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add a comment explaining why (and how) we keep SCREENWIDTH a multiple of 4

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Fabian Greffrath 2023-10-09 21:49:38 +02:00
parent 12ce2bbd65
commit 405f6c7683
1 changed files with 15 additions and 0 deletions
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15
src/i_video.c
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@ -930,6 +930,21 @@ void I_GetScreenDimensions(void)
SCREENWIDTH = w * ah / h;
// [crispy] make sure SCREENWIDTH is an integer multiple of 4 ...
// [FG] For performance reasons, SDL2 insists that the screen pitch,
// i.e. the *number of bytes* that one horizontal row of pixels
// occupy in memory, must be a multiple of 4.
// And for a paletted framebuffer with only one byte per pixel
// this means we need to keep the *number of pixels* a multiple of 4.
//
// Now, in widescreen mode, 240 px * 16 / 9 = 426.7 px.
// The widescreen status bar graphic of the Unity port is 426 px wide.
// This, however, is not a multiple of 4, so we have to *round up*
// to 428 px for it to fit on screen (with one blank px left and right).
// In hires mode, 480 px * 16 / 9 = 853.3 px.
// In order to fit the widescreen status bar graphic exactly twice on
// screen, we *round down* to 2 * 426 px = 852 px.
if (hires)
{
SCREENWIDTH = ((2 * SCREENWIDTH) & (int)~3) / 2;