library: fix vertex2d_xy y direction

src/library/doc.texi

@@ -1925,7 +1925,8 @@ or smaller than that area, proportionally.
 @subsection vertexxy
 
 Given an index of a vertex in a batch, returns its X and Y position on
-the screen, in pixel coordinates.
+the screen, in pixel coordinates. As with all pixel coordinates in the
+Bolt API, this is relative to the top-left pixel of the screen.
 
 @node batch2d-vertexatlasxy
 @subsection vertexatlasxy
@@ -1943,7 +1944,13 @@ its associated image in the batch's texture atlas, in pixel coordinates.
 @subsection vertexuv
 
 Given an index of a vertex in a batch, returns the vertex's associated
-"UV" coordinates.
+"UV" coordinates. The values will be floating-point numbers in the range
+0.0 - 1.0.
+
+Unlike with 3D render events, these UVs are relative to the entire
+texture atlas, not to the sub-image. However they may fall outside the
+bounds of the sub-image, in which case they're usually expected to wrap
+around within it.
 
 The values will be floating-point numbers in the range 0.0 - 1.0. They
 are relative to image in the texture atlas.
@@ -2202,8 +2209,10 @@ local x, y, w, h = event:atlasxywh(meta)
 
 Given a vertex number, returns the vertex's associated "UV" coordinates.
 
-The values will be floating-point numbers in the range 0.0 - 1.0. They
-are relative to image in the texture atlas.
+The values will be floating-point numbers, usually in the range 0.0 -
+1.0. They are relative to image in the texture atlas. They may fall
+outside the image (and therefore outside the range 0.0 - 1.0), in which
+case they're expected to wrap around within the image.
 
 @node render3d-vertexcolour
 @subsection vertexcolour

src/library/gl.c

@@ -1445,13 +1445,14 @@ void _bolt_gl_onDrawElements(GLenum mode, GLsizei count, GLenum type, const void
             vertex_userdata.atlas_size = &attributes[c->bound_program->loc_aTextureUVAtlasExtents];
             vertex_userdata.tex_uv = &attributes[c->bound_program->loc_aTextureUV];
             vertex_userdata.colour = &attributes[c->bound_program->loc_aVertexColour];
+            vertex_userdata.screen_height = roundf(2.0 / projection_matrix[5]);
 
             struct GLPluginTextureUserData tex_userdata;
             tex_userdata.tex = tex;
 
             struct RenderBatch2D batch;
             batch.screen_width = roundf(2.0 / projection_matrix[0]);
-            batch.screen_height = roundf(2.0 / projection_matrix[5]);
+            batch.screen_height = vertex_userdata.screen_height;
             batch.index_count = count;
             batch.vertices_per_icon = 6;
             batch.is_minimap = tex_target && tex_target->is_minimap_tex_small;
@@ -1687,11 +1688,13 @@ void _bolt_gl_onViewport(GLint x, GLint y, GLsizei width, GLsizei height) {
 
 static void _bolt_gl_plugin_drawelements_vertex2d_xy(size_t index, void* userdata, int32_t* out) {
     struct GLPluginDrawElementsVertex2DUserData* data = userdata;
-    if (!_bolt_get_attr_binding_int(data->c, data->position, data->indices[index], 2, out)) {
+    if (_bolt_get_attr_binding_int(data->c, data->position, data->indices[index], 2, out)) {
+        out[1] = (int32_t)data->screen_height - (out[1] + 1);
+    } else {
         float pos[2];
         _bolt_get_attr_binding(data->c, data->position, data->indices[index], 2, pos);
         out[0] = (int32_t)roundf(pos[0]);
-        out[1] = (int32_t)roundf(pos[1]);
+        out[1] = (int32_t)data->screen_height - ((int32_t)roundf(pos[1]) + 1);
     }
 }
 

src/library/gl.h

@@ -336,6 +336,7 @@ struct GLPluginDrawElementsVertex2DUserData {
     struct GLAttrBinding* atlas_size;
     struct GLAttrBinding* tex_uv;
     struct GLAttrBinding* colour;
+    uint32_t screen_height;
 };
 
 struct GLPluginDrawElementsVertex3DUserData {