ( dim.Width == 0 || dim.Height == 0 )
		return;

	u8* data = (u8*)image->getData();
	if (!data) 
		return;

	const u32 bpp = image->getBytesPerPixel();
	const u32 pitch = image->getPitch();

	if ( xAxis )
	{
		for ( u32 i=0; i<dim.Height/2; ++i)
		{
			// Reverse bottom/top lines
			u8* l1 = data+i*pitch;
			u8* l2 = data+(dim.Height-1-i)*pitch;
			for ( u32 b=0; b<pitch; ++b)
			{
				irr::u8 dummy = *l1;
				*l1 = *l2;
				*l2 = dummy;
				++l1;
				++l2;
			}
		}
	}
	if ( yAxis )
	{
		for ( u32 i=0; i<dim.Height; ++i)
		{
			// Reverse left/right for each line
			u8* l1 = data+i*pitch;
			u8* l2 = l1+(dim.Width-1)*bpp;
			for ( u32 p=0; p<dim.Width/2; ++p)
			{
				for ( u32 b=0; b<bpp; ++b)
				{
					irr::u8 dummy = l1[b];
					l1[b] = l2[b];
					l2[b] = dummy;
				}
				l1 += bpp;
				l2 -= bpp;
			}
		}
	}
}

Note that for real mirrors you probably have to render the scene another time (from the direction of the mirror) and put the result in a rendertargettexture. Then put that texture on some plane mesh.

Or there is also that EMT_SPHERE_MAP material.. which does some kind of mirroring in some magic way (sorry, I haven't read up yet on how exactly that one does it's magic).

[Andrey01]

Thanks for reply! I used the second way that you described me and it really helped me to solve the problem. I scaled the node by -1 along X, then enabled frontface culling and disabled backface one.