Is it possible to render text with transparency or blending modes?

Question

Ryan McCombe · Accepted Answer

Yes, it is possible to render text with transparency and apply various blending modes using SDL2 and SDL_ttf. SDL2 provides several blending modes that can be applied to textures, including text rendered as textures. Here's how you can achieve this:

Render text to a texture with an alpha channel.
Set the blending mode for the texture.
Render the texture with the desired opacity.

Here's an example implementation demonstrating text rendering with transparency and different blending modes:

1#include <SDL.h>
2#include <SDL_ttf.h>
3
4#include <string>
5
6class TransparentTextRenderer {
7public:
8  TransparentTextRenderer(
9    SDL_Renderer* renderer, TTF_Font* font)
10    : renderer(renderer), font(font) {}
11
12  SDL_Texture* createTextTexture(
13    const std::string& text, SDL_Color color) {
14    SDL_Surface* surface =
15      TTF_RenderText_Blended(
16        font, text.c_str(), color);
17    if (!surface) return nullptr;
18
19    SDL_Texture* texture =
20      SDL_CreateTextureFromSurface(
21        renderer, surface);
22    SDL_FreeSurface(surface);
23
24    return texture;
25  }
26
27  void renderText(SDL_Texture* texture, int x,
28                  int y, Uint8 alpha,
29                  SDL_BlendMode blendMode) {
30    SDL_SetTextureAlphaMod(texture, alpha);
31    SDL_SetTextureBlendMode(texture, blendMode);
32
33    int w, h;
34    SDL_QueryTexture(texture, nullptr, nullptr,
35                     &w, &h);
36    SDL_Rect dstRect = {x, y, w, h};
37
38    SDL_RenderCopy(renderer, texture, nullptr,
39                   &dstRect);
40  }
41
42private:
43  SDL_Renderer* renderer;
44  TTF_Font* font;
45};
46
47// Usage
48SDL_Renderer* renderer =
49  SDL_CreateRenderer(window, -1,
50                     SDL_RENDERER_ACCELERATED);
51TTF_Font* font = TTF_OpenFont("font.ttf", 24);
52TransparentTextRenderer textRenderer(
53  renderer, font);
54
55SDL_Texture* normalText =
56  textRenderer.createTextTexture(
57    "Normal Text", {255, 255, 255, 255});
58SDL_Texture* addText =
59  textRenderer.createTextTexture(
60    "Additive Blend", {255, 0, 0, 255});
61SDL_Texture* multiplyText =
62  textRenderer.createTextTexture(
63    "Multiply Blend", {0, 255, 0, 255});
64
65// In render loop
66SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);
67SDL_RenderClear (renderer);
68
69// Normal blending with 50% opacity
70textRenderer.renderText(
71  normalText, 100, 100, 128,
72  SDL_BLENDMODE_BLEND
73);
74
75// Additive blending
76textRenderer.renderText(
77  addText, 100, 150, 255,
78  SDL_BLENDMODE_ADD
79);
80
81// Multiply blending
82textRenderer.renderText(
83  multiplyText, 100, 200, 255,
84  SDL_BLENDMODE_MOD
85);
86
87SDL_RenderPresent (renderer);
88
89// Clean up
90SDL_DestroyTexture (normalText);
91SDL_DestroyTexture (addText);
92SDL_DestroyTexture (multiplyText);
93TTF_CloseFont (font);
94SDL_DestroyRenderer (renderer);

This example demonstrates three different blending modes:

SDL_BLENDMODE_BLEND: Standard alpha blending (default)
SDL_BLENDMODE_ADD: Additive blending, useful for glow effects
SDL_BLENDMODE_MOD: Multiply blending, useful for shadow effects

You can also create custom blend modes using SDL_ComposeCustomBlendMode() for more advanced effects.

Some tips for working with transparent text:

Ensure your window and renderer are created with alpha channel support.
Use TTF_RenderText_Blended() or TTF_RenderText_Blended_Wrapped() for high-quality anti-aliased text with an alpha channel.
Be mindful of performance when using complex blending modes, especially with large amounts of text.
Experiment with different blend modes and alpha values to achieve the desired visual effect.

By combining these techniques, you can create dynamic, visually appealing text effects in your SDL2 applications.

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