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Creating surfaces in SDL and alpha channel problems

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6 replies to this topic

#1 Gortal   Members   


Posted 19 June 2012 - 06:49 AM

Hello everyone Posted Image

I'm trying to make a simple game. I need to render the text, but I cannot actually use TTF_font cause I am not using any specified font - I've just created my own, little modified one. Anyway I try to render it in this way:

I have class Text which load many font grids. Then I have a method:
[source lang="cpp"]SDL_Surface* Text::Prepare_Text_Image (string txt, int type)[/source]
where obviously txt is the string wanted to get and type is the type of font I'd like to use.

In this function I load wanted font grid to
[source lang="cpp"]SDL_Surface* temp;[/source]
and then I create surface where the text will be rendered:
[source lang="cpp"]SDL_Surface* txt_image = SDL_CreateRGBSurface(SDL_SWSURFACE, spacing*txt.length(), spacing, 32, 0,0,0,0);[/source]
where spacing is just the size of a letter in grid.
Then I blit it like this:
[source lang="cpp"]SDL_BlitSurface(temp, &letter_clip, txt_image, &offset);[/source]

And I end up with an image like this one: Posted Image

The problem is that even when i use colorkeying then that ugly black background disappers but there is this color around each letter. Of course grid is in PNG file without background. I tried also to fill this txt_image just after creating it and before putting letters onto it with other color and it is ok. I mean there is no black background but red around each letter. So it seems that problem is with creating surface with channel alpha.

I've read dozens of topics but none gave me an answer because either they sent me to non-existing pages or the answer was really too complicated for me. I've even found this one: http://www.gamedev.n...litting-in-sdl/ but I don't understand anything from the answer from Sean Ridenour.

PS: Do you have any idea why around this red ball - the avatar - there is that ugly background? It seems that channel alpha doesn't work there either. I need also to rescale it cause it doesn't appear in proper circle, but nevermind ;)

Thanks for any answers Posted Image

#2 Goran Milovanovic   Members   


Posted 19 June 2012 - 02:21 PM

Can you show the actual image where the letters are outlined in the way that you describe?

As for the red ball - Sounds like the alpha in the image needs to be re-touched around the edges, becuase I think those few black pixels are there because their alpha is greater than 0.


| Game Dev video tutorials  ->   http://www.youtube.com/goranmilovano |

#3 Gortal   Members   


Posted 20 June 2012 - 04:05 AM

You mean when I use colorkey? Yes everything appears but there is that colour around these letters which I would like not to have. I just create surface with alpha channel... completly blank, transparent etc. then on that surface put a letter which has also transparent background so that there will be only the text.

#4 szecs   Members   


Posted 20 June 2012 - 04:51 AM

Blending and color-keying are different stuff. The former uses a specific color value (the rgb thing) do determine which pixel is fully opaque or fully transparent. The latter usually uses the alpha channel, so with the appropriate blending function, that will produce smooth blending (smooth "edges" for example). That's why you have that color around your letters, because the font is created with "smooth edges" in the image editor, so there are a lot of transitional-color pixels around edges, but color-keying works with one specific color value.. Just try this: open the font images in Paint, and use the bucket tool for colouring the background. The same ugly outline will show.

For alpha mapping, the image needs to have an alpha channel. This requires specific file formats and an image editor capable of adding/editing alpha channels (you can get around this with ugly hacks).

Do you save the images with appropriate file formats and with the alpha channel? PNG should be fine, but you have to select the option to save the alpha channel, is it has it at all.

Maybe I'm misunderstanding you...

Edited by szecs, 20 June 2012 - 05:00 AM.

#5 Gortal   Members   


Posted 20 June 2012 - 12:22 PM

Actually I produced all of these images in Gimp. They're all PNG files. When I open them in editor the background is transparent. Of course I made a little blur effect around the edges so that it looks smoother when I put it on the surface of the image. Is there any specific option to turn on or off when I'm saving these files? Of course I deleted the background and in PNG files are only letters or this avatar.

As I said, I believe that the problem with the text is that I cannot create a surface with alpha channel, because putting letters on the surface don't change background - I've checked it with filling the surface txt_image with red color just after creating and before putting letters, then the background is red as I've filled it. I also created this surface with extra space adding 1 to the text.lentgh() and then multiplying it by spacing. Then this black rectangle appers in a place for that "ghost" letter so it seems that this surface doesn't have channel alpha at all and is covered with black colour after creating.

#6 Spirrwell   Members   


Posted 20 June 2012 - 01:37 PM

It's hard to tell what you could be doing wrong without the whole function. Either way, when you loaded your images, I would guess you converted them to the display format using the SDL_DisplayFormat(). Of course that's a guess, but if you did do this, you'd want to convert it with SDL_DisplayFormatAlpha() instead. Another thing to do would be to check if the same problem occurs blitting a letter individually. If there is, the problem happens when it gets loaded. If there isn't a problem when you blit a standalone letter, then the problem is occurring when you're loading the letters as a string. That could help narrow down the problem.

#7 szecs   Members   


Posted 20 June 2012 - 10:43 PM

Yup, post the whole code.

"As I said, I believe that the problem with the text is that I cannot create a surface with alpha channel, because putting letters on the surface don't change background"

What does that mean? you can 't create it in GIMP or in your application?
What does "background" mean?

Does that mean that in GIMP, you have an alpha channel (you call it background) that you can't edit? Isn't it possible to at least copy stuff to the alpha channel?

Anyway, usually images with alpha channel and blending should have a solid background (by this I mean the surrounding) color of the border's color. It's a bit hard to express, and every blending related stuff, includig antialiasing should go into the alpha channel. You have to use a solid background color avoid halo artefacts around the borders of the "avatar".
It's hard to express, here's an example:
left: rgb channel of texture. middle: alpha channel of texture. right: texture rendered in application in front of a white surface.

Maybe I'm still misunderstanding.

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