docs: UseImageMarker (#55)

* Add UseImageMarker manual
* Add a TimelineClip picture showing UseImageMarker
* Update StreamingImageSequencePlayableAsset inspector image
* Minor change to the inspector GUI of StreamingImageSequencePlayableAsset
* Add Timeline version recommendation in the top readme.

Author: sindharta

Documentation~/en/StreamingImageSequencePlayableAsset.md

@@ -43,11 +43,25 @@ For other ways for importing images, see [ImportingImages](ImportingImages.md)
 
 # Curve
 
-We can also change the timing of the playback by adjusting the curve of the asset.   
-To do this, open the curve section, and right click on the curve to start adding keys.
+The timing of the playback is determined by the curve of the asset, which can be adjusted by
+opening the curve section, right clicking on the curve to add keys, and moving them accordingly.
 
 ![StreamingImageSequenceCurve](../images/StreamingImageSequenceCurve.png)
 
+# UseImageMarker
+
+Each frame has UseImageMarker, which can be used to skip the image assigned to that particular frame, and show the last used image instead.
+
+![UseImageMarker](../images/UseImageMarker.png)
+
+Keyboard shortcuts:  
+
+1. u: toggle use/skip. This shortcut can be changed in the StreamingImageSequence category in the Shortcuts window.  
+1. left/right arrow: move to the prev/next UseImageMarker.
+
+> If the UseImageMarkers are not shown correctly, click on the PlayableDirector GameObject in the hierarchy window to refresh the timeline window.
+
+
 # Inspector
 ![StreamingImageSequencePlayableAsset](../images/StreamingImageSequencePlayableAsset.png)
 
@@ -56,10 +70,14 @@ To do this, open the curve section, and right click on the curve to start adding
 * **Folder**  
   The folder where the image files are located
 * **Images**  
-  The images inside the folder.
-  We can reorder the images by dragging the image file name up/down.
+  The images inside the folder. 
+  Can be reordered by dragging the image file name up/down.
 * **Reset Curve**.  
   Reset the curve timing in the PlayableAsset to be linear.
+* **Show UseImageMarkers**.  
+  Show/hide UseImageMarker of each frame.
+* **Reset UseImageMarkers**.  
+  Reset UseImageMarkers of all frames.
 
 
 

Documentation~/images/StreamingImageSequencePlayableAsset.png

@@ -3,6 +3,8 @@
 Streaming Image Sequence is a package for playing sequential image sequences in Unity Timeline 
 easily without making Unity 2D Sprites.
 
+**Timeline 1.4.x or above is recommended.**
+
 # Supported Platforms
 
 1. Windows

Documentation~/images/UseImageMarker.png

@@ -81,8 +81,9 @@ public override void OnInspectorGUI() {
 
             }
 
-            m_asset.SetUseImageMarkerVisibility(GUILayout.Toggle(m_asset.GetUseImageMarkerVisibility(), "Show Use Image Markers"));
-            if (GUILayout.Button("Reset Use Image Markers")) {
+            GUILayout.Space(15);
+            m_asset.SetUseImageMarkerVisibility(GUILayout.Toggle(m_asset.GetUseImageMarkerVisibility(), "Show UseImageMarkers"));
+            if (GUILayout.Button("Reset UseImageMarkers")) {
                 m_asset.ResetPlayableFrames();
             }
 

Documentation~/index.md

@@ -3,6 +3,8 @@
 Streaming Image Sequence is a package for playing sequential image sequences in Unity Timeline 
 easily without making Unity 2D Sprites.
 
+**Timeline 1.4.x or above is recommended.**
+
 # Other Languages
 - [日本語](README_JP.md)