driver: Add (Graphics).HasHighPrecisionFlaot

This enables to determine whether vertices should be adjusted or
not.

Fixes #879

internal/driver/graphics.go

@@ -35,6 +35,7 @@ type Graphics interface {
 	VDirection() VDirection
 	NeedsRestoring() bool
 	IsGL() bool
+	HasHighPrecisionFloat() bool
 }
 
 type Image interface {

internal/graphicscommand/command.go

@@ -183,14 +183,15 @@ func (q *commandQueue) Flush() {
 		fmt.Println("--")
 	}
 
+	if theGraphicsDriver.HasHighPrecisionFloat() {
 		// Adjust texels.
-	// TODO: texelAdjustmentFactor can vary depends on the highp precisions (#879).
 		const texelAdjustmentFactor = 1.0 / 512.0
 		for i := 0; i < q.nvertices/graphics.VertexFloatNum; i++ {
 			s := q.dstSizes[i]
 			vs[i*graphics.VertexFloatNum+6] -= 1.0 / s.width * texelAdjustmentFactor
 			vs[i*graphics.VertexFloatNum+7] -= 1.0 / s.height * texelAdjustmentFactor
 		}
+	}
 
 	theGraphicsDriver.Begin()
 	for len(q.commands) > 0 {

internal/graphicsdriver/metal/driver.go

@@ -703,6 +703,10 @@ func (d *Driver) IsGL() bool {
 	return false
 }
 
+func (d *Driver) HasHighPrecisionFloat() bool {
+	return true
+}
+
 type Image struct {
 	driver  *Driver
 	width   int

internal/graphicsdriver/opengl/context.go

@@ -16,6 +16,7 @@ package opengl
 
 import (
 	"fmt"
+	"sync"
 
 	"github.com/hajimehoshi/ebiten/internal/graphics"
 	"github.com/hajimehoshi/ebiten/internal/thread"
@@ -49,6 +50,8 @@ type context struct {
 	lastViewportHeight int
 	lastCompositeMode  graphics.CompositeMode
 	maxTextureSize     int
+	highp              bool
+	highpOnce          sync.Once
 
 	t *thread.Thread
 
@@ -102,3 +105,13 @@ func (c *context) getMaxTextureSize() int {
 	}
 	return c.maxTextureSize
 }
+
+// highpPrecision represents an enough mantissa of float values in a shader.
+const highpPrecision = 23
+
+func (c *context) hasHighPrecisionFloat() bool {
+	c.highpOnce.Do(func() {
+		c.highp = c.getShaderPrecisionFormatPrecision() >= highpPrecision
+	})
+	return c.highp
+}

internal/graphicsdriver/opengl/context_desktop.go

@@ -493,6 +493,12 @@ func (c *context) maxTextureSizeImpl() int {
 	return size
 }
 
+func (c *context) getShaderPrecisionFormatPrecision() int {
+	// glGetShaderPrecisionFormat is not defined at OpenGL 2.0. Assume that desktop environments always have
+	// enough highp precision.
+	return highpPrecision
+}
+
 func (c *context) flush() {
 	_ = c.t.Call(func() error {
 		gl.Flush()

internal/graphicsdriver/opengl/context_js.go

@@ -73,6 +73,7 @@ var (
 	framebuffer_        = contextPrototype.Get("FRAMEBUFFER")
 	framebufferBinding  = contextPrototype.Get("FRAMEBUFFER_BINDING")
 	framebufferComplete = contextPrototype.Get("FRAMEBUFFER_COMPLETE")
+	highFloat           = contextPrototype.Get("HIGH_FLOAT")
 	linkStatus          = contextPrototype.Get("LINK_STATUS")
 	maxTextureSize      = contextPrototype.Get("MAX_TEXTURE_SIZE")
 	nearest             = contextPrototype.Get("NEAREST")
@@ -451,6 +452,12 @@ func (c *context) maxTextureSizeImpl() int {
 	return gl.Call("getParameter", maxTextureSize).Int()
 }
 
+func (c *context) getShaderPrecisionFormatPrecision() int {
+	c.ensureGL()
+	gl := c.gl
+	return gl.Call("getShaderPrecisionFormat", js.ValueOf(int(fragmentShader)), highFloat).Get("precision").Int()
+}
+
 func (c *context) flush() {
 	c.ensureGL()
 	gl := c.gl

internal/graphicsdriver/opengl/context_mobile.go

@@ -377,6 +377,12 @@ func (c *context) maxTextureSizeImpl() int {
 	return gl.GetInteger(mgl.MAX_TEXTURE_SIZE)
 }
 
+func (c *context) getShaderPrecisionFormatPrecision() int {
+	gl := c.gl
+	_, _, p := gl.GetShaderPrecisionFormat(mgl.FRAGMENT_SHADER, mgl.HIGH_FLOAT)
+	return p
+}
+
 func (c *context) flush() {
 	gl := c.gl
 	gl.Flush()

internal/graphicsdriver/opengl/driver.go

@@ -138,3 +138,7 @@ func (d *Driver) NeedsRestoring() bool {
 func (d *Driver) IsGL() bool {
 	return true
 }
+
+func (d *Driver) HasHighPrecisionFloat() bool {
+	return d.context.hasHighPrecisionFloat()
+}