Hello,
In Mesa-3.0, glReadPixels doesn't always return the correct alpha values. The
bug is related to read_fast_rgba_pixels(). In cases where
(*ctx->Driver.ReadRGBASpan) calls read_color_span, the final returned alpha
values are all 255, even if there is a local enabled alpha buffer with other
values. I think a fix might be to in readpix.c change line 507 and beyond from:
for (row=0; row<readHeight; row++) {
(*ctx->Driver.ReadRGBASpan)(ctx, readWidth, srcX, srcY,
(void *) dest);
dest += rowLength * 4;
srcY++;
}
to:
for (row=0; row<readHeight; row++) {
(*ctx->Driver.ReadRGBASpan)(ctx, readWidth, srcX, srcY,
(void *) dest);
if (ctx->RasterMask & ALPHABUF_BIT) {
gl_read_alpha_span( ctx, readWidth, srcX, srcY, dest );
}
dest += rowLength * 4;
srcY++;
}
An example program that shows the problem on linux (assuming Mesa installed in
/opt) follows. It writes one pixel with rgba of 1, 2, 3, 4; reads the pixel
back, and prints the value. Compile with
gcc -g3 -ansi -pedantic -Wall -o temp temp.c -I/usr/X11R6/include \
-I/opt/include -L/usr/X11R6/lib -L/opt -lGL -lGLU -L/usr/X11R6/lib \
-lX11 -lXext -lXmu -lXt -lm
------------- bug.c --------
#include <stdio.h>
#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glx.h>
#include <X11/Xlib.h>
#include <assert.h>
XVisualInfo* choose_depth_buffer_visual(Display* display) {
int configuration[] = {GLX_RGBA,
GLX_RED_SIZE, 8,
GLX_GREEN_SIZE, 8,
GLX_BLUE_SIZE, 8,
GLX_ALPHA_SIZE, 8,
None};
static XVisualInfo* vi = 0;
if(!glXQueryExtension(display, 0, 0)) return 0;
vi = glXChooseVisual(display, DefaultScreen(display), configuration);
assert(vi);
return vi;
}
int main() {
Display* display;
XVisualInfo* vi;
GLXContext pixmap_context = 0;
Pixmap pixmap;
GLXPixmap glxpixmap;
unsigned buf;
unsigned char color[4] = {1, 2, 3, 4};
display = XOpenDisplay(0);
vi = choose_depth_buffer_visual(display);
pixmap = XCreatePixmap(display, RootWindow(display, vi->screen),
1, 1, vi->depth);
glxpixmap = glXCreateGLXPixmap(display, vi, pixmap);
pixmap_context = glXCreateContext(display, vi, 0, True);
glXMakeCurrent(display, glxpixmap, pixmap_context);
glDrawBuffer(GL_FRONT);
glClear(GL_COLOR_BUFFER_BIT);
/* Either way fails. */
#if 1
glColor4ubv(color);
glBegin(GL_POINTS);
glVertex3d(0.0, 0.0, 0.0);
glEnd();
#else
glDrawPixels(1, 1, GL_RGBA, GL_UNSIGNED_BYTE, color);
#endif
glPixelStorei(GL_PACK_ALIGNMENT, 1);
glReadBuffer(GL_FRONT);
glPixelTransferf(GL_RED_SCALE, 1.0);
glPixelTransferf(GL_GREEN_SCALE, 1.0);
glPixelTransferf(GL_BLUE_SCALE, 1.0);
glPixelTransferf(GL_ALPHA_SCALE, 1.0);
glReadPixels(0, 0, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, &buf);
glXDestroyGLXPixmap(display, glxpixmap);
glXDestroyContext(display, pixmap_context);
XFreePixmap(display, pixmap);
printf("buf: %0x.\nShould have gotten 04030201 (or maybe 01020304 on a
litte-endian machine).\n ", buf);
return 0;
}
---------------------
Instead of getting 04030201 you will likely get ff030201. With the "fix" I do
get 0430201.
Doug
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_______________________________________________
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