Hi,
I am wondering if i am fully understanding how texture value
should be computed. I am refering here to section 3.8.13
of opengl specification and specialy to table 3.21.
My understanding is that when you got an RGB texture the
As = 1 but when computing you use Ap which equal to the
primary color component of incoming fragment if you are
texture 0, or to the previous A value if you are texture > 0
ie if you are texture 1 you use A from texture 0, if you are
texture 2 you use A from texture 1 right ?
According to fragment program extension, TEX, TXP, ... should
give you the right A value (Ap depending on which texture unit
you are using). I got the feeling that r300/r400 hardware doesn't
follow this. At least fglrx driver (one packaged in ubuntu edgy)
fail at pixel format test using TEX instruction. I also attach a
small app which should test that. If you launch it res should
be (255, 0, 0, 0) but i will be (255, 0, 0, 255). If anyone could
test that on a windows with r300 hardware.
I may also don't fully understand this part of the specification :)
Btw the Ap value seems a bit hard things to follow i guess that
ATI might have choose to save some silicon and simplify design
than to conform to this specification.
best,
Jerome Glisse
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define GL_GLEXT_PROTOTYPES
#include <GL/glut.h>
#define W 64
#define H 64
static GLubyte my_image[W*H*4];
static GLuint alphaBits;
#define USE_FRAG_PROG 1
void makeCheckImages(void)
{
int i, j, c;
for (i = 0; i < H; i++) {
for (j = 0; j < W; j++) {
c = ((((i&0x8)==0)^((j&0x8)==0)))*255;
c = 255;
my_image[j * 4 + i * W * 4 + 0] = (GLubyte) c;
my_image[j * 4 + i * W * 4 + 1] = (GLubyte) 0;
my_image[j * 4 + i * W * 4 + 2] = (GLubyte) 0;
my_image[j * 4 + i * W * 4 + 3] = (GLubyte) 0;
}
}
}
void init(void)
{
glGetIntegerv(GL_ALPHA_BITS, &alphaBits);
printf("GL_ALPHA_BITS: %d\n", alphaBits);
glDrawBuffer(GL_FRONT);
glReadBuffer(GL_FRONT);
glClearColor(0.0, 0.0, 0.0, 0.0);
makeCheckImages();
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, W, H, 0,
GL_RGBA, GL_UNSIGNED_BYTE, my_image);
#if USE_FRAG_PROG
{
GLuint prog;
static const char *progText =
"!!ARBfp1.0\n"
"TEX result.color, fragment.texcoord[0], "
"texture[0], 2D; \n"
"END \n"
;
if (!glutExtensionSupported("GL_ARB_fragment_program")) {
printf("Error: GL_ARB_fragment_program "
"not supported!\n");
exit(1);
}
glGenProgramsARB(1, &prog);
glBindProgramARB(GL_FRAGMENT_PROGRAM_ARB, prog);
glProgramStringARB(GL_FRAGMENT_PROGRAM_ARB,
GL_PROGRAM_FORMAT_ASCII_ARB,
strlen(progText),
(const GLubyte *) progText);
if (glGetError()) {
fprintf(stderr, "Bad fragment program, error: %s\n",
glGetString(GL_PROGRAM_ERROR_STRING_ARB));
exit(0);
}
}
#endif
}
void display(void)
{
GLubyte *res = (GLubyte *)malloc(W * H * 4);
glClear(GL_COLOR_BUFFER_BIT);
glViewport(0, 0, W, H);
glColor4f(0.0, 0.0, 0.0, 0.0);
glEnable(GL_TEXTURE_2D);
#if USE_FRAG_PROG
glEnable(GL_FRAGMENT_PROGRAM_ARB);
#endif
glBegin(GL_QUADS);
glTexCoord2f(0.0, 0.0); glVertex2f(-1.0, -1.0);
glTexCoord2f(1.0, 0.0); glVertex2f( 1.0, -1.0);
glTexCoord2f(1.0, 1.0); glVertex2f( 1.0, 1.0);
glTexCoord2f(0.0, 1.0); glVertex2f(-1.0, 1.0);
glEnd();
#if USE_FRAG_PROG
glDisable(GL_FRAGMENT_PROGRAM_ARB);
#endif
glDisable(GL_TEXTURE_2D);
glFlush();
// glutSwapBuffers();
glReadPixels(0, 0, W, H, GL_RGBA, GL_UNSIGNED_BYTE, res);
printf("pixel(0,0)=(%d, %d, %d, %d)\n",
my_image[0],my_image[1],
my_image[2],my_image[3]);
printf("->res(0,0)=(%d, %d, %d, %d)\n",
res[0],res[1],
res[2],res[3]);
free(res);
}
void reshape(int w, int h)
{
glLoadIdentity();
}
void keyboard(unsigned char key, int x, int y)
{
switch ( key ) {
case 27:
exit(0);
break;
default:
break;
}
}
void idle(void)
{
// glutPostRedisplay();
}
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_RGBA);
glutInitWindowSize(W, H);
glutCreateWindow("BUG 7235");
init();
glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutKeyboardFunc(keyboard);
glutIdleFunc(idle);
glutMainLoop();
return 0;
}
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