# Making a transparent image in SDL

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I have a little problem with an image: I have an opend window (created with SDL) and the window has a background picture, that can variate. On this background, I want to place an other picture (a black dot). So I now I want to know if it is possible to set a transparent collor? Because it only has to show the dot, and not the other pixels around it. Here's how I try to do it:
SDL_Surface *screen      // The screen surface (fully init)
SDL_Surface *background  // The background picture
SDL_Surface *ball        // The black dot

SDL_Rect rball = {160,160, 10, 10};

SDL_BlitSurface(background, NULL, screen, NULL);
SDl_BlitSurface(ball, NULL, screen, &rball);
SDl_Flip(screen);

Now I see the dot on the background, but also the surounding pixels from the ball.bmp.

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Did you try SDL_SetColorKey()?

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Can you give an example on how it works?
Because the Reference is not very clear to me (I'm not english)

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The first parameter is the bitmap surface that you want to set the color key for.

The second parameter is for flags. This is set to SDL_SRCCOLORKEY.

The third parameter is an RGB value. use the SDL_MapRGB function to obtain a value for this parameter. The first parameter of the SDL_MapRGB function is the format. I would use the format member of the surface being used. the next three parameters are values for R, G, and B respectively.

In the end, it should look like this:

SDL_SetColorKey( ball, SDL_SRCCOLORKEY, SDL_MapRGB ( ball->format, 0x00, 0x00, 0x00));

Further note: I used all 0 values for the color key, you should use whatever floats your boat.

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The basic example looks like:
SDL_Surface * surface = /* get a SDL_Surface somehow */;SDL_SetColorKey(surface, SDL_SRCCOLORKEY, SDL_MapRGB(surface->format, red, green, blue));

You can skip the SDL_MapRGB() if you already have the color you want to use as a transparent color in a the correct format for the surface.

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hmmm, link doesn't work for me.

i've tried this:

SDL_Surface *screen      // The screen surface (fully init)SDL_Surface *background  // The background pictureSDL_Surface *ball        // The black dotbackground = SDL_LoadBMP("images/bg.bmp");ball = SDL_LoadBMP("images/ball.bmp");SDL_Rect rball = {160,160, 10, 10};SDL_Color color = {225,128,64};int SDL_SetColorKey(screen, 0, color);SDL_BlitSurface(background, NULL, screen, NULL);SDl_BlitSurface(ball, NULL, screen, &rball);SDl_Flip(screen);

And it gives me this error:

Quote:
 initializer list being treated as compound expression

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if I use
Quote:
 SDL_SetColorKey(scherm, SDL_SRCCOLORKEY, SDL_MapRGB(scherm->format, 225, 128, 64));

It gives me no error, but I can still see the color that shoud be transparrent

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SDL_SetColorKey should be applied to the surface that you want to make transparent. In your case, it looks like you should ball instead of screen.

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To Clarify:
The surface you set the color key for should be the bitmap surface you are blitting to the screen.
Also:
I saw a stray 'int' in the line calling SDL_SetColorKey in your previous post.

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Yeah, thanx, I should have used the ball surface, not the screen surface. (It works now).

And for the 'int': I looked at the sdl reference, and it sais:
Quote:
 #include "SDL.h"int SDL_SetColorKey(SDL_Surface *surface, Uint32 flag, Uint32 key);

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that's what you see in the .h file. you don't need to put an int before you call the function.

*edit: what tentoid said

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Quote:
Original post by Spippo
Yeah, thanx, I should have used the ball surface, not the screen surface. (It works now).

And for the 'int': I looked at the sdl reference, and it sais:
Quote:
 #include "SDL.h"int SDL_SetColorKey(SDL_Surface *surface, Uint32 flag, Uint32 key);

The int means that the function will return an int (integer). It will tell you if the function was successfull or failed (see the error codes from the docs). So if you want to call the function and not to check the return value just use SDL_SetColorKey(...);

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It works great!!!!

For my ball, and even my paddle.

Thank you both.
You helped me realy good, and realy realy fast. (I think i'm going to rate you guys).

Spippo

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Well, I have a new problem now.

The ball is moving, but it doesn't delete the previous ball, so i see a thick line across my screen. I know 3 ways to handle this problem:

- Repaint my background every time. (this takes a lot of memory and the game doesn't go smooth after a few seconds)
- Repaint a part of the background around the ball. (after 2 minutes, I still got the same problem.
- Add previous cordinates in my class and repaint only this small part of the background.

Is there another way (and better way) to do this?

Spippo

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Quote:
 Original post by SpippoWell, I have a new problem now.The ball is moving, but it doesn't delete the previous ball, so i see a thick line across my screen. I know 3 ways to handle this problem:- Repaint my background every time. (this takes a lot of memory and the game doesn't go smooth after a few seconds)- Repaint a part of the background around the ball. (after 2 minutes, I still got the same problem.- Add previous cordinates in my class and repaint only this small part of the background.Is there another way (and better way) to do this?Spippo

Just use SDL_FillRect(). It's not that slow.. (use double buffering and hardware surfaces).
And just in case:
http://www.gamedev.net/community/forums/topic.asp?topic_id=284173&whichpage=1�

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I can't use the FillRect, because I have an image as a background, and not just one color.

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Quote:
 Original post by SpippoI can't use the FillRect, because I have an image as a background, and not just one color.

Clear.
Blit background.
Blit sprites.

What do u mean?

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Quote:
 Original post by SpippoWhat do u mean?

Taken from your post and modified:
SDL_FillRect(screen, NULL, 0); // fill the screen with black// draw other stuff:SDL_BlitSurface(background, NULL, screen, NULL);SDl_BlitSurface(ball, NULL, screen, &rball);// flip the buffersSDl_Flip(screen);

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I believe he's talking sbout the first option you listed in your previous post, Spippo. I'm dealing with a similar problem. I don't want to redraw the entire background, then redraw the sprites on each frame. I'll post here if you haven't found a solution before me.

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But then I have to draw the background every time. And that takes a lot of memory

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Quote:
 Original post by SpippoBut then I have to draw the background every time. And that takes a lot of memory
That's pretty much how things are done. Alternatively, you can save the area A of the background before blitting B to it, then blit A back at the beginning of the next time. However, depending on your machine, that might be a bit much for a ball moving around the screen. I recently worked on a multiplayer 2d space shooter with SDL, using the clear(), draw(), flip() method and it ran smoothly.

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Also, about the slowdown, if method 2 runs as slow as method 1 after a few minutes you may want to check your memory usage. That's rather indicative of a memory leak.

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Thanx Woodsman, That's what I was thinking of doing, I just didn't have the time to put it into words that I or anybody else could understand. I realize that's how it's normally done ( redrawing the background, then blitting the sprites over it on each frame), but I want to give myself a challenge.

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Quote:
 Original post by SpippoBut then I have to draw the background every time. And that takes a lot of memory

Did you try that? You won't probably notice any difference in the perfomance.. And it shouldn't affect memory usage. Clearing the screen is the way it should be done. Works fine in my game atleast :)

Where did you get the idea that it takes a lot of memory?