Mark 512x400 mode as decent quality.

I originally flagged this as a poor quality mode because it doesn't
include every original pixel at least once (ie. its dimensions are
not >= 640x400 - twice the original resolution). In practise, it's
actually okay though.

Add long comment adapted from my comment on #339 explaining what
the definition of a poor quality mode is.

1 files changed, 28 insertions, 2 deletions

diff --git a/src/i_video.h b/src/i_video.h
index ff720922..4a85f93e 100644
--- a/src/i_video.h
+++ b/src/i_video.h
@@ -64,9 +64,35 @@ typedef struct
 
     boolean (*DrawScreen)(int x1, int y1, int x2, int y2);
 
-    // If true, this is a "poor quality" mode.  The autoadjust 
-    // code should always attempt to use a different mode to this 
+    // If true, this is a "poor quality" mode.  The autoadjust
+    // code should always attempt to use a different mode to this
     // mode in fullscreen.
+    //
+    // Some notes about what "poor quality" means in this context:
+    //
+    // The aspect ratio correction works by scaling up to the larger
+    // screen size and then drawing pixels on the edges between the
+    // "virtual" pixels so that an authentic blocky look-and-feel is
+    // achieved.
+    //
+    // For a mode like 640x480, you can imagine the grid of the
+    // "original" pixels spaced out, with extra "blurry" pixels added
+    // in the space between them to fill it out. However, when you're
+    // running at a resolution like 320x240, this is not the case. In
+    // the small screen case, every single pixel has to be a blurry
+    // interpolation of two pixels from the original image.
+    //
+    // If you run in 320x240 and put your face up close to the screen
+    // you can see this: it's particularly visible in the small yellow
+    // status bar numbers for example. Overall it still looks "okay"
+    // but there's an obvious - albeit small - deterioration in
+    // quality.
+    //
+    // Once you get to 640x480, all the original pixels are there at
+    // least once and it's okay (the higher the resolution, the more
+    // accurate it is). When I first wrote the code I was expecting
+    // that even higher resolutions would be needed before it would
+    // look acceptable, but it turned out to be okay even at 640x480.
 
     boolean poor_quality;
 } screen_mode_t;