OpenGL 2DTexture from c++ unsigned byte array

Question

I'm trying to process a 2D array in a fragment shader - so to learn that, I started building my array:

int const _s = 512;
std::array<GLubyte,_s*_s*4> hitmap;
for(unsigned j = 0; j < _s; j++) {
  for(unsigned i = 0; i < _s; i+=4) {
   hitmap[i+j*_s] = j%256;                  //R     
   if(j>33 && j < 45) hitmap[i+j*_s] = 0;   //R
   hitmap[i+j*_s+1] = i%256;                //G
   if(i>33 && i < 45) hitmap[i+j*_s+1] = 0; //G
   hitmap[i+j*_s+2] = 0;                    //B
   hitmap[i+j*_s+3] = 255;                  //A
  }
}

And as a first step, just push that as a texture and display that on a surface.

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_MIRRORED_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_MIRRORED_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, _s, _s, 0, GL_RGBA, GL_UNSIGNED_BYTE, hitmap.data());

I'm drawing two triangles with following coordinates to get a square:

    // positions           // texture coords
     1.f,  1.f, 0.0f,      1.0f, 1.0f, // top right
     1.f, -1.f, 0.0f,      1.0f, 0.0f, // bottom right
    -1.f, -1.f, 0.0f,      0.0f, 0.0f, // bottom left
    -1.f,  1.f, 0.0f,      0.0f, 1.0f  // top left

And my fragment shader is pretty stupid:

#version 150 core
uniform sampler2D ourTexture;
out vec4 FragColor;
in vec2 TexCoord;
void main()
{
FragColor = texture(ourTexture, TexCoord);
};

(yes, I'm using a pretty old version here :/)

The result I get is the following:

There is some weird stuff right at the top, and the texture seems to repeat (although, not mirrored). I seriously can't figure out what I'm doing wrong - it looks like I don't provide enough data. I expect to get a 2x2 square pattern, given I'm mapping 512 px into the range of 256 via mod divisions.

Edit: Like this:

Rabbid76 · Accepted Answer · 2020-12-08 13:42:59Z

1

You've misunderstood the concept of GL_MIRRORED_REPEAT. See OpenGL wiki glTexParameter

GL_MIRRORED_REPEAT causes the s coordinate to be set to the fractional part of the texture coordinate if the integer part of s is even; if the integer part of s is odd, then the s texture coordinate is set to 1−frac(s), where frac(s) represents the fractional part of s.

This means that the texture is mirrored if the texture coordinates are in the range [1.0, 2.0], [3.0, 4.0], ...; If the texture coordinates are in range [0.0, 1.0], [2.0, 3.0], ... it will not be mirrored.
Since all of your texture coordinates are in the range [0.0, 1.0], nothing is mirrored at all.

Scale the the texture coordinates in the fragment shader to see the concept of GL_MIRRORED_REPEAT:

void main()
{
    FragColor = texture(ourTexture, TexCoord * 2.0);
};

There is also a mistake in the condition statement of the inner for-loop. The inner loop must run from 0 to _s*4 rather than form 0 to _s:

~~for(unsigned i = 0; i < _s; i+=4) {~~

for(unsigned i = 0; i < _s*4; i+=4) {

The size of a line in bytes is _s*4 and not _s:

int const _s = 512;
std::array<GLubyte,_s*_s*4> hitmap;
for(unsigned j = 0; j < _s; j++) {
    for(unsigned i = 0; i < _s*4; i+=4) {
        hitmap[i + j*_s*4] = j%256;                    //R     
        if(j>33 && j < 45) hitmap[i + j*_s*4] = 0;     //R
        hitmap[i + j*_s*4 + 1] = i%256;                //G
        if(i>33 && i < 45) hitmap[i + j*_s*4 + 1] = 0; //G
        hitmap[i + j*_s*4 + 2] = 0;                    //B
        hitmap[i + j*_s*4 + 3] = 255;                  //A
    }
}

edited Dec 8, 2020 at 13:42

answered Dec 8, 2020 at 12:57

Rabbid76

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5 Comments

user3895986 Over a year ago

Thank you for that clarification - but I'm still puzzled by the result I'm getting. I would expect to get a 2x2 "square" pattern, extending fully over the surface - w/o any repetition at all. But I do get a repeating pattern.

user3895986 Over a year ago

If I multiply the TexCoord*2.0 I get the texture mirrored in both directions - exactly what I would expect. I don't expect any mirroring in my initial example - the point which I don't understand is why my texture looks the way it looks. I'm building a 512x512 texture - and to scale that into a byte is just %256 (+a feature line). And I'm projecting that exactly once onto the whole surface. So why do I have this huge black area? (put a picture of what I'd expect as an edit into the post)

Rabbid76 Over a year ago

@user3895986 The size of a line in bytes is _s*4 and not _s. See the answer.

user3895986 Over a year ago

Thank you, yes, it was that indexing ... I feel very stupid now :)

Rabbid76 Over a year ago

@user3895986 I think you are not the first to make such a mistake (me too, for sure more than once). Thank you. You're welcome.

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