different screen sizes with corona sdk - lua

Question

I am making my first iphone game using corona sdk and would like it to run on as many devices as possible (phones + tablets). However I am not sure how to deal with different screen sizes and resolutions. I developed my game for the iPhone 5 using corona simulator and it works fine on that device. When I tried it on lower resolution devices like the iPhone 4 I get 2 black rectangles on each side. I tried creating 2 different backgrounds with different resolutions and added this in my config.lua:
imageSuffix = {
["#2x"] = 2
}
However this does not seem to change anything... I am not sure what height and width I should set in content in the config.lua file and what heights and widths I should set for the backgrounds. I am sorry if these questions are stupid, I am just starting.
Thanks in advance!

Answer 1

It sounds like you need to fully read up on config files and dynamic scaling.
The question is a little to broad as such I suggest you read this article about "the ultimate config/modernizing the config".
Some screens are wider while others are more narrow. If we take
resolution out of the equation, its easier to visualize the screens.
Corona makes it easy to take resolution out of the picture using
Dynamic Scaling. With Dynamic Scaling, you can use a common set of
screen coordinates and Corona will automatically scale the text and
graphics for different resolution screens. It can scale upwards or
downwards depending on your starting point. It also can substitute
higher resolution images when it needs to scale up. This is all
managed by a Lua file in your project folder called config.lua.
Since available resolutions vary considerably, it’s helpful to use the
same scale for each device. It doesn’t matter if you’re on an iPhone
3GS at 320×480 or a Retina iPad at 1536×2048, the location (0,0)
represents the top-left corner and (320,480), in vertical portrait
mode, is the bottom-right corner. The screen center is (160,240).
Each point, in this case, is one pixel on a lower-resolution device
like the 3GS, which has a native screen resolution of 320×480, while
each point is four pixels on a Retina iPad. Don’t worry about the math
— Corona will handle it for you.
Source: http://coronalabs.com/blog/2012/12/04/the-ultimate-config-lua-file/
This goes through the creation of a config file that fully utilize the dynamic scaling and image scaling.
local aspectRatio = display.pixelHeight / display.pixelWidth
application = {
content = {
width = aspectRatio > 1.5 and 320 or math.ceil( 480 / aspectRatio ),
height = aspectRatio < 1.5 and 480 or math.ceil( 320 * aspectRatio ),
scale = "letterBox",
fps = 30,
imageSuffix = {
["#2"] = 1.8,
["#4"] = 3.6,
},
},
}