Store alpha preserve of a layer in ORA files

For compatibility with Krita and maybe other applications. In Drawpile,
alpha preserve is implicit depending on the blend mode, but e.g. in
Krita, it has to be enabled separately. This adds an
`alpha-preserve="true"` attribute to layers with affected blend mdoes so
that Krita can restore them properly, once the attribute is implemented
on their side.

Drawpile doesn't read the attribute, since it can't do anything with the
information, except for the special case of the Normal blend mode, which
becomes Recolor when alpha preserve is enabled. Normally this is saved
as a separate blend mode, but since it's a valid way to represent it, we
handle it too.

Author: askmeaboutlo0m

ChangeLog

@@ -38,6 +38,7 @@ Unreleased Version 2.2.0-pre
  * Fix: Show own user marker again when using the laser pointer.
  * Fix: Use previous avatar when using the reconnect button after a disconnect.
  * Fix: Don't trigger a cacophony of notifications after joining a session, just play a single notification once catchup is done instead.
+ * Feature: Store alpha preserve state of layers in ORA files for better Krita compatibility (will be in Krita 5.2.1 probably.)
 
 2023-09-30 Version 2.2.0-beta.8
  * Fix: Apply color wheel direction to color dialogs too. Thanks Blozzom for reporting.

src/drawdance/libengine/dpengine/load.c

@@ -422,6 +422,17 @@ static DP_TransientLayerProps *ora_make_layer_props(DP_XmlElement *element,
     DP_BlendMode blend_mode = DP_blend_mode_by_svg_name(
         DP_xml_element_attribute(element, NULL, "composite-op"),
         DP_BLEND_MODE_NORMAL);
+
+    // Normal with alpha preserve is Recolor. Drawpile doesn't save it this way,
+    // but it's a valid way to represent it, so we handle it.
+    if (blend_mode == DP_BLEND_MODE_NORMAL) {
+        const char *alpha_preserve =
+            DP_xml_element_attribute(element, NULL, "alpha-preserve");
+        if (DP_str_equal_lowercase(alpha_preserve, "true")) {
+            blend_mode = DP_BLEND_MODE_RECOLOR;
+        }
+    }
+
     DP_transient_layer_props_blend_mode_set(tlp, (int)blend_mode);
 
     const char *censored =

src/drawdance/libengine/dpengine/save.c

@@ -392,6 +392,15 @@ static void ora_write_layer_props_xml(DP_SaveOraContext *c, DP_Output *output,
                         DP_blend_mode_svg_name(blend_mode));
     }
 
+    // Drawpile doesn't itself need the alpha preserve property, since its alpha
+    // preserve behavior depends on the blend mode, but other software cares.
+    // The Recolor blend mode is saved as src-atop, which is alpha-preserving
+    // per its definition, so it doesn't get the extra attribute.
+    if (DP_blend_mode_preserves_alpha(blend_mode)
+        && blend_mode != DP_BLEND_MODE_RECOLOR) {
+        DP_OUTPUT_PRINT_LITERAL(output, " alpha-preserve=\"true\"");
+    }
+
     if (DP_layer_props_censored(lp)) {
         DP_OUTPUT_PRINT_LITERAL(output, " drawpile:censored=\"true\"");
     }

src/drawdance/libmsg/dpmsg/blend_mode.c

@@ -28,6 +28,7 @@
 #define DECREASE_OPACITY (1 << 2)
 #define INCREASE_OPACITY (1 << 3)
 #define BLEND_BLANK      (1 << 4)
+#define PRESERVES_ALPHA  (1 << 5)
 
 // The Krita name for the linear light blend mode contains a space, which isn't
 // supported in draw dabs messages, since they already use the curly brace body
@@ -66,63 +67,63 @@ static const DP_BlendModeAttributes mode_attributes[DP_BLEND_MODE_COUNT] = {
         },
     [DP_BLEND_MODE_MULTIPLY] =
         {
-            LAYER | BRUSH,
+            LAYER | BRUSH | PRESERVES_ALPHA,
             "DP_BLEND_MODE_MULTIPLY",
             "svg:multiply",
             "Multiply",
         },
     [DP_BLEND_MODE_DIVIDE] =
         {
-            LAYER | BRUSH,
+            LAYER | BRUSH | PRESERVES_ALPHA,
             "DP_BLEND_MODE_DIVIDE",
             "krita:divide",
             "Divide",
         },
     [DP_BLEND_MODE_BURN] =
         {
-            LAYER | BRUSH,
+            LAYER | BRUSH | PRESERVES_ALPHA,
             "DP_BLEND_MODE_BURN",
             "svg:color-burn",
             "Burn",
         },
     [DP_BLEND_MODE_DODGE] =
         {
-            LAYER | BRUSH,
+            LAYER | BRUSH | PRESERVES_ALPHA,
             "DP_BLEND_MODE_DODGE",
             "svg:color-dodge",
             "Dodge",
         },
     [DP_BLEND_MODE_DARKEN] =
         {
-            LAYER | BRUSH,
+            LAYER | BRUSH | PRESERVES_ALPHA,
             "DP_BLEND_MODE_DARKEN",
             "svg:darken",
             "Darken",
         },
     [DP_BLEND_MODE_LIGHTEN] =
         {
-            LAYER | BRUSH,
+            LAYER | BRUSH | PRESERVES_ALPHA,
             "DP_BLEND_MODE_LIGHTEN",
             "svg:lighten",
             "Lighten",
         },
     [DP_BLEND_MODE_SUBTRACT] =
         {
-            LAYER | BRUSH,
+            LAYER | BRUSH | PRESERVES_ALPHA,
             "DP_BLEND_MODE_SUBTRACT",
             "krita:subtract",
             "Subtract",
         },
     [DP_BLEND_MODE_ADD] =
         {
-            LAYER | BRUSH,
+            LAYER | BRUSH | PRESERVES_ALPHA,
             "DP_BLEND_MODE_ADD",
             "svg:plus",
             "Add",
         },
     [DP_BLEND_MODE_RECOLOR] =
         {
-            LAYER | BRUSH,
+            LAYER | BRUSH | PRESERVES_ALPHA,
             "DP_BLEND_MODE_RECOLOR",
             "svg:src-atop",
             "Recolor",
@@ -143,7 +144,7 @@ static const DP_BlendModeAttributes mode_attributes[DP_BLEND_MODE_COUNT] = {
         },
     [DP_BLEND_MODE_SCREEN] =
         {
-            LAYER | BRUSH,
+            LAYER | BRUSH | PRESERVES_ALPHA,
             "DP_BLEND_MODE_SCREEN",
             "svg:screen",
             "Screen",
@@ -157,70 +158,70 @@ static const DP_BlendModeAttributes mode_attributes[DP_BLEND_MODE_COUNT] = {
         },
     [DP_BLEND_MODE_LUMINOSITY_SHINE_SAI] =
         {
-            LAYER | BRUSH,
+            LAYER | BRUSH | PRESERVES_ALPHA,
             "DP_BLEND_MODE_LUMINOSITY_SHINE_SAI",
             "krita:luminosity_sai",
             "Luminosity/Shine (SAI)",
         },
     [DP_BLEND_MODE_OVERLAY] =
         {
-            LAYER | BRUSH,
+            LAYER | BRUSH | PRESERVES_ALPHA,
             "DP_BLEND_MODE_OVERLAY",
             "svg:overlay",
             "Overlay",
         },
     [DP_BLEND_MODE_HARD_LIGHT] =
         {
-            LAYER | BRUSH,
+            LAYER | BRUSH | PRESERVES_ALPHA,
             "DP_BLEND_MODE_HARD_LIGHT",
             "svg:hard-light",
             "Hard Light",
         },
     [DP_BLEND_MODE_SOFT_LIGHT] =
         {
-            LAYER | BRUSH,
+            LAYER | BRUSH | PRESERVES_ALPHA,
             "DP_BLEND_MODE_SOFT_LIGHT",
             "svg:soft-light",
             "Soft Light",
         },
     [DP_BLEND_MODE_LINEAR_BURN] =
         {
-            LAYER | BRUSH,
+            LAYER | BRUSH | PRESERVES_ALPHA,
             "DP_BLEND_MODE_LINEAR_BURN",
             "krita:linear_burn",
             "Linear Burn",
         },
     [DP_BLEND_MODE_LINEAR_LIGHT] =
         {
-            LAYER | BRUSH,
+            LAYER | BRUSH | PRESERVES_ALPHA,
             "DP_BLEND_MODE_LINEAR_LIGHT",
             "krita:linear light",
             "Linear Light",
         },
     [DP_BLEND_MODE_HUE] =
         {
-            LAYER | BRUSH,
+            LAYER | BRUSH | PRESERVES_ALPHA,
             "DP_BLEND_MODE_HUE",
             "svg:hue",
             "Hue",
         },
     [DP_BLEND_MODE_SATURATION] =
         {
-            LAYER | BRUSH,
+            LAYER | BRUSH | PRESERVES_ALPHA,
             "DP_BLEND_MODE_SATURATION",
             "svg:saturation",
             "Saturation",
         },
     [DP_BLEND_MODE_LUMINOSITY] =
         {
-            LAYER | BRUSH,
+            LAYER | BRUSH | PRESERVES_ALPHA,
             "DP_BLEND_MODE_LUMINOSITY",
             "svg:luminosity",
             "Luminosity",
         },
     [DP_BLEND_MODE_COLOR] =
         {
-            LAYER | BRUSH,
+            LAYER | BRUSH | PRESERVES_ALPHA,
             "DP_BLEND_MODE_COLOR",
             "svg:color",
             "Color",
@@ -311,6 +312,11 @@ bool DP_blend_mode_blend_blank(int blend_mode)
     return get_attributes(blend_mode)->flags & BLEND_BLANK;
 }
 
+bool DP_blend_mode_preserves_alpha(int blend_mode)
+{
+    return get_attributes(blend_mode)->flags & PRESERVES_ALPHA;
+}
+
 DP_BlendMode DP_blend_mode_by_svg_name(const char *svg_name,
                                        DP_BlendMode not_found_value)
 {

src/drawdance/libmsg/dpmsg/blend_mode.h

@@ -79,6 +79,8 @@ bool DP_blend_mode_can_decrease_opacity(int blend_mode);
 
 bool DP_blend_mode_blend_blank(int blend_mode);
 
+bool DP_blend_mode_preserves_alpha(int blend_mode);
+
 DP_BlendMode DP_blend_mode_by_svg_name(const char *svg_name,
                                        DP_BlendMode not_found_value);