Re: [Mesa3d-users] SIGSEGV with glDrawPixels and COLOR_INDEX pixmap

[Brian P. <br...@va...>]

Christian Fremgen wrote:
> 
> Hi there!
> 
> I have written a little application under Windows 2000 which draws
> a color-indexed pixmap with predefined color-maps to the rendering
> context (provided by Qt), which is defined to use RGB.
> With the mentioned OS and its OpenGL-support it works
> really nice, so I tried to port it to Linux with Mesa-3.3.
> 
> The same code produces a SIGSEGV under Linux, when glDrawPixels
> is called.
> 
> A short code-description:
> initialization:
> ...
> float r[256], g[256], b[256];
> 
> // definition of the r, g and b colortable omitted
> 
> glPixelTransferi (GL_MAP_COLOR, TRUE);
> glPixelMapfv (GL_PIXEL_MAP_I_TO_R, 256, r);
> glPixelMapfv (GL_PIXEL_MAP_I_TO_G, 256, g);
> glPixelMapfv (GL_PIXEL_MAP_I_TO_B, 256, b);
> ...
> 
> painting:
> ....
> // pixelData is defined as: unsigned char* or Glubyte*
> glDrawPixels (512, 512, GL_COLOR_INDEX, GL_UNSIGNED_BYTE, pixelData);
> // never comes to this line.... but gets a SIGSEGV
> 
> After several hours of trying I tried the following in "painting":
> glPixelTransferi (GL_MAP_COLOR, FALSE);
> glDrawPixels (512, 512, GL_RED, GL_UNSIGNED_BYTE, pixelData);
> 
> and see.. it works this way... my whole information.. in red.. :)
> 
> The problem is that I need color-maps to do efficient manipulation of the
> colors
> in my pixelData. And it would be really nice to develop with Linux and
> Mesa-3.3
> and not with Windows.......


Well, I didn't get a segfault in my test, but I got a black image
instead of the expected result.  The problem was that Index->Index
mapping was happening by mistake, effectively converting all color
indexes to zero.

Here's a patch to fix the problem:


--- image.c.orig        Thu Oct  5 10:25:13 2000
+++ image.c     Thu Oct  5 10:14:12 2000
@@ -2281,7 +2281,7 @@
                               unpacking);
 
          if (applyTransferOps) {
-            if (ctx->Pixel.MapColorFlag) {
+            if (dstFormat == GL_COLOR_INDEX && ctx->Pixel.MapColorFlag) {
                _mesa_map_ci(ctx, n, indexes);
             }
             if (ctx->Pixel.IndexShift || ctx->Pixel.IndexOffset) {
@@ -2550,7 +2550,7 @@
                               unpacking);
 
          if (applyTransferOps) {
-            if (ctx->Pixel.MapColorFlag) {
+            if (dstFormat == GL_COLOR_INDEX && ctx->Pixel.MapColorFlag) {
                _mesa_map_ci(ctx, n, indexes);
             }
             if (ctx->Pixel.IndexShift || ctx->Pixel.IndexOffset) {


This next patch fixes a problem which prevented the fast-path
glDrawPixels operation from being used in this case:

--- drawpix.c.orig      Thu Oct  5 10:37:49 2000
+++ drawpix.c   Thu Oct  5 10:43:11 2000
@@ -122,11 +122,10 @@
        && ctx->ColorMatrix.type == MATRIX_IDENTITY
        && !ctx->Pixel.ColorTableEnabled
        && !ctx->Pixel.PostColorMatrixColorTableEnabled
-       && ctx->Pixel.PostConvolutionColorTableEnabled
+       && !ctx->Pixel.PostConvolutionColorTableEnabled
        && !ctx->Pixel.MinMaxEnabled
        && !ctx->Pixel.HistogramEnabled
        && ctx->Pixel.IndexShift==0 && ctx->Pixel.IndexOffset==0
-       && ctx->Pixel.MapColorFlag==0
        && ctx->Texture.ReallyEnabled == 0
        && unpack->Alignment==1
        && !unpack->SwapBytes
@@ -197,7 +196,8 @@
        * skip "skipRows" rows and skip "skipPixels" pixels/row.
        */
 
-      if (format==GL_RGBA && type==GL_UNSIGNED_BYTE) {
+      if (format==GL_RGBA && type==GL_UNSIGNED_BYTE
+          && ctx->Pixel.MapColorFlag==0) {
          if (ctx->Visual->RGBAflag) {
             GLubyte *src = (GLubyte *) pixels
                + (skipRows * rowLength + skipPixels) * 4;
@@ -224,7 +224,8 @@
          }
          return GL_TRUE;
       }
-      else if (format==GL_RGB && type==GL_UNSIGNED_BYTE) {
+      else if (format==GL_RGB && type==GL_UNSIGNED_BYTE
+               && ctx->Pixel.MapColorFlag==0) {
          if (ctx->Visual->RGBAflag) {
             GLubyte *src = (GLubyte *) pixels
                + (skipRows * rowLength + skipPixels) * 3;
@@ -250,7 +251,8 @@
          }
          return GL_TRUE;
       }
-      else if (format==GL_LUMINANCE && type==GL_UNSIGNED_BYTE) {
+      else if (format==GL_LUMINANCE && type==GL_UNSIGNED_BYTE
+               && ctx->Pixel.MapColorFlag==0) {
          if (ctx->Visual->RGBAflag) {
             GLubyte *src = (GLubyte *) pixels
                + (skipRows * rowLength + skipPixels);
@@ -291,7 +293,8 @@
          }
          return GL_TRUE;
       }
-      else if (format==GL_LUMINANCE_ALPHA && type==GL_UNSIGNED_BYTE) {
+      else if (format==GL_LUMINANCE_ALPHA && type==GL_UNSIGNED_BYTE
+               && ctx->Pixel.MapColorFlag==0) {
          if (ctx->Visual->RGBAflag) {
             GLubyte *src = (GLubyte *) pixels
                + (skipRows * rowLength + skipPixels)*2;


This will also be fixed in Mesa 3.4 (which I hope to release in a few weeks).

-Brian