09 shaders and post

Tutorial 09 — Shaders and post-processing

Copyright (c) 2024-2026 Jack Waechter. MIT licensed.

Two related paths:

1. Custom GLSL on the immediate 2D pass (AP_UseShader then AP_FillRect).
2. Post stack that runs at AP_Present when enabled (AP_SetPostEnabled).

Custom fragment on a quad

Pass NULL for the vertex stage to keep the builtin (window-pixel positions, u_resolution).

Immediate layout:

• location 0 vec2 a_position
• location 1 vec4 a_color
• location 2 vec2 a_uv
• uniform vec2 u_resolution
• uniform sampler2D u_texture (white 1×1 when untextured)

static const char *k_wave_fs =
    "#version 330 core\n"
    "in vec4 v_color;\n"
    "in vec2 v_uv;\n"
    "uniform sampler2D u_texture;\n"
    "uniform float u_time;\n"
    "out vec4 frag_color;\n"
    "void main() {\n"
    "  vec2 uv = v_uv;\n"
    "  uv.x += 0.04 * sin(uv.y * 24.0 + u_time * 3.0);\n"
    "  frag_color = v_color * texture(u_texture, uv);\n"
    "}\n";

AP_Shader *wave = AP_CreateShader(NULL, k_wave_fs);

/* in the loop, after AP_Clear: */
AP_UseShader(wave);
AP_SetUniformF("u_time", (float)AP_GetTime());
AP_SetDrawColor(1.0f, 1.0f, 1.0f, 1.0f);
AP_FillRect(&(AP_FRect){0.0f, 0.0f, 1280.0f, 720.0f});
AP_UseShader(NULL);

AP_DestroyShader(wave) at shutdown. AP_SetUniformF2 / F3 / F4 cover vectors.

Load from disk with AP_CreateShaderFromFile(vs_path, fs_path).

Post stack

When post is on, AP_Clear captures offscreen. Effects run inside AP_Present.

AP_SetPostEnabled(true);
AP_SetPostVignette(0.45f);
AP_SetPostBloom(0.7f, 0.35f);
AP_SetPostColorGrade(1.05f, 1.08f, 0.02f);
AP_SetPostGrain(0.08f);
AP_SetGuiLayer(AP_GUI_LAYER_OVERLAY); /* UI not processed */

The setters enable their flags when the amount is > 0. You can also:

AP_EnablePostFlag(AP_POST_CHROMATIC);
AP_SetPostChromatic(0.004f);

Flags: vignette, bloom, color grade, chromatic, grain, sharpen, custom shader.

Antialiasing

Two independent knobs, since they cover different rendering paths:

• AP_WINDOW_MSAA (window creation flag, msaa_samples in AP_WindowConfig) — antialiases direct-to-backbuffer rendering, i.e. everything drawn while post-processing is disabled.
• AP_SetPostMSAA(samples) — real multisampling for the offscreen scene and GUI capture used while post-processing is enabled. Without it, the scene/GUI textures behind the post stack are single-sample and every mesh edge, rounded rect, and glyph will look jagged no matter what AP_WINDOW_MSAA is set to.

AP_SetPostEnabled(true);
AP_SetPostMSAA(4); /* 0 disables; otherwise clamped to GL_MAX_SAMPLES */

AP_GetPostMSAA() reads back the clamped value actually in use.

Custom post shader

AP_SetPostShader(my_fullscreen_shader);
AP_EnablePostFlag(AP_POST_CUSTOM);

GUI overlay vs scene:

AP_SetPostIncludeGui(false); /* default: overlay after post */

Order reminder

AP_PumpEvents
AP_Clear                 /* starts post capture if enabled */
  AP_Begin3D … AP_End3D
  2D scene (AP_FillRect, AP_DrawTexture, …)
  AP_Gui*                /* overlay unless you change the layer */
AP_Present               /* post, then swap */

Next

Build something: Breakout, Top-down walker, or Desktop tool. For a deeper dive, see Camera rigs and cinematic post.