Screen coordinates

[Ashurek85]

Hi,

I have a couple of questions regarding the coordinate system and the default display output.

First, it looks like the coordinate system does not start at (0, 0). What is the actual origin of the coordinate system?

Empirically, I have found that (8, -16) seems to correspond to the top-left corner, while (312, -240) seems to correspond to the bottom-right corner. Is this expected? If so, could you clarify how the coordinate system is defined? This gives me an output resolution of 304x224. Shouldn't it be 320x224?

Second, when launching gngeo with the default configuration, I see vertical black bars on both the left and right sides of the screen. Is this the expected behavior with the default settings, or could it indicate a configuration or scaling issue?

In the image below, a 16x16 sprite is shown positioned at the edges of the visible area. The screenshot also shows the vertical black bars that appear on the left and right sides when running gngeo with the default configuration.

Thanks!

[dciabrin]

Hi,

I have a couple of questions regarding the coordinate system and the default display output.

First, it looks like the coordinate system does not start at (0, 0). What is the actual origin of the coordinate system?

So the hardware has a rendered/visible resolution of 320x224 pixels, although some details matter.

If you consider the fixed tiles only [1] you have 40x32 fixed tiles of 8x8 pixels, so 320x256 pixels worth. However, the first two horizontal rows (16 pixels total) and last two horizontal rows (16 additional pixels) are never rendered on screen.
In addition to that the leftmost and rightmost fixed tile columns are most of the time "black". Here, the hardware can render those tiles on screen, however most games render those two columns with black tiles. This is related to the fact that these columns are most of the time hidden on regular CRT TVs unlike arcade CRTs. So it is usually advised to leave those unused, and they are rendered black.

So in your example, the topmost visible coordinate would be 8,16.

Also, the tiles in RAM are laid out vertically. That is, the first tile in VRAM start on the topleft corner, the second tile in VRAM is directly the tile below. after 32 tiles, the next tile start from the top, one column to the right.

Second, when launching gngeo with the default configuration, I see vertical black bars on both the left and right sides of the screen. Is this the expected behavior with the default settings, or could it indicate a configuration or scaling issue?

So this reflects the fact that the emulator is correctly able to render all visible pixels, i.e. 320x224. The game themselves are responsible for hiding the leftmost and rightmost stripes of tiles.

If you run a rom in gngeo with the opengl blitter enabled (the one with pixel shaders), the render crops the leftmost and rightmost column, effectively only rendering 304*224 pixels. But practically, you have the full 320x224 pixels available for render if you so chose.

Hope that helps

[1] https://wiki.neogeodev.org/index.php/Fix_layer

[Ashurek85]

Thanks for the information.

Should I just accept the two vertical black bars on the left and right as normal and assume that the effective resolution is 304x224?

I've been using ngdevkit for a few weeks now, and I've been pleasantly surprised—I expected it to be a bit harder to use. You can tell a lot of work went into it.