I'm proposing that we eliminate the 9/4 horizontal aspect ratio correction code from CHDK as a start to cleaning up some of the 'gui' code. My reasoning behind this is outlined below; but in order to do this, the changes I am suggesting need to be tested on affected cameras, to ensure that my logic is correct and this won't cause other problems.

The affected cameras, and the changes I would like to get tested, are listed at the end. If anyone can try these changes and report back the results it would be appreciated.

In CHDK there is code that implements 'aspect ratio' correction for drawing to the screen bitmap buffer. This is controlled with the CAM_USES_ASPECT_CORRECTION, and ASPECT_XCORRECTION and ASPECT_YCORRECTION defines.

Currently no camera that I can see uses ASPECT_YCORRECTION so we can ignore that for now.

But there are 13 cameras that define the CAM_USES_ASPECT_CORRECTION setting and therefore use the ASPECT_XCORRECTION when drawing text, lines, rectangles, etc to the screen bitmap.

The screen bitmap buffer is what is used to display the OSD (CHDK and Canon). I'm not talking about the viewport buffer(s) here, only the screen bitmap. The screen bitmaps uses one byte per pixel which is translated via a palette lookup into RGB colors to display on the LCD.

From looking at the code it would seem that at some point Canon changed the screen bitmap hardware from being 360 x 240 pixels in size to being 720 x 240 pixels. One thing to note here is that in both cases the physical size of the screen bitmap does not match the 4:3 aspect ratio of the LCD, which simply means that screen bitmap pixels aren't square. In the case of the newer cameras they are in fact a lot taller than they are wide.

The CAM_USES_ASPECT_CORRECTION code is used to deal with these newer cameras by defining a 'virtual' screen width in CHDK (vid_get_bitmap_screen_width) and then setting multiple pixels in the screen bitmap for each CHDK 'virtual' pixel.

There are two different variations on this 'virtual' CHDK screen width.

The first variant, used by 7 of the 13 cameras, sets vid_get_bitmap_screen_width to 360 to match the older cameras and then doubles each pixel horizontally when rendering to the physical screen bitmap. This is controlled by setting ASPECT_XCORRECTION to (x<<1). This is relatively quick and efficient and matches how the older cameras with 360x240 bitmaps displayed things.

The cameras that use this are :- d10, g12, ixus100_sd780, ixus120_sd940, ixus95_sd1200, ixus980_sd990, and sx30.

The other 6 2 cameras set vid_get_bitmap_screen_width to 320 and then use the 9/4 scaling factor to render CHDK pixels into the screen bitmap. This is controlled by setting ASPECT_XCORRECTION to (((x<<3)+x)>>2). This means that each pixel that CHDK tries to draw to the screen bitmap has to be mapped to 2.25 physical pixels. Since you can't actually set 1/4 of a pixel there's a lot of work being done to do this scaling in the code and to make sure it doesn't leave holes. It does this at the expense of sometimes writing to the same screen bitmap address multiple times.

As far as I can tell this scaling only affects the CHDK OSD and menus - edge overlay, zebra, grids and games aren't affected. So the only real visible difference that I can see if that in the 9/4 scaling method the font is slightly wider and in the OSD squares are square and circles are round.

In my view this 9/4 scaling is not needed for any camera - what does it matter if the OSD & menus are slightly squashed horiontally, they were always like that on the older cameras anyway.

The cameras that use 9/4 scaling are :- g11, ~~s90, s95, sx130is, sx20~~ and sx200is.

To test out if the a camera currently using 9/4 scaling still works correctly with the X2 scaling the following changes should be made:
1. change the vid_get_bitmap_screen_width function to return 360 instead of 320
2. change ASPECT_XCORRECTION in platform_camera.h to ((x)<<1)
3. change ASPECT_GRID_XCORRECTION in platform_camera.h to (x)

Edit 24/5 Added change for ASPECT_GRID_XCORRECTION above so grids are scaled correctly.

Rebuild and test the result.
You should see:
- OSD & menu text is slightly narrower; but you now get 45 characters on each line instead of 40.
- Positions of OSD elements will shift left slightly.
- edge overlay, zebra, grids and games should still work the same.

Phil.