Having issues with texture pixel manipulation and alpha

Lazy_Foo · August 20, 2013, 3:00am

So what I am trying to do is manually color key a SDL_Texture. This is born out of the fact that I want to create an SDL_Texture from an SDL_Surface, but SDL_CreateTextureFromSurface() does not return a streamable texture.

So I
-load a surface and convert it to the display format
-create a streaming texture in the display format
-lock the texture to access its pixels
-memcpy the surface pixels to the texture pixels
-map the color key and the transparent pixel in the display format
-go through the texture pixels and substitute all the color key pixels with transparent pixels
-unlock the surface

So I can find the color key pixels and replace them with another color just fine. The problem is that I can’t seem to make the color transparent even though I am mapping the alpha to 0.

Here’s my code:

Code:
//The final texture
SDL_Texture* newTexture = NULL;

//Load image at specified path
SDL_Surface* loadedSurface = IMG_Load( path.c_str() );
if( loadedSurface == NULL )
{
	printf( "Unable to load image %s! SDL_image Error: %s\n", path.c_str(), IMG_GetError() );
}
else
{
	//Convert surface to display format
	SDL_Surface* formattedSurface = SDL_ConvertSurface( loadedSurface, SDL_GetWindowSurface( gWindow )->format, NULL );
	if( formattedSurface == NULL )
	{
		printf( "Unable to convert loaded surface to display format! %s\n", SDL_GetError() );
	}
	else
	{
		//Create blank streamable texture
		newTexture = SDL_CreateTexture( gRenderer, SDL_GetWindowPixelFormat( gWindow ), SDL_TEXTUREACCESS_STREAMING, formattedSurface->w, formattedSurface->h );
		if( newTexture == NULL )
		{
			printf( "Unable to create blank texture! SDL Error: %s\n", SDL_GetError() );
		}
		else
		{
			//Lock texture for manipulation
			SDL_LockTexture( newTexture, &formattedSurface->clip_rect, &mPixels, &mPitch );

			//Copy loaded/formatted surface pixels
			memcpy( mPixels, formattedSurface->pixels, formattedSurface->pitch * formattedSurface->h );

			//Get image dimensions
			mWidth = formattedSurface->w;
			mHeight = formattedSurface->h;

			//Get pixel data in editable format
			Uint32* pixels = (Uint32*)mPixels;
			int pixelCount = ( mPitch / 4 ) * mHeight;

			//Map colors
			Uint32 colorKey = SDL_MapRGB( SDL_GetWindowSurface( gWindow )->format, 0, 0xFF, 0xFF );
			Uint32 transparent = SDL_MapRGBA( SDL_GetWindowSurface( gWindow )->format, 0x00, 0xFF, 0xFF, 0x00 );
			
			//Color key pixels
			for( int i = 0; i < pixelCount; ++i )
			{
				if( pixels[ i ] == colorKey )
				{
					pixels[ i ] = transparent;
				}
			}

			//Unlock texture to update
			SDL_UnlockTexture( newTexture );
			mPixels = NULL;
		}

		//Get rid of old formatted surface
		SDL_FreeSurface( formattedSurface );
	}	
	
	//Get rid of old loaded surface
	SDL_FreeSurface( loadedSurface );
}

So what would be the best way to do manual color keying in SDL 2.0?

Sik_the_hedgehog · August 20, 2013, 3:32am

Alpha blending needs to be enabled explicitly.

Also are you sure it needs to be streamable? Otherwise it’d be
impossible to load the data for a new texture that isn’t streamable

Lazy_Foo · August 20, 2013, 4:22am

That doesn’t seem to be working. I tried enabling blending before the texture is locked and after and neither worked.

Code:
//The final texture
SDL_Texture* newTexture = NULL;

//Load image at specified path
SDL_Surface* loadedSurface = IMG_Load( path.c_str() );
if( loadedSurface == NULL )
{
	printf( "Unable to load image %s! SDL_image Error: %s\n", path.c_str(), IMG_GetError() );
}
else
{
	//Convert surface to display format
	SDL_Surface* formattedSurface = SDL_ConvertSurface( loadedSurface, SDL_GetWindowSurface( gWindow )->format, NULL );
	if( formattedSurface == NULL )
	{
		printf( "Unable to convert loaded surface to display format! %s\n", SDL_GetError() );
	}
	else
	{
		//Create blank streamable texture
		newTexture = SDL_CreateTexture( gRenderer, SDL_GetWindowPixelFormat( gWindow ), SDL_TEXTUREACCESS_STREAMING, formattedSurface->w, formattedSurface->h );
		if( newTexture == NULL )
		{
			printf( "Unable to create blank texture! SDL Error: %s\n", SDL_GetError() );
		}
		else
		{
			SDL_SetTextureBlendMode( newTexture, SDL_BLENDMODE_BLEND );

			//Lock texture for manipulation
			SDL_LockTexture( newTexture, &formattedSurface->clip_rect, &mPixels, &mPitch );

			//Copy loaded/formatted surface pixels
			memcpy( mPixels, formattedSurface->pixels, formattedSurface->pitch * formattedSurface->h );

			//Get image dimensions
			mWidth = formattedSurface->w;
			mHeight = formattedSurface->h;

			//Get pixel data in editable format
			Uint32* pixels = (Uint32*)mPixels;
			int pixelCount = ( mPitch / 4 ) * mHeight;

			//Map colors
			Uint32 colorKey = SDL_MapRGBA( SDL_GetWindowSurface( gWindow )->format, 0, 0xFF, 0xFF, 0xFF );
			Uint32 transparent = SDL_MapRGBA( SDL_GetWindowSurface( gWindow )->format, 0x00, 0xFF, 0xFF, 0x00 );
			
			//Color key pixels
			for( int i = 0; i < pixelCount; ++i )
			{
				if( pixels[ i ] == colorKey )
				{
					pixels[ i ] = transparent;
				}
			}

			//Unlock texture to update
			SDL_UnlockTexture( newTexture );
			mPixels = NULL;

			SDL_SetTextureBlendMode( newTexture, SDL_BLENDMODE_BLEND );
		}

		//Get rid of old formatted surface
		SDL_FreeSurface( formattedSurface );
	}	
	
	//Get rid of old loaded surface
	SDL_FreeSurface( loadedSurface );
}

//Return success
mTexture = newTexture;

Could it have to do with the display format not supporting alpha? I tried making everything SDL_PIXELFORMAT_RGBA4444, but that didn’t work either.------------------------
SDL 2.0 Tutorial (http://lazyfoo.net/tutorials/SDL/index.php) now under construction!

Lazy_Foo · August 20, 2013, 4:45am

…well I feel stupid.

Of course a 32bit RGBA format would be SDL_PIXELFORMAT_RGBA8888 not SDL_PIXELFORMAT_RGBA4444. 3 semester of calculus, linear algebra, 2 semesters of physics and still I screw up 2nd grade math. [Embarassed]------------------------
SDL 2.0 Tutorial (http://lazyfoo.net/tutorials/SDL/index.php) now under construction!

slouken · August 20, 2013, 3:35pm

Also check out SDL_ConvertColorkeyToAlpha() in SDL_surface.cOn Mon, Aug 19, 2013 at 9:45 PM, Lazy Foo’ wrote:

**
…well I feel stupid.

Of course a 32bit RGBA format would be SDL_PIXELFORMAT_RGBA8888 not
SDL_PIXELFORMAT_RGBA4444. 3 semester of calculus, linear algebra, 2
semesters of physics and still I screw up 2nd grade math. [image:
Embarassed]

SDL 2.0 Tutorial http://lazyfoo.net/tutorials/SDL/index.php now under
construction!

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tekman · August 19, 2017, 9:11am

Hello,
maybe it’s not the best first post but i now develop applications with SDL2, very nice work and touch screen support is very cool!
I tried the lazyfoo tutorial that is very nice too.
But on the tutorial 40 I tried the code and transparency doesn’t work.
If I change the background color it still stays white for the supposed transparent pixels…
Could you please help me? I’m trying to convert my SDL 1.2 perl app in SDL 2.05 c++ app.
Thanks and have nice holidays.

tekman · August 19, 2017, 4:35pm

Chapter 41 is working, so I will compare with the 40…