Fix for T53599: OpenGL render with transparent background

I had to make Eevee draw its scene in the scene pass (before it was doing it
in the background pass). This is not ideal since reference images require
a separation between scene and background.

But it's the best way to solve it now. Clay is working fine.

source/blender/draw/DRW_engine.h

@@ -95,6 +95,7 @@ void DRW_draw_render_loop_offscreen(
         struct Depsgraph *graph,
         struct RenderEngineType *engine_type,
         struct ARegion *ar, struct View3D *v3d,
+        const bool draw_background,
         struct GPUOffScreen *ofs);
 void DRW_draw_select_loop(
         struct Depsgraph *graph,

source/blender/draw/engines/eevee/eevee_engine.c

@@ -56,7 +56,7 @@ static void eevee_engine_init(void *ved)
 		/* Alloc transient pointers */
 		stl->g_data = MEM_callocN(sizeof(*stl->g_data), __func__);
 	}
-	stl->g_data->background_alpha = 1.0f;
+	stl->g_data->background_alpha = DRW_state_draw_background() ? 1.0f : 0.0f;
 	stl->g_data->valid_double_buffer = (txl->color_double_buffer != NULL);
 
 	DRWFboTexture tex = {&txl->color, DRW_TEX_RGBA_16, DRW_TEX_FILTER | DRW_TEX_MIPMAP};
@@ -414,8 +414,8 @@ DrawEngineType draw_engine_eevee_type = {
 	&eevee_cache_init,
 	&eevee_cache_populate,
 	&eevee_cache_finish,
+	NULL,
 	&eevee_draw_scene,
-	NULL, //&EEVEE_draw_scene
 	&eevee_view_update,
 	&eevee_id_update,
 };

source/blender/draw/intern/DRW_render.h

@@ -431,6 +431,7 @@ bool DRW_state_is_image_render(void);
 bool DRW_state_is_scene_render(void);
 bool DRW_state_show_text(void);
 bool DRW_state_draw_support(void);
+bool DRW_state_draw_background(void);
 
 struct DRWTextStore *DRW_state_text_cache_get(void);
 

source/blender/draw/intern/draw_manager.c

@@ -354,6 +354,7 @@ static struct DRWGlobalState {
 		unsigned int is_depth : 1;
 		unsigned int is_image_render : 1;
 		unsigned int is_scene_render : 1;
+		unsigned int draw_background : 1;
 	} options;
 
 	/* Current rendering context */
@@ -3416,7 +3417,10 @@ void DRW_draw_render_loop_ex(
 
 	/* Start Drawing */
 	DRW_state_reset();
+
+	if (DRW_state_draw_background()) {
 		drw_engines_draw_background();
+	}
 
 	/* WIP, single image drawn over the camera view (replace) */
 	bool do_bg_image = false;
@@ -3503,7 +3507,7 @@ void DRW_draw_render_loop(
 
 void DRW_draw_render_loop_offscreen(
         struct Depsgraph *graph, RenderEngineType *engine_type,
-        ARegion *ar, View3D *v3d, GPUOffScreen *ofs)
+        ARegion *ar, View3D *v3d, const bool draw_background, GPUOffScreen *ofs)
 {
 	RegionView3D *rv3d = ar->regiondata;
 
@@ -3517,6 +3521,7 @@ void DRW_draw_render_loop_offscreen(
 	/* Reset before using it. */
 	memset(&DST, 0x0, sizeof(DST));
 	DST.options.is_image_render = true;
+	DST.options.draw_background = draw_background;
 	DRW_draw_render_loop_ex(graph, engine_type, ar, v3d, NULL);
 
 	/* restore */
@@ -3817,6 +3822,17 @@ bool DRW_state_draw_support(void)
 	        ((v3d->flag2 & V3D_RENDER_OVERRIDE) == 0);
 }
 
+/**
+ * Whether we should render the background
+ */
+bool DRW_state_draw_background(void)
+{
+	if (DRW_state_is_image_render() == false) {
+		return true;
+	}
+	return DST.options.draw_background;
+}
+
 /** \} */
 
 

source/blender/editors/space_view3d/view3d_draw.c

@@ -2060,7 +2060,7 @@ void ED_view3d_draw_offscreen(
 		/* XXX, should take depsgraph as arg */
 		Depsgraph *depsgraph = BKE_scene_get_depsgraph(scene, view_layer, false);
 		BLI_assert(depsgraph != NULL);
-		DRW_draw_render_loop_offscreen(depsgraph, eval_ctx->engine_type, ar, v3d, ofs);
+		DRW_draw_render_loop_offscreen(depsgraph, eval_ctx->engine_type, ar, v3d, do_sky, ofs);
 	}
 
 	/* restore size */