OpenTTD: the OpenGL driver on GL 2.1, and SDL3's blue cast (2026-10-16 to 10-18)

Symptom. The research expected the GL mode to fail at init with no GPU risk, and it did: stock -v sdl-opengl refused to start on nv30 with “Single element texture formats not supported” (batch 2026-10-16-openttd-smokes). The software path started, but with a strong blue cast.

Cause.

• The OpenGL driver needs GL 3.0 or ARB_texture_rg for its single-channel textures; its VAO check wants ARB and APPLE vertex array objects below GL 3.0 (an upstream bug: Mesa has no APPLE one); and its sprite shaders use texture2DLod in the fragment stage, which needs ARB_shader_texture_lod (nv30 has no TXL).
• SDL3’s accelerated window framebuffer was BGRA8888 on big-endian, so OpenTTD’s 0xAARRGGBB words came out with red and blue swapped.

Fix.

• openttd 0001: the OpenGL driver on GL 2.1 without ARB_texture_rg or ARB_shader_texture_lod (GL_LUMINANCE8 for the single-channel textures, the VAO check fixed, no LOD in the shader), checked on llvmpipe forced to GL 2.1; 0002: the sprite recolour applied in the GLSL 1.10 sprite shader. The GL driver ran on the PS3 on 2026-10-18: 21 steady fps on the title screen (batch 2026-10-18-batch8).
• libsdl3 +ps3.242 fixed the colours but converted every frame twice on the CPU (about 2 fps, the audio starved); +ps3.254 (0002) uploads ARGB/XRGB8888 natively with GL_BGRA and 8_8_8_8_REV: 8 fps on the title game (batch 2026-10-17-ps3rsx-249-first-boot). The fix serves any SDL window-surface app.