https://bugs.freedesktop.org/show_bug.cgi?id=27901
Summary: GLSL cos/sin functions broken on Mesa R600 driver Product: Mesa Version: git Platform: All OS/Version: All Status: NEW Severity: normal Priority: medium Component: Drivers/DRI/R600 AssignedTo: dri-devel@lists.freedesktop.org ReportedBy: alain.perrot@gmail.com
As I was playing with OpenGL examples from Joe Groff's blog at http://duriansoftware.com/joe/An-intro-to-modern-OpenGL.-Table-of-Contents.h..., I noticed that the cos and sin GLSL functions are broken on the Mesa R600 driver. Other trigonometric functions may be broken as well.
The blog entries provide two versions of a sample program that blend two images together using GLSL. The blend factor is computed from a timestamp using the sin function.
There's no problem with the example from chapter 2 where the blend factor is computed on the CPU (source code at http://github.com/jckarter/hello-gl).
Mesa R600 driver fails to properly run the example from chapter 3 where the blend factor is computed on the GPU using the GLSL sin function. Mesa software renderer has no problem running this example (source code at http://github.com/jckarter/hello-gl-ch3).
Looking at GLSL 1.10 spec, R600 ISA doc and R600 driver code (function assemble_TRIG in r700_assembler.c), I guess there is two problems :
1. both the GLSL cos/sin functions and R600 hardware expects the angle to be specified in radians, but the assemble_TRIG function divides the specified angle by 2 * PI (the result is no more radians).
2. R600 hardware expects the angle to be in the range [-PI, PI] while GLSL does not specify a range, and the assemble_TRIG function does not clip the angle in the required range.
A quick and dirty hack to make the example from chapter 3 work with R600 driver is to clip the timestamp to the [-PI, PI] range and then to multiply it by 2 * PI :
--- hello-gl.c.old 2010-04-29 20:55:55.000000000 +0200 +++ hello-gl.c 2010-04-29 20:56:12.000000000 +0200 @@ -202,7 +202,10 @@ static void update_timer(void) { int milliseconds = glutGet(GLUT_ELAPSED_TIME); - g_resources.timer = (float)milliseconds * 0.001f; + g_resources.timer = fmodf((float)milliseconds * 0.001f, 2.0f * 3.1415926535897f); + if(g_resources.timer > 3.1415926535897f) + g_resources.timer -= 2.0f * 3.1415926535897f; + g_resources.timer *= 2.0f * 3.1415926535897f; glutPostRedisplay(); }
My config: RV670 (Radeon HD 3870) Kubuntu 10.04 "Lucid Lynx" 64-bit Linux kernel 2.6.34-rc5 from Ubuntu Mainline Kernel PPA libdrm, radeon, Mesa, Xorg from Xorg-edgers PPA