diff --git a/internal/graphics/command.go b/internal/graphics/command.go
index d4ce3d2e0..cdd80d93b 100644
--- a/internal/graphics/command.go
+++ b/internal/graphics/command.go
@@ -32,8 +32,6 @@ import (
 type command interface {
 	Exec(indexOffsetInBytes int) error
 	NumVertices() int
-	AddNumVertices(n int)
-	CanMerge(dst, src *Image, mode opengl.CompositeMode, filter Filter) bool
 }
 
 // commandQueue is a command queue for drawing commands.
@@ -75,11 +73,12 @@ func (q *commandQueue) EnqueueDrawImageCommand(dst, src *Image, vertices []float
 	q.m.Lock()
 	q.appendVertices(vertices)
 	if 0 < len(q.commands) {
-		last := q.commands[len(q.commands)-1]
-		if last.CanMerge(dst, src, clr, mode, filter) {
-			last.AddNumVertices(len(vertices))
-			q.m.Unlock()
-			return
+		if c, ok := q.commands[len(q.commands)-1].(*drawImageCommand); ok {
+			if c.canMerge(dst, src, clr, mode, filter) {
+				c.nvertices += len(vertices)
+				q.m.Unlock()
+				return
+			}
 		}
 	}
 	c := &drawImageCommand{
@@ -228,10 +227,6 @@ func (c *drawImageCommand) NumVertices() int {
 	return c.nvertices
 }
 
-func (c *drawImageCommand) AddNumVertices(n int) {
-	c.nvertices += n
-}
-
 // split splits the drawImageCommand c into two drawImageCommands.
 //
 // split is called when the number of vertices reaches of the maximum and
@@ -246,7 +241,7 @@ func (c *drawImageCommand) split(quadsNum int) [2]*drawImageCommand {
 	return [2]*drawImageCommand{&c1, &c2}
 }
 
-// CanMerge returns a boolean value indicating whether the other drawImageCommand can be merged
+// canMerge returns a boolean value indicating whether the other drawImageCommand can be merged
 // with the drawImageCommand c.
 func (c *drawImageCommand) canMerge(dst, src *Image, clr *affine.ColorM, mode opengl.CompositeMode, filter Filter) bool {
 	if c.dst != dst {
@@ -302,13 +297,6 @@ func (c *replacePixelsCommand) NumVertices() int {
 	return 0
 }
 
-func (c *replacePixelsCommand) AddNumVertices(n int) {
-}
-
-func (c *replacePixelsCommand) CanMerge(dst, src *Image, mode opengl.CompositeMode, filter Filter) bool {
-	return false
-}
-
 // disposeCommand represents a command to dispose an image.
 type disposeCommand struct {
 	target *Image
@@ -330,13 +318,6 @@ func (c *disposeCommand) NumVertices() int {
 	return 0
 }
 
-func (c *disposeCommand) AddNumVertices(n int) {
-}
-
-func (c *disposeCommand) CanMerge(dst, src *Image, mode opengl.CompositeMode, filter Filter) bool {
-	return false
-}
-
 // newImageCommand represents a command to create an empty image with given width and height.
 type newImageCommand struct {
 	result *Image
@@ -379,13 +360,6 @@ func (c *newImageCommand) NumVertices() int {
 	return 0
 }
 
-func (c *newImageCommand) AddNumVertices(n int) {
-}
-
-func (c *newImageCommand) CanMerge(dst, src *Image, mode opengl.CompositeMode, filter Filter) bool {
-	return false
-}
-
 // newScreenFramebufferImageCommand is a command to create a special image for the screen.
 type newScreenFramebufferImageCommand struct {
 	result *Image
@@ -406,10 +380,3 @@ func (c *newScreenFramebufferImageCommand) Exec(indexOffsetInBytes int) error {
 func (c *newScreenFramebufferImageCommand) NumVertices() int {
 	return 0
 }
-
-func (c *newScreenFramebufferImageCommand) AddNumVertices(n int) {
-}
-
-func (c *newScreenFramebufferImageCommand) CanMerge(dst, src *Image, mode opengl.CompositeMode, filter Filter) bool {
-	return false
-}