Water rendering glitches with SDL1

[RandomBrushes]

If building with SDL1 (the default), water surfaces (here on LibreQuake's lq_e1m4) can be rendered with apparently the wrong texture buffer:

fancy-water.mp4

(Integrated AMD Radeon Vega graphics, on Linux with Mesa drivers. Also happens on integrated Intel graphics, on Linux, with Mesa drivers.)

The SDL2 compile is unaffected and renders water surfaces properly.

[lavenderdotpet]

have also experienced this a few year back

screenshot of it happening on my machine

[andrey-budko]

I tried reproducing this glitch with some random changes. For example, this produces exactly the same effect as in the video above:

diff --git a/Quake/gl_warp.c b/Quake/gl_warp.c
index 3c6ea5e9..43bef9ef 100644
--- a/Quake/gl_warp.c
+++ b/Quake/gl_warp.c
@@ -240,7 +240,7 @@ void R_UpdateWarpTextures (void)
 			continue;
 
 		//render warp
-		GL_SetCanvas (CANVAS_WARPIMAGE);
+		//GL_SetCanvas (CANVAS_WARPIMAGE);
 		GL_Bind (tx->gltexture);
 		for (x=0.0; x<128.0; x=x2)
 		{

Or this:

diff --git a/Quake/gl_draw.c b/Quake/gl_draw.c
index d60db57e..4c2a9efb 100644
--- a/Quake/gl_draw.c
+++ b/Quake/gl_draw.c
@@ -724,7 +724,7 @@ void GL_SetCanvas (canvastype newcanvas)
 		break;
 	case CANVAS_WARPIMAGE:
 		glOrtho (0, 128, 0, 128, -99999, 99999);
-		glViewport (glx, gly+glheight-gl_warpimagesize, gl_warpimagesize, gl_warpimagesize);
+		glViewport (glx, gly+glheight, 0, 0);
 		break;
 	case CANVAS_CROSSHAIR: //0,0 is center of viewport
 		s = CLAMP (1.0f, scr_crosshairscale.value, 10.0f);

Have no idea why this actually happens, or how it could depend on the SDL version.