graphicsdriver/opengl: Use glBufferSubData instead of glMapBuffer

We have confirmed that this does not slow down ReplacePixels.

Fixes #993
Fixes #1040

internal/graphicsdriver/opengl/context_desktop.go

@@ -548,22 +548,19 @@ func (c *context) newPixelBufferObject(width, height int) buffer {
 	return bf
 }
 
-func (c *context) mapPixelBuffer(buffer buffer) uintptr {
-	var ptr uintptr
+func (c *context) replacePixelsWithPBO(buffer buffer, t textureNative, width, height int, args []*driver.ReplacePixelsArgs) {
+	c.bindTexture(t)
 	_ = c.t.Call(func() error {
 		gl.BindBuffer(gl.PIXEL_UNPACK_BUFFER, uint32(buffer))
-		// Even though the buffer is partly updated, GL_WRITE_ONLY is fine.
-		// https://stackoverflow.com/questions/30248594/write-only-glmapbuffer-what-if-i-dont-write-it-all
-		ptr = gl.MapBuffer(gl.PIXEL_UNPACK_BUFFER, gl.WRITE_ONLY)
-		return nil
-	})
-	return ptr
-}
 
-func (c *context) unmapPixelBuffer(buffer buffer, t textureNative, width, height int) {
-	c.bindTexture(t)
-	_ = c.t.Call(func() error {
-		gl.UnmapBuffer(gl.PIXEL_UNPACK_BUFFER)
+		stride := 4 * width
+		for _, a := range args {
+			offset := 4 * (a.Y*width + a.X)
+			for j := 0; j < a.Height; j++ {
+				gl.BufferSubData(gl.PIXEL_UNPACK_BUFFER, offset+stride*j, 4*a.Width, gl.Ptr(a.Pixels[4*a.Width*j:4*a.Width*(j+1)]))
+			}
+		}
+
 		gl.TexSubImage2D(gl.TEXTURE_2D, 0, 0, 0, int32(width), int32(height), gl.RGBA, gl.UNSIGNED_BYTE, nil)
 		gl.BindBuffer(gl.PIXEL_UNPACK_BUFFER, 0)
 		return nil

internal/graphicsdriver/opengl/pbo_desktop.go

@@ -20,9 +20,6 @@
 package opengl
 
 import (
-	"reflect"
-	"unsafe"
-
 	"github.com/hajimehoshi/ebiten/internal/driver"
 )
 
@@ -37,23 +34,5 @@ func drawPixelsWithPBO(img *Image, args []*driver.ReplacePixelsArgs) {
 		panic("opengl: newPixelBufferObject failed")
 	}
 
-	mappedPBO := img.driver.context.mapPixelBuffer(img.pbo)
-	if mappedPBO == 0 {
-		panic("opengl: mapPixelBuffer failed")
-	}
-
-	var mapped []byte
-	sh := (*reflect.SliceHeader)(unsafe.Pointer(&mapped))
-	sh.Data = mappedPBO
-	sh.Len = 4 * w * h
-	sh.Cap = 4 * w * h
-
-	for _, a := range args {
-		stride := 4 * w
-		offset := 4 * (a.Y*w + a.X)
-		for j := 0; j < a.Height; j++ {
-			copy(mapped[offset+stride*j:offset+stride*j+4*a.Width], a.Pixels[4*a.Width*j:4*a.Width*(j+1)])
-		}
-	}
-	img.driver.context.unmapPixelBuffer(img.pbo, img.textureNative, w, h)
+	img.driver.context.replacePixelsWithPBO(img.pbo, img.textureNative, w, h, args)
 }