SDL_ttf white blended glyphs are visible on white background

SDL Development
1

Hi, I’m trying to render text in SDL2, so I came up with the idea of rendering every glyph that I will use in white on a Texture, then set the color mod and RenderCopy the glyphs on the screen for rendering any text in any color. But I have a problem, Somehow, I can see the white glyphs on white background, while it’s not the case on other apps such as paint or word.
This is my code (I removed the parts which weren’t related to the problem)

void load(SDL_Renderer *renderer, std::string filepath, int size)
{
	const int m_size = 95;
	const char* m_chars = " !\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~";
	std::vector<SDL_Rect> m_rects = {};
	SDL_Texture m_texture;

	TTF_Font *font = TTF_OpenFont(filepath.c_str(), size);

	SDL_Rect rect = { 0, 0, 0, 0 };
	std::vector<SDL_Surface*> glyphs;
	for (int i = 0; i < m_size; i++)
	{
		char ch = m_chars[i];

		SDL_Surface *glyph = TTF_RenderGlyph_Blended(font, ch, { 255, 255, 255 });
		glyphs.push_back(glyph);

		rect.w = glyph->w;
		rect.h = glyph->h;
		m_rects.push_back(rect);
		rect.x += rect.w;
	}

	int width = 0, height = 0;
	for (int i = 0; i < m_size; i++)
	{
		width += (int)rect.w;
		height = std::max(height, (int)rect.h);
	}

	SDL_Surface *atlas = SDL_CreateRGBSurface(0, width, height, 32,
#if SDL_BYTEORDER == SDL_BIG_ENDIAN
	0xff000000, 0x00ff0000, 0x0000ff00, 0x000000ff
#else
	0x000000ff, 0x0000ff00, 0x00ff0000, 0xff000000
#endif
	);

	for (int i = 0; i < m_size; i++)
	{
		SDL_Rect dst = { (int)m_rects[i].x, (int)m_rects[i].y };
		SDL_BlitSurface(glyphs[i], NULL, atlas, &dst);
		SDL_FreeSurface(glyphs[i]);
	}

	m_texture = SDL_CreateTextureFromSurface(renderer, atlas);
	SDL_FreeSurface(atlas);
	TTF_CloseFont(font);
}

But when I render m_texture on a window with white background, I’m not supposed to see the texture, it should seem invisible, but I’m seeing:
image
Whats the problem? what am I doing wrong?

2

I guess you need to blend result texture atlas to screen surface in premultiplied alpha blending mode.
There is a bunch of such questions on stackoverflow…

3

Where is your code for copying from glyphs to atlas? I don’t see it in your listing.

4

Sorry I accidentally removed it while trying to get rid of the unrelated code, it should be edited in now

5

OK, thanks. Try changing the blend mode of glyphs[i] before the blit:

for (int i = 0; i < m_size; i++)
{
	SDL_Rect dst = { (int)m_rects[i].x, (int)m_rects[i].y };
	SDL_SetSurfaceBlendMode(glyphs[i], SDL_BLENDMODE_NONE);
	SDL_BlitSurface(glyphs[i], NULL, atlas, &dst);
	SDL_FreeSurface(glyphs[i]);
}
6

I tried a solution which I found here and it seems to have fixed the problem. Thank you!

7

I tried your solution too and it worked as well, Thank you too!