partial implementation of double buffered render

src/rendering/twn_draw.c

@@ -374,11 +374,11 @@ void render(void) {
         }
     }
 
+    start_render_frame(); {
         render_space();
-    render_skybox(); /* after space, as to use depth buffer for early rejection */
+        render_skybox(); /* after space, as to use depth buffer for early z rejection */
         render_2d();
-    swap_buffers();
-    clear_draw_buffer();
+    } end_render_frame();
 }
 
 

src/rendering/twn_draw_c.h

@@ -165,6 +165,8 @@ void text_cache_reset_arena(TextCache *cache);
 
 VertexBuffer create_vertex_buffer(void);
 
+VertexBuffer get_scratch_vertex_array(void);
+
 void delete_vertex_buffer(VertexBuffer buffer);
 
 void specify_vertex_buffer(VertexBuffer buffer, void const *data, size_t bytes);
@@ -187,7 +189,7 @@ void swap_buffers(void);
 
 void set_depth_range(double low, double high);
 
-void bind_quad_element_buffer(void);
+VertexBuffer get_quad_element_buffer(void);
 
 void render_circle(const CirclePrimitive *circle);
 
@@ -238,4 +240,8 @@ void pop_fog(void);
 
 void finally_pop_fog(void);
 
+void start_render_frame(void);
+
+void end_render_frame(void);
+
 #endif

src/rendering/twn_gl_15_rendering.c

@@ -95,6 +95,58 @@ typedef struct ElementIndexedQuadWithoutColorWithoutTexture {
 } ElementIndexedQuadWithoutColorWithoutTexture;
 
 
+typedef struct {
+    size_t offset;
+    GLenum type;
+    GLsizei stride;
+    GLuint buffer;
+    uint8_t arity; /* leave at 0 to signal pointer as unused */
+} AttributeArrayPointer;
+
+
+/* allows us to have generic way to issue draws as well as */
+/* deferring new draw calls while previous frame is still being drawn */
+typedef struct {
+    AttributeArrayPointer vertices;
+    AttributeArrayPointer texcoords;
+
+    bool constant_colored;
+    union {
+        AttributeArrayPointer colors;        
+        Color color;
+    };
+
+    bool textured, texture_repeat;
+    TextureKey texture;
+    TextureMode texture_mode;
+
+    GLuint element_buffer;
+    GLsizei element_count;
+    GLsizei range_start, range_end;
+} DeferredCommandDrawElements;
+
+
+typedef struct {
+    Color color;
+    bool clear_color;
+    bool clear_depth;
+    bool clear_stencil;
+} DeferredCommandClear;
+
+
+typedef struct {
+    enum DeferredCommandType {
+        DEFERRED_COMMAND_TYPE_DRAW_ELEMENTS,
+        DEFERRED_COMMAND_TYPE_CLEAR,
+    } type;
+
+    union {
+        DeferredCommandDrawElements draw_elements;
+        DeferredCommandClear clear;
+    };
+} DeferredCommand;
+
+
 typedef enum {
     PIPELINE_NO,
     PIPELINE_SPACE,
@@ -108,6 +160,186 @@ static Pipeline pipeline_last_used = PIPELINE_NO;
 #define CIRCLE_VERTICES_MAX 2048
 
 
+/* potentially double buffered array of vertex array handles */
+/* we assume they will be refilled fully each frame */
+static size_t scratch_va_front_used, scratch_va_back_used;
+static GLuint *front_scratch_vertex_arrays, *back_scratch_vertex_arrays;
+static GLuint **current_scratch_vertex_array = &front_scratch_vertex_arrays;
+static void restart_scratch_vertex_arrays(void) {
+    scratch_va_front_used = 0;
+    scratch_va_back_used = 0;
+
+    if (ctx.render_double_buffered)
+        current_scratch_vertex_array = current_scratch_vertex_array == &front_scratch_vertex_arrays ?
+                &back_scratch_vertex_arrays : &front_scratch_vertex_arrays;
+}
+
+
+GLuint get_scratch_vertex_array(void) {
+    size_t *used = current_scratch_vertex_array == &front_scratch_vertex_arrays ?
+            &scratch_va_front_used : &scratch_va_back_used;
+
+    if (arrlenu(*current_scratch_vertex_array) <= *used) {
+        GLuint handle;
+        glGenBuffers(1, &handle);
+        arrpush(*current_scratch_vertex_array, handle);
+    }
+
+    (*used)++;
+    return (*current_scratch_vertex_array)[arrlenu(*current_scratch_vertex_array) - 1];
+}
+
+
+static DeferredCommand *deferred_commands;
+static void issue_deferred_draw_commands(void) {
+    use_2d_pipeline();
+
+    for (size_t i = 0; i < arrlenu(deferred_commands); ++i) {
+        switch (deferred_commands[i].type) {
+            case DEFERRED_COMMAND_TYPE_CLEAR: {
+                glClearColor((1.0f / 255) * deferred_commands[i].clear.color.r,
+                             (1.0f / 255) * deferred_commands[i].clear.color.g,
+                             (1.0f / 255) * deferred_commands[i].clear.color.b,
+                             (1.0f / 255) * deferred_commands[i].clear.color.a);
+
+                /* needed as we might mess with it */
+                glDepthRange(0.0, 1.0);
+                glDepthMask(GL_TRUE);
+
+                glClear((deferred_commands[i].clear.clear_color   ? GL_COLOR_BUFFER_BIT   : 0) |
+                        (deferred_commands[i].clear.clear_depth   ? GL_DEPTH_BUFFER_BIT   : 0) |
+                        (deferred_commands[i].clear.clear_stencil ? GL_STENCIL_BUFFER_BIT : 0) );
+                break;
+            }
+
+            case DEFERRED_COMMAND_TYPE_DRAW_ELEMENTS: {
+                DeferredCommandDrawElements const command = deferred_commands[i].draw_elements;
+
+                /* TODO: don't assume a single vertex array ? */
+                SDL_assert(command.vertices.arity != 0);
+                SDL_assert(command.vertices.buffer);
+                SDL_assert(command.element_count != 0);
+                SDL_assert(command.element_buffer);
+
+                glBindBuffer(GL_ARRAY_BUFFER, command.vertices.buffer);
+                glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, command.element_buffer);
+
+                glEnableClientState(GL_VERTEX_ARRAY);
+                glVertexPointer(command.vertices.arity,
+                                command.vertices.type,
+                                command.vertices.stride,
+                                (void *)command.vertices.offset);
+
+                use_texture_mode(command.texture_mode);
+
+                if (command.texcoords.arity != 0) {
+                    SDL_assert(command.texcoords.buffer == command.vertices.buffer);
+
+                    glEnableClientState(GL_TEXTURE_COORD_ARRAY);
+                    glClientActiveTexture(GL_TEXTURE0);
+                    glTexCoordPointer(command.texcoords.arity,
+                                      command.texcoords.type,
+                                      command.texcoords.stride,
+                                      (void *)command.texcoords.offset);
+                }
+
+                if (command.colors.arity != 0) {
+                    SDL_assert(command.colors.buffer == command.vertices.buffer);
+
+                    glEnableClientState(GL_COLOR_ARRAY);
+                    glColorPointer(command.colors.arity,
+                                   command.colors.type,
+                                   command.colors.stride,
+                                   (void *)command.colors.offset);
+                } else if (command.constant_colored)
+                    glColor4ub(command.color.r,
+                               command.color.g,
+                               command.color.b,
+                               command.color.a);
+
+                if (command.textured) {
+                    if (command.texture_repeat)
+                        textures_bind_repeating(&ctx.texture_cache, command.texture);
+                    else
+                        textures_bind(&ctx.texture_cache, command.texture);
+                }
+
+                if (command.range_start == command.range_end)
+                    glDrawElements(GL_TRIANGLES, command.element_count, GL_UNSIGNED_SHORT, NULL);
+                else
+                    glDrawRangeElements(GL_TRIANGLES,
+                                        command.range_start,
+                                        command.range_end,
+                                        command.element_count,
+                                        GL_UNSIGNED_SHORT,
+                                        NULL);
+
+                /* state clearing */
+
+                if (command.textured)
+                    glBindTexture(GL_TEXTURE_2D, 0);
+
+                if (command.colors.arity != 0)
+                    glDisableClientState(GL_COLOR_ARRAY);
+
+                if (command.texcoords.arity != 0)
+                    glDisableClientState(GL_TEXTURE_COORD_ARRAY);
+
+                glDisableClientState(GL_VERTEX_ARRAY);
+
+                glBindBuffer(GL_ARRAY_BUFFER, 0);
+                glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
+
+                break;
+            }
+
+            default:
+                SDL_assert(false);
+        }
+    }
+}
+
+
+void clear_draw_buffer(void) {
+    /* TODO: we can optimize a rectangle drawn over whole window to a clear color call*/
+
+    DeferredCommand command = {
+        .type = DEFERRED_COMMAND_TYPE_CLEAR,
+        .clear = (DeferredCommandClear) {
+            .clear_color = true,
+            .clear_depth = true,
+            .clear_stencil = true,
+            .color = (Color) { 230, 230, 230, 1 }
+        }
+    };
+
+    arrpush(deferred_commands, command);
+}
+
+
+void start_render_frame(void) {
+    restart_scratch_vertex_arrays();
+    clear_draw_buffer();
+}
+
+
+void end_render_frame(void) {
+    if (!ctx.render_double_buffered || ctx.game.frame_number == 0) {
+        issue_deferred_draw_commands();
+        SDL_GL_SwapWindow(ctx.window);
+        arrsetlen(deferred_commands, 0);
+    } else {
+        /* instead of waiting for frame to finish for the swap, we continue */
+        /* while issuing new state for the next call, but deferring any fragment emitting calls for later  */
+        /* actual swap will happen when next frame is fully finished, introducing a delay */
+        SDL_GL_SwapWindow(ctx.window);
+        issue_deferred_draw_commands();
+        glFlush();
+        arrsetlen(deferred_commands, 0);
+    }
+}
+
+
 void use_space_pipeline(void) {
     if (pipeline_last_used == PIPELINE_SPACE)
         return;
@@ -301,7 +533,7 @@ void finally_render_quads(const Primitive2D primitives[],
                           const struct QuadBatch batch,
                           const VertexBuffer buffer)
 {
-    (void)buffer;
+    DeferredCommandDrawElements command = {0};
 
     GLsizei off = 0, voff = 0, uvoff = 0, coff = 0;
 
@@ -323,63 +555,55 @@ void finally_render_quads(const Primitive2D primitives[],
         voff  = offsetof(ElementIndexedQuadWithoutColorWithoutTexture, v0);
     }
 
-    /* vertex specification */
-    glEnableClientState(GL_VERTEX_ARRAY);
-    glVertexPointer(2,
-                    GL_FLOAT,
-                    off,
-                    (void *)(size_t)voff);
+    command.vertices = (AttributeArrayPointer) {
+        .arity = 2,
+        .type = GL_FLOAT,
+        .stride = off,
+        .offset = voff,
+        .buffer = buffer
+    };
 
-    if (batch.textured) {
-        glEnableClientState(GL_TEXTURE_COORD_ARRAY);
-        glClientActiveTexture(GL_TEXTURE0);
-        glTexCoordPointer(2,
-                          GL_FLOAT,
-                          off,
-                          (void *)(size_t)uvoff);
-    }
+    if (batch.textured)
+        command.texcoords = (AttributeArrayPointer) {
+            .arity = 2,
+            .type = GL_FLOAT,
+            .stride = off,
+            .offset = uvoff,
+            .buffer = buffer
+        };
 
     if (!batch.constant_colored) {
-        glEnableClientState(GL_COLOR_ARRAY);
-        glColorPointer(4,
-                       GL_UNSIGNED_BYTE,
-                       off,
-                       (void *)(size_t)coff);
-    } else
-        glColor4ub(primitives[0].sprite.color.r,
-                   primitives[0].sprite.color.g,
-                   primitives[0].sprite.color.b,
-                   primitives[0].sprite.color.a);
+        command.colors = (AttributeArrayPointer) {
+            .arity = 4,
+            .type = GL_UNSIGNED_BYTE,
+            .stride = off,
+            .offset = coff,
+            .buffer = buffer
+        };
+    } else {
+        command.constant_colored = true;
+        command.color = primitives[0].sprite.color;
+    }
 
     if (batch.textured) {
-        if (!batch.repeat)
-            textures_bind(&ctx.texture_cache, primitives->sprite.texture_key);
-        else
-            textures_bind_repeating(&ctx.texture_cache, primitives->sprite.texture_key);
+        /* TODO: bad, don't */
+        command.textured = true;
+        command.texture = primitives->sprite.texture_key;
+        command.texture_repeat = batch.repeat;
     }
 
-    bind_quad_element_buffer();
+    command.element_buffer = get_quad_element_buffer();
+    command.element_count = 6 * (GLsizei)batch.size;
+    command.range_end = 6 * (GLsizei)batch.size;
 
-    glDrawElements(GL_TRIANGLES, 6 * (GLsizei)batch.size, GL_UNSIGNED_SHORT, NULL);
+    command.texture_mode = batch.mode;
 
-    /* clear the state */
-    {
-        glBufferData(GL_ARRAY_BUFFER, 0, NULL, GL_STREAM_DRAW);
-        glBindBuffer(GL_ARRAY_BUFFER, 0);
+    DeferredCommand final_command = {
+        .type = DEFERRED_COMMAND_TYPE_DRAW_ELEMENTS,
+        .draw_elements = command
+    };
 
-        if (batch.textured)
-            glDisableClientState(GL_TEXTURE_COORD_ARRAY);
-
-        glDisableClientState(GL_VERTEX_ARRAY);
-
-        if (!batch.constant_colored)
-            glDisableClientState(GL_COLOR_ARRAY);
-
-        if (batch.textured)
-            glBindTexture(GL_TEXTURE_2D, 0);
-
-        glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
-    }
+    arrpush(deferred_commands, final_command);
 }
 
 
@@ -553,7 +777,7 @@ void finally_draw_text(FontData const *font_data,
                       offsetof(ElementIndexedQuadWithoutColor, v1),
                       (void *)(size_t)offsetof(ElementIndexedQuadWithoutColor, uv0));
 
-    bind_quad_element_buffer();
+    get_quad_element_buffer();
 
     use_texture_mode(TEXTURE_MODE_GHOSTLY);
 

src/rendering/twn_gl_any_rendering.c

@@ -32,7 +32,7 @@ void specify_vertex_buffer(VertexBuffer buffer, void const *data, size_t bytes)
 }
 
 
-void bind_quad_element_buffer(void) {
+VertexBuffer get_quad_element_buffer(void) {
     static GLuint buffer = 0;
 
     /* it's only generated once at runtime */
@@ -62,32 +62,8 @@ void bind_quad_element_buffer(void) {
 
     } else
         glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, buffer);
-}
 
-
-void clear_draw_buffer(void) {
-    /* TODO: we can optimize a rectangle drawn over whole window to a clear color call*/
-    glClearColor((1.0f / 255) * 230,
-                 (1.0f / 255) * 230,
-                 (1.0f / 255) * 230, 1);
-
-    glDepthRange(0.0, 1.0);
-    glDepthMask(GL_TRUE);
-
-    /* TODO: don't clear color when skybox is applied? */
-    /*       for that window should match framebuffer */
-    /*       also it is driver dependent, from what i can gather */
-    /* INFO: also, based on below, driver might prefer it staying this way */
-    /*       https://gamedev.stackexchange.com/questions/90344/render-with-const-depth-value */
-    /*       we could optionally load ARB_invalidate_subdata extension if it's available instead */
-    glClear(GL_COLOR_BUFFER_BIT |
-            GL_DEPTH_BUFFER_BIT |
-            GL_STENCIL_BUFFER_BIT);
-}
-
-
-void swap_buffers(void) {
-    SDL_GL_SwapWindow(ctx.window);
+    return buffer;
 }
 
 

src/rendering/twn_rects.c

@@ -72,9 +72,7 @@ void render_rect_batch(const Primitive2D primitives[],
     SDL_assert(primitives[0].type == PRIMITIVE_2D_RECT);
 
     /* single vertex array is used for every batch with NULL glBufferData() trick at the end */
-    static VertexBuffer vertex_array = 0;
-    if (vertex_array == 0)
-        vertex_array = create_vertex_buffer();
+    VertexBuffer const vertex_array = get_scratch_vertex_array();
 
     use_texture_mode(batch.mode);
 

src/rendering/twn_sprites.c

@@ -125,12 +125,7 @@ void render_sprite_batch(const Primitive2D primitives[],
     SDL_assert(primitives && batch.size != 0);
     SDL_assert(primitives[0].type == PRIMITIVE_2D_SPRITE);
 
-    /* single vertex array is used for every batch with NULL glBufferData() trick at the end */
-    static VertexBuffer vertex_array = 0;
-    if (vertex_array == 0)
-        vertex_array = create_vertex_buffer();
-
-    use_texture_mode(batch.mode);
+    VertexBuffer const vertex_array = get_scratch_vertex_array();
 
     const Rect dims =
         textures_get_dims(&ctx.texture_cache, primitives->sprite.texture_key);

src/rendering/twn_text.c

@@ -171,9 +171,7 @@ static void text_destroy_font_data(FontData *font_data) {
 
 
 static void text_draw_with(FontData* font_data, char* text, Vec2 position, Color color) {
-    VertexBuffer vertex_array = 0;
-    if (vertex_array == 0)
-        vertex_array = create_vertex_buffer();
+    VertexBuffer const vertex_array = get_scratch_vertex_array();
 
     const size_t len = SDL_strlen(text);
 

src/rendering/twn_triangles.c

@@ -45,9 +45,7 @@ void draw_triangle(const char *path,
 void draw_uncolored_space_traingle_batch(struct MeshBatch *batch,
                                          TextureKey texture_key)
 {
-    static VertexBuffer vertex_array = 0;
-    if (vertex_array == 0)
-        vertex_array = create_vertex_buffer();
+    VertexBuffer const vertex_array = get_scratch_vertex_array();
 
     const size_t primitives_len = arrlenu(batch->primitives);
 

src/twn_engine_context_c.h

@@ -77,6 +77,7 @@ typedef struct EngineContext {
     bool window_size_has_changed;
     bool resync_flag;
     bool was_successful;
+    bool render_double_buffered;
 } EngineContext;
 
 /* TODO: does it need to be marked with TWN_API? */

src/twn_loop.c

@@ -658,6 +658,8 @@ static bool initialize(void) {
     }
     */
 
+    ctx.render_double_buffered = true;
+
     return true;
 
 fail: