love.graphics.newScreenshot replaces alpha with full opacity by default (issue #374, thanks Boolsheet)

Some additional overhead was added to newScreenshot because it doesn't use glPixelTransfer (not supported in ES2 / core GL3+, and like all GL functions has the slight potential to be buggy on some drivers). This choice might be revisited in the future.

src/modules/graphics/opengl/Graphics.cpp

@@ -1070,7 +1070,7 @@ void Graphics::polygon(DrawMode mode, const float *coords, size_t count)
 	}
 }
 
-love::image::ImageData *Graphics::newScreenshot(love::image::Image *image)
+love::image::ImageData *Graphics::newScreenshot(love::image::Image *image, bool copyAlpha)
 {
 	int w = getWidth();
 	int h = getHeight();
@@ -1084,16 +1084,21 @@ love::image::ImageData *Graphics::newScreenshot(love::image::Image *image)
 
 	glReadPixels(0, 0, w, h, GL_RGBA, GL_UNSIGNED_BYTE, pixels);
 
+	if (!copyAlpha)
+	{
+		// Replace alpha values with full opacity.
+		for (int i = 3; i < size; i += 4)
+			pixels[i] = 255;
+	}
+
 	// OpenGL sucks and reads pixels from the lower-left. Let's fix that.
 
 	GLubyte *src = pixels - row, *dst = screenshot + size;
 
 	for (int i = 0; i < h; ++i)
-	{
 		memcpy(dst-=row, src+=row, row);
-	}
 
-	love::image::ImageData *img = image->newImageData(w, h, (void *)screenshot);
+	love::image::ImageData *img = image->newImageData(w, h, (void *) screenshot);
 
 	delete [] pixels;
 	delete [] screenshot;

src/modules/graphics/opengl/Graphics.h

@@ -461,8 +461,9 @@ public:
 	/**
 	 * Creates a screenshot of the view and saves it to the default folder.
 	 * @param image The love.image module.
+	 * @param copyAlpha If the alpha channel should be copied or set to full opacity (255).
 	 **/
-	love::image::ImageData *newScreenshot(love::image::Image *image);
+	love::image::ImageData *newScreenshot(love::image::Image *image, bool copyAlpha = true);
 
 	void push();
 	void pop();

src/modules/graphics/opengl/wrap_Graphics.cpp

@@ -839,7 +839,8 @@ int w_getMaxPointSize(lua_State *L)
 int w_newScreenshot(lua_State *L)
 {
 	love::image::Image *image = luax_getmodule<love::image::Image>(L, "image", MODULE_IMAGE_T);
-	love::image::ImageData *i = instance->newScreenshot(image);
+	bool copyAlpha = luax_optboolean(L, 1, false);
+	love::image::ImageData *i = instance->newScreenshot(image, copyAlpha);
 	luax_newtype(L, "ImageData", IMAGE_IMAGE_DATA_T, (void *)i);
 	return 1;
 }