CSFML multi-pass blur shader example

blur.c

/*
 * Compile with:
 * gcc -oblur blur.c -lcsfml-graphics
 *
 * XXX #1:
 * How to use shaders in CSFML:
 * - https://en.sfml-dev.org/forums/index.php?topic=9767.0
 *
 * XXX #2:
 * Gaussian Blur with link inside a thread to a book about SFML:
 * - https://en.sfml-dev.org/forums/index.php?topic=20155.0
 *
 * XXX #3:
 * "The texture coordinates are inverted because the texture produced by
 * sf::RenderTexture is inverted (because of FBO).":
 * - https://en.sfml-dev.org/forums/index.php?topic=11178.0
 *
 * */

#include <SFML/Graphics/RenderWindow.h>
#include <SFML/Graphics/RenderTexture.h>
#include <SFML/Graphics/VertexArray.h>
#include <SFML/Graphics/Shader.h>
#include <SFML/Graphics/Sprite.h>
#include <SFML/Graphics/Texture.h>
#include <stdlib.h>

#define UIBGCOLOR (sfColor){26,  26,  26,  255}

char *fragshader = " \
uniform sampler2D   source; \
uniform vec2        offsetFactor; \
 \
void main() \
{ \
    vec2 textureCoordinates = gl_TexCoord[0].xy; \
    vec4 color = vec4(0.0); \
    color += texture2D(source, textureCoordinates - 4.0 * offsetFactor) * 0.0162162162; \
    color += texture2D(source, textureCoordinates - 3.0 * offsetFactor) * 0.0540540541; \
    color += texture2D(source, textureCoordinates - 2.0 * offsetFactor) * 0.1216216216; \
    color += texture2D(source, textureCoordinates - offsetFactor) * 0.1945945946; \
    color += texture2D(source, textureCoordinates) * 0.2270270270; \
    color += texture2D(source, textureCoordinates + offsetFactor) * 0.1945945946; \
    color += texture2D(source, textureCoordinates + 2.0 * offsetFactor) * 0.1216216216; \
    color += texture2D(source, textureCoordinates + 3.0 * offsetFactor) * 0.0540540541; \
    color += texture2D(source, textureCoordinates + 4.0 * offsetFactor) * 0.0162162162; \
    gl_FragColor = color; \
} \
";

sfRenderWindow     *window;
sfVertexArray      *vertices;
sfRenderStates      states;
sfRenderTexture    *render_tex;
sfShader           *shader;
sfTexture          *texture;
sfSprite           *sprite;

static void handleWindowEvents()
{
    sfEvent event;
    while (sfRenderWindow_pollEvent(window, &event))
        if (event.type == sfEvtClosed)
            sfRenderWindow_close(window);
}

void main_loop()
{
    sfRenderWindow_clear(window, UIBGCOLOR);

    sfShader_setTextureParameter(shader, "source", texture);
    sfShader_setVector2Parameter(shader, "offsetFactor", (sfVector2f){0.f, 1.f/300.f});
    sfRenderTexture_drawVertexArray(render_tex, vertices, &states);
    sfRenderTexture_display(render_tex);

    const sfTexture *tex1 = sfRenderTexture_getTexture(render_tex);
    sfShader_setTextureParameter(shader, "source", tex1);
    sfShader_setVector2Parameter(shader, "offsetFactor", (sfVector2f){1.f/300.f, 0.f});
    sfRenderTexture_drawVertexArray(render_tex, vertices, &states);
    sfRenderTexture_display(render_tex);

    const sfTexture *tex2 = sfRenderTexture_getTexture(render_tex);
    sfSprite_setTexture(sprite, tex2, sfTrue);
    sfRenderWindow_drawSprite(window, sprite, NULL);
    sfRenderWindow_display(window);
}


int main()
{
    sfVideoMode mode = (sfVideoMode){300, 300, 32};
    window = sfRenderWindow_create(mode, "CSFML Blur Shader",
            sfResize | sfClose, NULL);
    if (!window)
        exit(EXIT_FAILURE);
    sfRenderWindow_setFramerateLimit(window, 60);

    // XXX #3: .texCoords `y` coord is inverted relatively to .position `y` coord
    sfVertex vert0 = {
        .position = (sfVector2f){0, 0},
        .texCoords = (sfVector2f){0, 1}
    };
    sfVertex vert1 = {
        .position = (sfVector2f){300, 0},
        .texCoords = (sfVector2f){1, 1}
    };
    sfVertex vert2 = {
        .position = (sfVector2f){0, 300},
        .texCoords = (sfVector2f){0, 0}
    };
    sfVertex vert3 = {
        .position = (sfVector2f){300, 300},
        .texCoords = (sfVector2f){1, 0}
    };

    vertices = sfVertexArray_create();
    sfVertexArray_append(vertices, vert0);
    sfVertexArray_append(vertices, vert1);
    sfVertexArray_append(vertices, vert2);
    sfVertexArray_append(vertices, vert3);
    sfVertexArray_setPrimitiveType(vertices, sfTrianglesStrip);

    shader = sfShader_createFromMemory(NULL, NULL, fragshader);
    states = (sfRenderStates){
        .shader = shader,
        .blendMode = sfBlendAlpha,
        .transform = sfTransform_Identity,
        .texture = NULL,
    };

    render_tex = sfRenderTexture_create(300, 300, sfFalse);
    sfIntRect area = (sfIntRect){0, 0, 300, 300};
    texture = sfTexture_createFromFile("./image.png", &area);
    sprite = sfSprite_create();
    // XXX #3: needs to be flipped upside down
    sfSprite_setScale(sprite, (sfVector2f){1.f, -1.f});
    sfSprite_setOrigin(sprite, (sfVector2f){0.f, 300.f});

    while (sfRenderWindow_isOpen(window)) {
        handleWindowEvents();
        main_loop();
    }

    sfRenderTexture_destroy(render_tex);
    sfTexture_destroy(texture);
    sfSprite_destroy(sprite);
    sfRenderWindow_destroy(window);

    return EXIT_SUCCESS;
}

image.png

result.png