Support double width low resolution

README.md

@@ -25,6 +25,7 @@ Portable emulator of an Apple II+. Written in Go.
     - Text 40 columns
     - text 80 columns (Apple //e only)
     - Low-Resolution graphics
+    - Double-Width Low-Resolution graphics (Apple //e only)
     - High-Resolution graphics
     - Mixed mode
 - Displays:

screen.go

@@ -47,12 +47,7 @@ func Snapshot(a *Apple2) *image.RGBA {
 		if isHiResMode {
 			snap = snapshotHiResModeMonoShift(a, pageIndex, isMixMode, lightColor)
 		} else {
-			if isDoubleResMode {
-				//snap = snapshotLoResDoubleModeMono(a, false /*isSecondPage*/, isMixMode, lightColor)
-				snap = snapshotLoResModeMono(a, isSecondPage, isMixMode, lightColor)
-			} else {
-				snap = snapshotLoResModeMono(a, isSecondPage, isMixMode, lightColor)
-			}
+			snap = snapshotLoResModeMono(a, isDoubleResMode, isSecondPage, isMixMode, lightColor)
 		}
 
 		if isMixMode {

screenLoRes.go

@@ -6,8 +6,9 @@ import (
 )
 
 const (
-	loResPixelWidth  = charWidth * 2
-	loResPixelHeight = charHeight / 2
+	loResPixelWidth       = charWidth * 2
+	doubleLoResPixelWidth = charWidth
+	loResPixelHeight      = charHeight / 2
 )
 
 func getColorPatterns(light color.Color) [16][16]color.Color {
@@ -37,14 +38,18 @@ func getColorPatterns(light color.Color) [16][16]color.Color {
 
 }
 
-func snapshotLoResModeMono(a *Apple2, isSecondPage bool, isMixMode bool, light color.Color) *image.RGBA {
-	text, columns, lines := getActiveText(a, false, isSecondPage, false)
+func snapshotLoResModeMono(a *Apple2, isDoubleResMode bool, isSecondPage bool, isMixMode bool, light color.Color) *image.RGBA {
+	text, columns, lines := getActiveText(a, isDoubleResMode, isSecondPage, false)
 	if isMixMode {
 		lines -= textLinesMix
 	}
 	grLines := lines * 2
+	pixelWidth := loResPixelWidth
+	if isDoubleResMode {
+		pixelWidth = doubleLoResPixelWidth
+	}
 
-	size := image.Rect(0, 0, columns*loResPixelWidth, grLines*loResPixelHeight)
+	size := image.Rect(0, 0, columns*pixelWidth, grLines*loResPixelHeight)
 	img := image.NewRGBA(size)
 
 	patterns := getColorPatterns(light)
@@ -55,10 +60,12 @@ func snapshotLoResModeMono(a *Apple2, isSecondPage bool, isMixMode bool, light c
 			if l%2 == 0 {
 				grPixel = char & 0xf
 			}
-			offset := (c % 2) * 2 // 2 pixel offset for odd lores pixels, 0 for even pixels
+			// We place pixelWidth mono pixels per graphic pixel.
+			// The groups of 4 mono pixels need to be alligned with an offset to get plain surfaces
+			offset := (c * pixelWidth) % 4
 
-			// Insert the 14 half pixels required
-			for i := 0; i < loResPixelWidth; i++ {
+			// Insert the pixelWidth pixels required
+			for i := 0; i < pixelWidth; i++ {
 				v := patterns[grPixel][i+offset]
 				// Repeat the same color for 4 lines
 				for r := 0; r < loResPixelHeight; r++ {