Fragment shaders on a texture - objective-c

Question

I am trying to add some post-processing capabilities to a program. The rendering is done using openGL. I just want to allow the program to load some home made fragment shader and use them on the video stream.
I wrote a little piece of shader using "OpenGL Shader Builder" that just turns a texture in grayscale. The shaders works well in the shader builder but I can't make it work in the main program. The screens stays all black.
Here is the setup :
#implementation PluginGLView
- (id) initWithCoder: (NSCoder *) coder
{
const GLubyte * strExt;
if ((self = [super initWithCoder:coder]) == nil)
return nil;
glLock = [[NSLock alloc] init];
if (nil == glLock) {
[self release];
return nil;
}
// Init pixel format attribs
NSOpenGLPixelFormatAttribute attrs[] =
{
NSOpenGLPFAAccelerated,
NSOpenGLPFANoRecovery,
NSOpenGLPFADoubleBuffer,
0
};
// Get pixel format from OpenGL
NSOpenGLPixelFormat* pixFmt = [[NSOpenGLPixelFormat alloc] initWithAttributes:attrs];
if (!pixFmt)
{
NSLog(#"No Accelerated OpenGL pixel format found\n");
NSOpenGLPixelFormatAttribute attrs2[] =
{
NSOpenGLPFANoRecovery,
0
};
// Get pixel format from OpenGL
pixFmt = [[NSOpenGLPixelFormat alloc] initWithAttributes:attrs2];
if (!pixFmt) {
NSLog(#"No OpenGL pixel format found!\n");
[self release];
return nil;
}
}
[self setPixelFormat:[pixFmt autorelease]];
/*
long swapInterval = 1 ;
[[self openGLContext]
setValues:&swapInterval
forParameter:NSOpenGLCPSwapInterval];
*/
[glLock lock];
[[self openGLContext] makeCurrentContext];
// Init object members
strExt = glGetString (GL_EXTENSIONS);
texture_range = gluCheckExtension ((const unsigned char *)"GL_APPLE_texture_range", strExt) ? GL_TRUE : GL_FALSE;
texture_hint = GL_STORAGE_SHARED_APPLE ;
client_storage = gluCheckExtension ((const unsigned char *)"GL_APPLE_client_storage", strExt) ? GL_TRUE : GL_FALSE;
rect_texture = gluCheckExtension((const unsigned char *)"GL_EXT_texture_rectangle", strExt) ? GL_TRUE : GL_FALSE;
// Setup some basic OpenGL stuff
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);
glColor4f(1.0f, 1.0f, 1.0f, 1.0f);
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);
// Loads the shaders
shader=LoadShader(GL_FRAGMENT_SHADER,"/Users/alexandremathieu/fragment.fs");
program=glCreateProgram();
glAttachShader(program, shader);
glLinkProgram(program);
glUseProgram(program);
[NSOpenGLContext clearCurrentContext];
[glLock unlock];
image_width = 1024;
image_height = 512;
image_depth = 16;
image_type = GL_UNSIGNED_SHORT_1_5_5_5_REV;
image_base = (GLubyte *) calloc(((IMAGE_COUNT * image_width * image_height) / 3) * 4, image_depth >> 3);
if (image_base == nil) {
[self release];
return nil;
}
// Create and load textures for the first time
[self loadTextures:GL_TRUE];
// Init fps timer
//gettimeofday(&cycle_time, NULL);
drawBG = YES;
// Call for a redisplay
noDisplay = YES;
PSXDisplay.Disabled = 1;
[self setNeedsDisplay:true];
return self;
}
And here is the "render screen" function which basically...renders the screen.
- (void)renderScreen
{
int bufferIndex = whichImage;
glBindTexture(GL_TEXTURE_RECTANGLE_EXT, bufferIndex+1);
glUseProgram(program);
int loc=glGetUniformLocation(program, "texture");
glUniform1i(loc,bufferIndex+1);
glTexSubImage2D(GL_TEXTURE_RECTANGLE_EXT, 0, 0, 0, image_width, image_height, GL_BGRA, image_type, image[bufferIndex]);
glBegin(GL_QUADS);
glTexCoord2f(0.0f, 0.0f);
glVertex2f(-1.0f, 1.0f);
glTexCoord2f(0.0f, image_height);
glVertex2f(-1.0f, -1.0f);
glTexCoord2f(image_width, image_height);
glVertex2f(1.0f, -1.0f);
glTexCoord2f(image_width, 0.0f);
glVertex2f(1.0f, 1.0f);
glEnd();
[[self openGLContext] flushBuffer];
[NSOpenGLContext clearCurrentContext];
//[glLock unlock];
}
and finally here's the shader.
uniform sampler2DRect texture;
void main() {
vec4 color, texel;
color = gl_Color;
texel = texture2DRect(texture, gl_TexCoord[0].xy);
color *= texel;
// Begin Shader
float gray=0.0;
gray+=(color.r + color.g + color.b)/3.0;
color=vec4(gray,gray,gray,color.a);
// End Shader
gl_FragColor = color;
}
The loading and using of shaders works since I am able to turn the screen all red with this shader
void main(){
gl_FragColor=vec4(1.0,0.0,0.0,1.0);
}
If the shader contains a syntax error I get an error message from the LoadShader function etc. If I remove the use of the shader, everything works normally.
I think the problem comes from the "passing the texture as a uniform parameter" thing. But these are my very firsts step with openGL and I can't be sure of anything.

Answer 1

Texture samplers have to be set to the number of the active texture unit. So for example with glActiveTexture(GL_TEXTURE3) the sampler must be set to 3 as well. In your case the number should be 0.