townengine/src/rendering/sprites.h

/* a rendering.c mixin */
#ifndef SPRITES_H
#define SPRITES_H

#include "../textures.h"
#include "../rendering.h"
#include "../context.h"
#include "quad_element_buffer.h"

#include <stb_ds.h>

#include <stdbool.h>
#include <stddef.h>

/*
 * an implementation note:
 * try to avoid doing expensive work in the push functions,
 * because they will be called multiple times in the main loop
 * before anything is really rendered
 */
/* TODO: it might make sense to infer alpha channel presence / meaningfulness for textures in atlas */
/* 		 so that they are rendered with no blend / batched in a way to reduce overdraw automatically */
void push_sprite(char *path, t_frect rect) {
    struct sprite_primitive sprite = {
        .rect = rect,
        .color = (t_color) { 255, 255, 255, 255 },
        .rotation = 0.0,
        .texture_key = textures_get_key(&ctx.texture_cache, path),
        .flip_x = false,
        .flip_y = false,
        .blend = true,
    };

    struct primitive_2d primitive = {
        .type = PRIMITIVE_2D_SPRITE,
        .sprite = sprite,
    };

    arrput(ctx.render_queue_2d, primitive);
}


void push_sprite_ex(t_frect rect, t_push_sprite_args args) {
    struct sprite_primitive sprite = {
        .rect = rect,
        .color = args.color,
        .rotation = args.rotation,
        .texture_key = textures_get_key(&ctx.texture_cache, args.path),
        .flip_x = args.flip_x,
        .flip_y = args.flip_y,
        .blend = args.blend,
    };

    struct primitive_2d primitive = {
        .type = PRIMITIVE_2D_SPRITE,
        .sprite = sprite,
    };

    arrput(ctx.render_queue_2d, primitive);
}


static struct sprite_batch {
    int atlas_id;
    size_t size; /* how many primitives are in current batch */
    bool blend;  /* whether it's blended or not */
} collect_sprite_batch(const struct primitive_2d *primitives, size_t len) {
    /* assumes that first primitive is already a sprite */
    struct sprite_batch result = {
        .atlas_id =
            textures_get_atlas_id(&ctx.texture_cache, primitives[0].sprite.texture_key),
        .blend = primitives[0].sprite.blend,
    };

    /* batch size is clamped so that reallocated short indices could be used */
    if (len >= QUAD_ELEMENT_BUFFER_LENGTH)
        len = QUAD_ELEMENT_BUFFER_LENGTH;

    for (size_t i = 0; i < len; ++i) {
        const struct primitive_2d *const current = &primitives[i];

        /* don't touch things other than sprites */
        if (current->type != PRIMITIVE_2D_SPRITE)
            break;

        /* only collect the same blend modes */
        if (current->sprite.blend != result.blend)
            break;

        /* only collect the same texture atlases */
        if (textures_get_atlas_id(&ctx.texture_cache, current->sprite.texture_key)
                != result.atlas_id)
            break;

        ++result.size;
    }

    return result;
}


/* assumes that orthogonal matrix setup is done already */
static void render_sprites(const struct primitive_2d primitives[],
                           const size_t len,
                           const bool   reversed)
{
	/* single vertex array is used for every batch with NULL glBufferData() trick at the end */
    static GLuint vertex_array = 0;
    if (vertex_array == 0)
        glGenBuffers(1, &vertex_array);

    glBindBuffer(GL_ARRAY_BUFFER, vertex_array);
    glBufferData(GL_ARRAY_BUFFER,
                 sizeof (struct sprite_primitive_payload) * len,
                 NULL,
                 GL_STREAM_DRAW);

    const t_rect srcrect =
        textures_get_srcrect(&ctx.texture_cache, primitives->sprite.texture_key);
    const t_rect dims =
        textures_get_dims(&ctx.texture_cache, primitives->sprite.texture_key);

    /* vertex population over a mapped buffer */
    {
        /* TODO: check errors, ensure alignment ? */
        struct sprite_primitive_payload *const payload =
            glMapBuffer(GL_ARRAY_BUFFER, GL_WRITE_ONLY);

        for (size_t i = 0; i < len; ++i) {
            const size_t cur = reversed ? len - i - 1: i;
            const struct sprite_primitive sprite = primitives[cur].sprite;

	        const float wr = (float)srcrect.w / (float)dims.w;
	        const float hr = (float)srcrect.h / (float)dims.h;
	        const float xr = (float)srcrect.x / (float)dims.w;
	        const float yr = (float)srcrect.y / (float)dims.h;

            payload[i] = (struct sprite_primitive_payload) {
                /* upper-left */
                .v0 = {
                    sprite.rect.x,
                    sprite.rect.y },
                .uv0 = {
                    xr + wr * sprite.flip_x,
                    yr + hr * sprite.flip_y, },

                /* bottom-left */
                .v1 = {
                    (sprite.rect.x),
                    (sprite.rect.y + sprite.rect.h) },
                .uv1 = {
                    xr + wr * sprite.flip_x,
                    yr + hr * !sprite.flip_y, },

                /* bottom-right */
                .v2 = {
                    (sprite.rect.x + sprite.rect.w),
                    (sprite.rect.y + sprite.rect.h) },
                .uv2 = {
                    xr + wr * !sprite.flip_x,
                    yr + hr * !sprite.flip_y, },

                /* upper-right */
                .v3 = {
                    (sprite.rect.x + sprite.rect.w),
                    (sprite.rect.y) },
                .uv3 = {
                    xr + wr * !sprite.flip_x,
                    yr + hr * sprite.flip_y, },

                /* equal for all (flat shaded) */
                .c0 = sprite.color,
                .c1 = sprite.color,
                .c2 = sprite.color,
                .c3 = sprite.color,
            };
        }

        glUnmapBuffer(GL_ARRAY_BUFFER);
    }

    /* vertex specification */
    glEnableClientState(GL_VERTEX_ARRAY);
    glVertexPointer(2,
                    GL_FLOAT,
                    offsetof(struct sprite_primitive_payload, v1),
                    (void *)offsetof(struct sprite_primitive_payload, v0));

    glEnableClientState(GL_TEXTURE_COORD_ARRAY);
    glClientActiveTexture(GL_TEXTURE0);
    glTexCoordPointer(2,
                      GL_FLOAT,
                      offsetof(struct sprite_primitive_payload, v1),
                      (void *)offsetof(struct sprite_primitive_payload, uv0));

    glEnableClientState(GL_COLOR_ARRAY);
    glColorPointer(4,
                   GL_UNSIGNED_BYTE,
                   offsetof(struct sprite_primitive_payload, v1),
                   (void *)offsetof(struct sprite_primitive_payload, c0));

    textures_bind(&ctx.texture_cache, primitives->sprite.texture_key, GL_TEXTURE_2D);

    bind_quad_element_buffer();

    glDrawElements(GL_TRIANGLES, 6 * (GLsizei)len, GL_UNSIGNED_SHORT, NULL);

    /* clear the state */
    glBufferData(GL_ARRAY_BUFFER, 0, NULL, GL_STREAM_DRAW);
    glBindBuffer(GL_ARRAY_BUFFER, 0);

    glDisableClientState(GL_TEXTURE_COORD_ARRAY);
    glDisableClientState(GL_VERTEX_ARRAY);
    glDisableClientState(GL_COLOR_ARRAY);

    glBindTexture(GL_TEXTURE_2D, 0);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
}

#endif