transparency: alpha vs. add_color

morris · Post by **morris** » Mon Feb 04, 2008 1:32 pm

hi,

i've got a problem with transparency. look at the following screenshot:

http://fahrschule-piepenbrock.de/morris ... arency.jpg

the left one is a *.png file with an alpha channel, the right is a jpg with black background. i think you can see that with alpha, the borders are quite glitchy in my opinion, and with additive color it looks pretty neat to me. of course, additive color takes away all darker colors, which i'd really hate

but on the other hand its just too ugly.

does anybody know why this is the case? i need a way to make alpha transparency look better. i've already tried to play around with Material::MaterialTypeParam, which changes something in the alpha, but all values look bad.

JP · Post by JP » Mon Feb 04, 2008 1:47 pm

Are the dimensions of the textures you're using powers of two?

i.e. something like 256x128 instead of 300x200.

If they're not power of two then they're scaled up to the nearest power of two which causes some artifacts.

morris · Post by **morris** » Mon Feb 04, 2008 1:54 pm

yes, they are both 256x256. i should have mentioned that they are not GUIImages, and are not drawn using draw2dImage. they are used as materials on a face (which is, at the time of the screenshot, parallel to the viewport). they are both scaled as if they were textures on a mesh, but still additive color is looking a lot better than alpha transparency.

xDan · Post by **xDan** » Mon Feb 04, 2008 2:08 pm

I think this needs to be remedied in your image editor. Use alpha, but paint nice transparencies... e.g. paint those shapes slightly transparent, or just the edges of them slightly transparent... (in a nice gradient)

morris · Post by **morris** » Mon Feb 04, 2008 2:38 pm

hm i dont know how to do that, but i'll give it a try now. hope it won't be too complicated because then every image i use will have to be like that

would be cooler to have a programmed solution to this.

arras · Post by **arras** » Mon Feb 04, 2008 2:46 pm

xDan probably meant anti-aliasing. This may help if your textures aren't anti-aliased already.

Also you may try setting SMaterial::MaterialTypeParam and play with it:

SMaterial::MaterialTypeParam. If set to 0, the alpha ref gets its default value which is 0.5f and which means that pixels with an alpha value >127 will be written, others not. In other, simple words: this value controls how sharp the edges become when going from a transparent to a solid spot on the texture.

And you may want to try out EMT_TRANSPARENT_ALPHA_CHANNEL_REF material type too.

morris · Post by **morris** » Mon Feb 04, 2008 2:57 pm

arras wrote:xDan probably meant anti-aliasing. This may help if your textures aren't anti-aliased already.

they ARE anti-aliased, they're not done in paint

they wouldnt look good with additive color if they weren't, i think.

arras wrote:Also you may try setting SMaterial::MaterialTypeParam and play with it: SMaterial::MaterialTypeParam. If set to 0, the alpha ref gets its default value which is 0.5f and which means that pixels with an alpha value >127 will be written, others not. In other, simple words: this value controls how sharp the edges become when going from a transparent to a solid spot on the texture.

as i said before, i've already tried that. in fact, it helps for lighter colors, but everything dark will have weird highlighted borders (opengl):

http://fahrschule-piepenbrock.de/morris ... rency2.jpg

and this wouldnt convince me to use alpha. EMT_TRANSPARENT_ALPHA_CHANNEL_REF is _much_ worse than both other options, it has hard and ugly edges and still some highlighted borders.

arras · Post by **arras** » Mon Feb 04, 2008 3:04 pm

Did you tried to turn off mip-map generation before loading those textures? It can be done in IVideoDriver::setTextureCreationFlag() but must be done before textures are loaded from file. If I remember right there is somewhere also way how to check if texture have mip-maps.
Not sure if it can help however...

xDan · Post by **xDan** » Mon Feb 04, 2008 4:21 pm

they ARE anti-aliased, they're not done in paint Wink they wouldnt look good with additive color if they weren't, i think.

But are they anti-aliased to transparent (if that makes sense...)?

(I mean, you could have done the anti aliasing while you still have a black background or something, then filled the background as transparent afterwards. That would look bad with alpha but ok as additive)

Sorry I'm repeating myself, it's just that it looks like a problem I had once.

You could upload the texture...

morris · Post by **morris** » Mon Feb 04, 2008 4:37 pm

@xDan:
anti-aliasing is done automatically by photoshop; i delete the background layer and draw things on an empty layer. this way i get anti-aliasing to "transparency".

also, if i put the png into a GUIImage it looks perfect - so anti-aliasing should be alright.

@arras: turning off mipmaps didnt make any difference

edit: here's the texture: http://fahrschule-piepenbrock.de/morris/test.png

jingquan · Post by **jingquan** » Tue Feb 05, 2008 3:02 am

I notice this problem as well. Using the alpha channel will leave strange opaque border around my alpha image. Addictive color sloves this problem, but as morris mentioned, it have a bit of color problem, but quite fine to me.

hybrid · Post by **hybrid** » Tue Feb 05, 2008 9:54 am

Try to change the MaterialTypeParam to something small (e.g. 0.1 or 0.01).

morris · Post by **morris** » Tue Feb 05, 2008 2:28 pm

@hybrid: setting the param to 0.01f e.g. lets white things look good, but i get weird light borders (see screenshot above, transparency2.jpg) for darker colors. i think this is a bug but i dont have any experience with opengl so i didnt succeed in investigating that any further.

hybrid · Post by **hybrid** » Tue Feb 05, 2008 7:07 pm

Ah sorry, I seem to have missed that part. I'm not sure why this would happen.

morris · Post by **morris** » Fri Feb 08, 2008 5:27 pm

i wonder if this is considered a bug? maybe i should open a topic in "Bugs" with stand-alone code that reproduces the issue.