UI: Previews for screen layouts

Basically all this does is drawing layout previews into the opened layout search menu.
https://youtu.be/RHYWtZP7pyA

The previews are drawn using offscreen rendering so they can't use multi-threading (yet!). But that shouldn't be an issue since only a handful of previews are drawn at the same time. Normally we only need to redraw the preview if a screen layout was changed. Would be nice if PreviewImage could store if it supports threaded rendering.
Previews are saved in files, might be useful if you later want to support appending layouts.
Adds a new file screen_draw.c.

1 files changed, 111 insertions, 0 deletions

diff --git a/source/blender/editors/screen/screen_draw.c b/source/blender/editors/screen/screen_draw.c
new file mode 100644
index 00000000000..4332112abe6
--- /dev/null
+++ b/source/blender/editors/screen/screen_draw.c
@@ -0,0 +1,111 @@
+/*
+ * ***** BEGIN GPL LICENSE BLOCK *****
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * ***** END GPL LICENSE BLOCK *****
+ */
+
+/** \file blender/editors/screen/screen_draw.c
+ *  \ingroup edscr
+ */
+
+#include "ED_screen.h"
+
+#include "GPU_framebuffer.h"
+#include "GPU_immediate.h"
+
+#include "WM_api.h"
+
+
+/**
+ * Calculates a scale factor to squash the preview for \a screen into a rectangle of given size and aspect.
+ */
+static void screen_preview_scale_get(
+        const bScreen *screen, float size_x, float size_y,
+        const float asp[2],
+        float r_scale[2])
+{
+	float max_x = 0, max_y = 0;
+
+	for (ScrArea *sa = screen->areabase.first; sa; sa = sa->next) {
+		max_x = MAX2(max_x, sa->totrct.xmax);
+		max_y = MAX2(max_y, sa->totrct.ymax);
+	}
+	r_scale[0] = (size_x * asp[0]) / max_x;
+	r_scale[1] = (size_y * asp[1]) / max_y;
+}
+
+static void screen_preview_draw_areas(const bScreen *screen, const float scale[2], const float col[4],
+                                      const float ofs_between_areas)
+{
+	const float ofs_h = ofs_between_areas * 0.5f;
+	unsigned int pos = add_attrib(immVertexFormat(), "pos", GL_FLOAT, 2, KEEP_FLOAT);
+	rctf rect;
+
+	immBindBuiltinProgram(GPU_SHADER_2D_UNIFORM_COLOR);
+	immUniformColor4fv(col);
+
+	for (ScrArea *sa = screen->areabase.first; sa; sa = sa->next) {
+		rect.xmin = sa->totrct.xmin * scale[0] + ofs_h;
+		rect.xmax = sa->totrct.xmax * scale[0] - ofs_h;
+		rect.ymin = sa->totrct.ymin * scale[1] + ofs_h;
+		rect.ymax = sa->totrct.ymax * scale[1] - ofs_h;
+
+		immBegin(PRIM_TRIANGLE_FAN, 4);
+		immVertex2f(pos, rect.xmin, rect.ymin);
+		immVertex2f(pos, rect.xmax, rect.ymin);
+		immVertex2f(pos, rect.xmax, rect.ymax);
+		immVertex2f(pos, rect.xmin, rect.ymax);
+		immEnd();
+	}
+
+	immUnbindProgram();
+}
+
+static void screen_preview_draw(const bScreen *screen, int size_x, int size_y)
+{
+	const float asp[2] = {1.0f, 0.8f}; /* square previews look a bit ugly */
+	/* could use theme color (tui.wcol_menu_item.text), but then we'd need to regenerate all previews when changing */
+	const float col[4] = {1.0f, 1.0f, 1.0f, 1.0f};
+	float scale[2];
+
+	wmOrtho2(0.0f, size_x, 0.0f, size_y);
+	/* center */
+	glTranslatef(size_x * (1.0f - asp[0]) * 0.5f, size_y * (1.0f - asp[1]) * 0.5f, 0.0f);
+
+	screen_preview_scale_get(screen, size_x, size_y, asp, scale);
+	screen_preview_draw_areas(screen, scale, col, 1.5f);
+}
+
+/**
+ * Render the preview for a screen layout in \a screen.
+ */
+void ED_screen_preview_render(const bScreen *screen, int size_x, int size_y, unsigned int *r_rect)
+{
+	char err_out[256] = "unknown";
+	GPUOffScreen *offscreen = GPU_offscreen_create(size_x, size_y, 0, err_out);
+
+	GPU_offscreen_bind(offscreen, true);
+	glClearColor(0.0, 0.0, 0.0, 0.0);
+	glClear(GL_COLOR_BUFFER_BIT);
+
+	screen_preview_draw(screen, size_x, size_y);
+
+	GPU_offscreen_read_pixels(offscreen, GL_UNSIGNED_BYTE, r_rect);
+	GPU_offscreen_unbind(offscreen, true);
+
+	GPU_offscreen_free(offscreen);
+}