[gthumb] image viewer: use a checked pattern as background for transparent images

[Paolo Bacchilega <paobac src gnome org>]

commit 038846658b0ca654a6d79f531b425d0687728448
Author: Paolo Bacchilega <paobac src gnome org>
Date:   Mon Apr 8 20:44:29 2019 +0200

    image viewer: use a checked pattern as background for transparent images

 gthumb/cairo-utils.c      | 34 ++++++++++++++++++++++++++++++++++
 gthumb/cairo-utils.h      |  1 +
 gthumb/gth-image-viewer.c |  6 +++++-
 3 files changed, 40 insertions(+), 1 deletion(-)
---
diff --git a/gthumb/cairo-utils.c b/gthumb/cairo-utils.c
index 6af4de1c..530ffbb0 100644
--- a/gthumb/cairo-utils.c
+++ b/gthumb/cairo-utils.c
@@ -1170,3 +1170,37 @@ _cairo_paint_grid (cairo_t               *cr,
 
        cairo_restore (cr);
 }
+
+
+cairo_pattern_t *
+_cairo_create_checked_pattern (int size)
+{
+       int              h_size;
+       cairo_surface_t *surface;
+       cairo_t         *ctx;
+       cairo_pattern_t *pattern;
+
+       h_size = size / 2;
+       surface = cairo_image_surface_create (CAIRO_FORMAT_RGB24, size, size);
+       ctx = cairo_create (surface);
+
+       cairo_set_source_rgba (ctx, 0.4, 0.4, 0.4, 1.0);
+       cairo_rectangle (ctx, 0, 0, h_size, h_size);
+       cairo_fill (ctx);
+       cairo_rectangle (ctx, h_size, h_size, h_size, h_size);
+       cairo_fill (ctx);
+
+       cairo_set_source_rgba (ctx, 0.5, 0.5, 0.5, 1.0);
+       cairo_rectangle (ctx, h_size, 0, h_size, h_size);
+       cairo_fill (ctx);
+       cairo_rectangle (ctx, 0, h_size, h_size, h_size);
+       cairo_fill (ctx);
+
+       pattern = cairo_pattern_create_for_surface (surface);
+       cairo_pattern_set_extend (pattern, CAIRO_EXTEND_REPEAT);
+
+       cairo_surface_destroy (surface);
+       cairo_destroy (ctx);
+
+       return pattern;
+}
diff --git a/gthumb/cairo-utils.h b/gthumb/cairo-utils.h
index c83d3b2e..83888114 100644
--- a/gthumb/cairo-utils.h
+++ b/gthumb/cairo-utils.h
@@ -265,5 +265,6 @@ void              _cairo_draw_slide                         (cairo_t
 void              _cairo_paint_grid                         (cairo_t               *cr,
                                                             cairo_rectangle_int_t *rectangle,
                                                             GthGridType            grid_type);
+cairo_pattern_t * _cairo_create_checked_pattern                    (int                    size);
 
 #endif /* CAIRO_UTILS_H */
diff --git a/gthumb/gth-image-viewer.c b/gthumb/gth-image-viewer.c
index 97214a63..923c3fc9 100644
--- a/gthumb/gth-image-viewer.c
+++ b/gthumb/gth-image-viewer.c
@@ -41,6 +41,7 @@
                               * delay use this delay instead. */
 #define STEP_INCREMENT  20.0  /* Scroll increment. */
 #define BLACK_VALUE 0.2
+#define CHECKED_PATTERN_SIZE 20
 
 
 enum {
@@ -72,6 +73,7 @@ struct _GthImageViewerPrivate {
 
        GthImage               *image;
        cairo_surface_t        *surface;
+       cairo_pattern_t        *background_pattern;
        GdkPixbufAnimation     *animation;
        int                     original_width;
        int                     original_height;
@@ -175,6 +177,7 @@ gth_image_viewer_finalize (GObject *object)
        _g_clear_object (&self->priv->iter);
        _cairo_clear_surface (&self->priv->iter_surface);
        _cairo_clear_surface (&self->priv->surface);
+       cairo_pattern_destroy (self->priv->background_pattern);
 
        G_OBJECT_CLASS (gth_image_viewer_parent_class)->finalize (object);
 }
@@ -1518,6 +1521,7 @@ gth_image_viewer_init (GthImageViewer *self)
 
        self->priv = gth_image_viewer_get_instance_private (self);
 
+       self->priv->background_pattern = _cairo_create_checked_pattern (CHECKED_PATTERN_SIZE);
        self->priv->is_animation = FALSE;
        self->priv->play_animation = TRUE;
        self->priv->cursor_visible = TRUE;
@@ -2660,7 +2664,7 @@ gth_image_viewer_paint_frame (GthImageViewer *self,
 
        /* background */
 
-       cairo_set_source_rgba (cr, 1.0, 1.0, 1.0, 1.0);
+       cairo_set_source (cr, self->priv->background_pattern);
        cairo_rectangle (cr,
                         self->image_area.x,
                         self->image_area.y,