Fix coordinate mapping in ScreenToGLCoords and GLToScreenCoords

Fixes #64.

mgl32/shapes.go

@@ -263,29 +263,30 @@ func ReticulateSplines(ranges [][][2]float32, cPoints [][][]Vec2, withLlamas boo
 // Transform from pixel coordinates to GL coordinates.
 //
 // This assumes that your pixel coordinate system considers its origin to be in the top left corner (GL's is in the bottom left).
-// The coordinates x and y may be out of the range [0,screenWidth] and [0,screeneHeight].
+// The coordinates x and y may be out of the range [0,screenWidth-1] and [0,screeneHeight-1].
 //
-// GL's coordinate system maps [screenWidth,0] to [1.0,1.0] and [0,screenHeight] to [-1.0,-1.0]. If x and y are out of the range, they'll still
-// be mapped correctly, just off the screen. (e.g. if y = 2*screenHeight you'll get -2.0 for yOut)
+// GL's coordinate system maps [screenWidth-1,0] to [1.0,1.0] and [0,screenHeight-1] to [-1.0,-1.0]. If x and y are out of the range, they'll still
+// be mapped correctly, just off the screen. (e.g. if y = 2*(screenHeight-1) you'll get -3.0 for yOut)
 //
 // This is similar to Unproject, except for 2D cases and much simpler (especially since an inverse may always be found)
 func ScreenToGLCoords(x, y int, screenWidth, screenHeight int) (xOut, yOut float32) {
-	xOut = 2.0*float32(x)/float32(screenWidth) - 1.0
-	yOut = -2.0*float32(y)/float32(screenHeight) + 1.0
+	xOut = 2.0*float32(x)/float32(screenWidth-1) - 1.0
+	yOut = -2.0*float32(y)/float32(screenHeight-1) + 1.0
 
 	return
 }
 
-// Transform from GL's proportional system to pixel coordinates
+// Transform from GL's proportional system to pixel coordinates.
+//
 // Assumes the pixel coordinate system has its origin in the top left corner. (GL's is in the bottom left)
 //
-// GL's coordinate system maps [screenWidth,0] to [1.0,1.0] and [0,screenHeight] to [-1.0,-1.0]. If x and y are out of the range, they'll still
-// be mapped correctly, just off the screen. (e.g. if y=-2.0, you'll get 2*screenHeight for yOut)
+// GL's coordinate system maps [screenWidth-1,0] to [1.0,1.0] and [0,screenHeight-1] to [-1.0,-1.0]. If x and y are out of the range, they'll still
+// be mapped correctly, just off the screen. (e.g. if y=-3.0, you'll get 2*(screenHeight-1) for yOut)
 //
 // This is similar to Project, except for 2D cases and much simpler
 func GLToScreenCoords(x, y float32, screenWidth, screenHeight int) (xOut, yOut int) {
-	xOut = int((x + 1.0) * float32(screenWidth) / 2.0)
-	yOut = int((y - 1.0) * float32(screenHeight) / 2.0)
+	xOut = int((x + 1.0) * float32(screenWidth-1) / 2.0)
+	yOut = int((1.0 - y) * float32(screenHeight-1) / 2.0)
 
 	return
 }

mgl32/shapes_test.go

@@ -0,0 +1,54 @@
+// Copyright 2018 The go-gl Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.package mgl64_test
+
+package mgl32
+
+import (
+	"testing"
+)
+
+func TestScreenToGLCoords(t *testing.T) {
+	// use a small screen size in order to minimize errors due to fp rounding.
+	const (
+		sw = 100
+		sh = 100
+	)
+	x, y := ScreenToGLCoords(0, sh-1, sw, sh)
+	if x != -1 {
+		t.Errorf("x = %f, expected -1.0", x)
+	}
+	if y != -1 {
+		t.Errorf("y = %f, expected -1.0", y)
+	}
+
+	x, y = ScreenToGLCoords(sw-1, 0, sw, sh)
+	if x != 1 {
+		t.Errorf("x = %f, expected 1.0", x)
+	}
+	if y != 1 {
+		t.Errorf("y = %f, expected 1.0", y)
+	}
+}
+
+func TestGLToScreenCoords(t *testing.T) {
+	const (
+		sw = 100
+		sh = 100
+	)
+	x, y := GLToScreenCoords(-1, -1, sw, sh)
+	if x != 0 {
+		t.Errorf("x = %d, expected 0", x)
+	}
+	if y != sh-1 {
+		t.Errorf("y = %d, expected %d", y, sh-1)
+	}
+
+	x, y = GLToScreenCoords(1, 1, sw, sh)
+	if x != sw-1 {
+		t.Errorf("x = %d, expected %d", x, sw-1)
+	}
+	if y != 0 {
+		t.Errorf("y = %d, expected 0", y)
+	}
+}

mgl64/shapes.go

@@ -265,29 +265,30 @@ func ReticulateSplines(ranges [][][2]float64, cPoints [][][]Vec2, withLlamas boo
 // Transform from pixel coordinates to GL coordinates.
 //
 // This assumes that your pixel coordinate system considers its origin to be in the top left corner (GL's is in the bottom left).
-// The coordinates x and y may be out of the range [0,screenWidth] and [0,screeneHeight].
+// The coordinates x and y may be out of the range [0,screenWidth-1] and [0,screeneHeight-1].
 //
-// GL's coordinate system maps [screenWidth,0] to [1.0,1.0] and [0,screenHeight] to [-1.0,-1.0]. If x and y are out of the range, they'll still
-// be mapped correctly, just off the screen. (e.g. if y = 2*screenHeight you'll get -2.0 for yOut)
+// GL's coordinate system maps [screenWidth-1,0] to [1.0,1.0] and [0,screenHeight-1] to [-1.0,-1.0]. If x and y are out of the range, they'll still
+// be mapped correctly, just off the screen. (e.g. if y = 2*(screenHeight-1) you'll get -3.0 for yOut)
 //
 // This is similar to Unproject, except for 2D cases and much simpler (especially since an inverse may always be found)
 func ScreenToGLCoords(x, y int, screenWidth, screenHeight int) (xOut, yOut float64) {
-	xOut = 2.0*float64(x)/float64(screenWidth) - 1.0
-	yOut = -2.0*float64(y)/float64(screenHeight) + 1.0
+	xOut = 2.0*float64(x)/float64(screenWidth-1) - 1.0
+	yOut = -2.0*float64(y)/float64(screenHeight-1) + 1.0
 
 	return
 }
 
-// Transform from GL's proportional system to pixel coordinates
+// Transform from GL's proportional system to pixel coordinates.
+//
 // Assumes the pixel coordinate system has its origin in the top left corner. (GL's is in the bottom left)
 //
-// GL's coordinate system maps [screenWidth,0] to [1.0,1.0] and [0,screenHeight] to [-1.0,-1.0]. If x and y are out of the range, they'll still
-// be mapped correctly, just off the screen. (e.g. if y=-2.0, you'll get 2*screenHeight for yOut)
+// GL's coordinate system maps [screenWidth-1,0] to [1.0,1.0] and [0,screenHeight-1] to [-1.0,-1.0]. If x and y are out of the range, they'll still
+// be mapped correctly, just off the screen. (e.g. if y=-3.0, you'll get 2*(screenHeight-1) for yOut)
 //
 // This is similar to Project, except for 2D cases and much simpler
 func GLToScreenCoords(x, y float64, screenWidth, screenHeight int) (xOut, yOut int) {
-	xOut = int((x + 1.0) * float64(screenWidth) / 2.0)
-	yOut = int((y - 1.0) * float64(screenHeight) / 2.0)
+	xOut = int((x + 1.0) * float64(screenWidth-1) / 2.0)
+	yOut = int((1.0 - y) * float64(screenHeight-1) / 2.0)
 
 	return
 }

mgl64/shapes_test.go

@@ -0,0 +1,56 @@
+// This file is generated from mgl32/shapes_test.go; DO NOT EDIT
+
+// Copyright 2018 The go-gl Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.package mgl64_test
+
+package mgl64
+
+import (
+	"testing"
+)
+
+func TestScreenToGLCoords(t *testing.T) {
+	// use a small screen size in order to minimize errors due to fp rounding.
+	const (
+		sw = 100
+		sh = 100
+	)
+	x, y := ScreenToGLCoords(0, sh-1, sw, sh)
+	if x != -1 {
+		t.Errorf("x = %f, expected -1.0", x)
+	}
+	if y != -1 {
+		t.Errorf("y = %f, expected -1.0", y)
+	}
+
+	x, y = ScreenToGLCoords(sw-1, 0, sw, sh)
+	if x != 1 {
+		t.Errorf("x = %f, expected 1.0", x)
+	}
+	if y != 1 {
+		t.Errorf("y = %f, expected 1.0", y)
+	}
+}
+
+func TestGLToScreenCoords(t *testing.T) {
+	const (
+		sw = 100
+		sh = 100
+	)
+	x, y := GLToScreenCoords(-1, -1, sw, sh)
+	if x != 0 {
+		t.Errorf("x = %d, expected 0", x)
+	}
+	if y != sh-1 {
+		t.Errorf("y = %d, expected %d", y, sh-1)
+	}
+
+	x, y = GLToScreenCoords(1, 1, sw, sh)
+	if x != sw-1 {
+		t.Errorf("x = %d, expected %d", x, sw-1)
+	}
+	if y != 0 {
+		t.Errorf("y = %d, expected 0", y)
+	}
+}