bloom.c

#define BLOOM_LEVELS 6

int bloom_size[BLOOM_LEVELS][2];

GLuint bloom_fbo[BLOOM_LEVELS][2];
GLuint bloom_tex[2];

void bloomInit(void) {
    for(int j = 0; j < 2; j++) {
        glBindTexture(GL_TEXTURE_2D, bloom_tex[j]);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);

        for(int i = 0; i < BLOOM_LEVELS; i++) {
            glBindFramebuffer(GL_FRAMEBUFFER, bloom_fbo[i][j]);
            glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, bloom_tex[j], i);
        }
    }
}

void bloomResize(int width, int height) {
    // Initial size is half resolution.
    bloom_size[0][0] = w / 2;
    bloom_size[0][1] = h / 2;

    // Calculate mip sizes.
    for(int i = 1; i < BLOOM_LEVELS; i++) {
        bloom_size[i][0] = MAX(bloom_size[i - 1][0] / 2, 1);
        bloom_size[i][1] = MAX(bloom_size[i - 1][1] / 2, 1);
    }

    // Resize texture storage.
    for(int j = 0; j < 2; j++) {
        glBindTexture(GL_TEXTURE_2D, bloom_tex[j]);
        glTexImage2D(GL_TEXTURE_2D, 0, GL_R11F_G11F_B10F, bloom_size[0][0], bloom_size[0][1], 0, GL_RGB, GL_FLOAT, NULL);

        // Ideally, this should be accomplished by `glTexStorage2D`,
        // but that would require GL 4.2 or `GL_EXT_texture_storage`.
        glGenerateMipmap(GL_TEXTURE_2D);
    }
}

void bloomDraw(void) {
    // ====================
    //
    // Extract color with threshold from main scene color attachment.

    glUseProgram(program_bloom_extract);

    glViewport(0, 0, bloom_size[0][0], bloom_size[0][1]);
    glBindFramebuffer(GL_DRAW_FRAMEBUFFER, bloom_fbo[0][1]);

    glActiveTexture(GL_TEXTURE0);
    glBindTexture(GL_TEXTURE_2D, main_color_attachment);

    drawFullscreenQuad();

    // ====================
    //
    // Ping pong separable blur.

    glUseProgram(program_bloom_blur);

    for(int i = 0; i < BLOOM_LEVELS; i++) {
        glViewport(0, 0, bloom_size[i][0], bloom_size[i][1]);

        // ====================
        //
        // X axis horizontal blur.

        glBindFramebuffer(GL_DRAW_FRAMEBUFFER, bloom_fbo[i][0]);

        glActiveTexture(GL_TEXTURE0);
        glBindTexture(GL_TEXTURE_2D, bloom_tex[1]);

        // XY = pixel UV step, Z = mip level of source texture.
        glUniform3f("u_step", 1.0 / bloom_size[i][0], 0.0, (float) MAX(i - 1, 0));

        drawFullscreenQuad();

        // ====================
        //
        // Y axis vertical blur.

        glBindFramebuffer(GL_DRAW_FRAMEBUFFER, bloom_fbo[i][1]);

        glActiveTexture(GL_TEXTURE0);
        glBindTexture(GL_TEXTURE_2D, bloom_tex[0]);

        glUniform3f("u_step", 0.0, 1.0 / bloom_size[i][1], (float) i);

        drawFullscreenQuad();
    }

    // ====================
    //
    // Composite bloom layers together.

    glUseProgram(program_bloom_composite);

    glViewport(0, 0, bloom_size[0][0], bloom_size[0][1]);
    glBindFramebuffer(GL_DRAW_FRAMEBUFFER, bloom_fbo[0][0]);

    glActiveTexture(GL_TEXTURE0);
    glBindTexture(GL_TEXTURE_2D, bloom_tex[1]);

    drawFullscreenQuad();
}

bloom_blur.frag

in vec2 v_uv;

out vec4 rt0;

uniform sampler2D us_tex0;
uniform vec3 u_step;

void main() {
    vec2 a = u_step.xy * 1.3846;
    vec2 b = u_step.xy * 3.2307;

    rt0 = textureLod(us_tex0, v_uv, u_step.z) * 0.2270;
    rt0 += textureLod(us_tex0, v_uv - a, u_step.z) * 0.3162;
    rt0 += textureLod(us_tex0, v_uv + a, u_step.z) * 0.3162;
    rt0 += textureLod(us_tex0, v_uv - b, u_step.z) * 0.0702;
    rt0 += textureLod(us_tex0, v_uv + b, u_step.z) * 0.0702;
};

bloom_composite.frag

in vec2 v_uv;

out vec4 rt0;

uniform sampler2D us_tex0;

void main() {
    rt0 = sampleBicubic(us_tex0, v_uv, 1.0) * 1.0;
    rt0 += sampleBicubic(us_tex0, v_uv, 2.0) * 0.95;
    rt0 += sampleBicubic(us_tex0, v_uv, 3.0) * 0.9;
    rt0 += sampleBicubic(us_tex0, v_uv, 4.0) * 0.7;
    rt0 += sampleBicubic(us_tex0, v_uv, 5.0) * 0.4;
    rt0 *= 0.3;
};

bloom_extract.frag

in vec2 v_uv;

out vec4 rt0;

uniform sampler2D us_tex0;

void main() {
    rt0 = textureLod(us_tex0, v_uv, 0.0);
    rt0.rgb *= smoothstep(3.0, 4.0, dot(rt0.rgb, vec3(0.2126, 0.7152, 0.0722)));
};