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probe/fbconfig: Texture-map a quadrangle using the "legacy" APIs

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* This was... obnoxious to figure out.  Need to set up a small
pile of state in the context, with very little in the way of
diagnostic advice when all you get for output is a white rectangle
and no texture.

  * The legacy (no shader!) APIs are based on the fixed-function
pipeline, and apparently not supported by certain OpenGL
implementations such as the Intel open-source Linux drivers.

  * Apparently, the "modern" way to do things is to set up a
"vertex buffer", store the coordinates there, and then use shaders
to do the geometry mangling and texture mapping.
This commit is contained in:
Alastair Bridgewater 2019-09-07 16:36:20 -04:00
parent 617f4554ba
commit 21a37af414
1 changed files with 54 additions and 3 deletions
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57
probe/fbconfig/fbconfig.c
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@ -6,6 +6,15 @@
#include <unistd.h>
#define CHECK_ERROR(context) \
do { \
int error = glGetError(); \
if (error) { \
fprintf(stderr, "GL error 0x%x while " context "\n", \
error); \
} \
} while (0)
typedef void (APIENTRY *DEBUGPROC)(GLenum source,
GLenum type,
GLuint id,
@ -86,16 +95,58 @@ int main(int argc, char *argv[])
glXMakeContextCurrent(dpy, glxWin, glxWin, context);
glViewport(0, 0, 256, 256);
glEnable(GL_DEBUG_OUTPUT);
glDebugMessageCallback(debug_message, NULL);
GLuint textures = 0;
glGenTextures(1, &textures);
glBindTexture(GL_TEXTURE_2D, textures);
CHECK_ERROR("making texture");
/* All texture operations function in terms of a "current
* texture". Set it here */
glBindTexture(GL_TEXTURE_2D, textures);
CHECK_ERROR("binding texture");
/* Prep a test texture image, grabbing a small chunk of the root
* window */
{
XImage *img = XGetImage(dpy, DefaultRootWindow(dpy), 0, 0, 100, 100, AllPlanes, ZPixmap);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 100, 100, 0, GL_RGBA, GL_UNSIGNED_BYTE, img->data);
CHECK_ERROR("loading texture");
XDestroyImage(img);
}
glEnable(GL_TEXTURE_2D);
/* Set up the display coordinate space as orthographic */
glOrtho(0.0d, 256.0d, 256.0d, 0.0d, -1.0d, 1.0d);
CHECK_ERROR("setting transforms");
/* If we don't set the mapping filters, we get a blank image */
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
/* Use the "legacy" (deprecated, possibly no longer supported by
* open-source drivers) "immediate mode" APIs to render a
* window-sized rectangle from the texture image */
glBegin(GL_QUADS);
/* Texture coordinates are from 0 to 1, output coordinates are
* from 0 to width/height */
glTexCoord2i(0, 0);
glVertex2i(0, 0);
glTexCoord2i(1, 0);
glVertex2i(256, 0);
glTexCoord2i(1, 1);
glVertex2i(256, 256);
glTexCoord2i(0, 1);
glVertex2i(0, 256);
glEnd();
CHECK_ERROR("rasterizing the quadrangle");
glClearColor(1.0f, 0.0f, 0.0f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);
glFlush();
CHECK_ERROR("flush");
glXSwapBuffers(dpy, glxWin);
sleep(10);