Texture coordinates near 1 behave oddly

Question

I'm using (Py)OpenGL to display 256 colors indexed images. I use a shader together with a 1D texture containing the palette. Here's the Fragment shader code :

#version 330
uniform sampler2D texture; 
uniform sampler1D palette;

void main() 
{ 
    vec2 uv = gl_TexCoord[0].xy; 
    vec4 color = texture2D(texture, uv); 
    gl_FragColor = texture1D(palette, color.a) ;
}

To avoid rounding errors, all MAG and MIN filters are set to NEAREST.

The way I was seeing the texture coordinates for the 1D texture was :

• color 0 lies in interval [0 ; 1/256[
• color 1 lies in interval [1/256 ; 2/256[ ...
• color 255 lies in interval [255/256 ; 1[

I converted myself integer indexes to float between 0 and 1, to make sure what was happening, using the formula x_float = (x_int + .4)/256, that is x_float is inside the before mentioned intervals, a little before its center (to avoid the result being rounded at the wrong side of the interval).

But it doesn't work. I made a checkboard of 256 celles, with colors indexes 0 to 255 and a palette of levels of grey (from 0x000000 to 0xFFFFFF). The code is given below. I then made a screen snapshot and edited it in Paint.NET to look if colors were right, and noticed a jump at color 0xE0 : I get twice color 0xDF, and from this one, everything is shifted by one : last color is 0xFE instead of 0xFF.

I suspect some kind of rounding error, but don't see how...

Here's the complete code :

from OpenGL.GL import *
from OpenGL.GLU import *
from OpenGL.GLUT import *

from OpenGL.arrays import vbo
from OpenGL.GL import shaders
from numpy import *

def checkboard(size = 512, cell = 32):
    bitmap = zeros(size * size, 'u8')
    bitmap.shape = (size, size)
    current_color = 0

    for y in range(0, size, cell):
        for x in range(0, size, cell):
            bitmap[y : y + cell, x : x + cell] = current_color
            current_color += 1

    palette = array([[a, a, a, 255] for a in range(256)], 'u8')

    return bitmap, palette

def reshape(w, h):
    glutDisplayFunc(lambda: display(w, h))
    glutPostRedisplay();

glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE | GLUT_DEPTH)
glutInitWindowSize(512, 512)
glutCreateWindow("Hello World :'D")

### init code

# vbo
quad = (0.0, 0.0, 512.0, 512.0)
tex = (0., 0., 1., 1.)

my_vbo = vbo.VBO(
        array( [
            ( quad[0], quad[1], 0, tex[0], tex[1]),
            ( quad[0], quad[3], 0, tex[0], tex[3]),
            ( quad[2], quad[3], 0, tex[2], tex[3]),
            ( quad[2], quad[1], 0, tex[2], tex[1])
        ],'f,f,f,f,f')
    )

# texture
bitmap, palette = checkboard()
height, width = bitmap.shape
f_image = (array(bitmap, 'f') + .4) / 256.0

# Image to be displayed
image_id = glGenTextures(1)
glEnable(GL_TEXTURE_2D)
glBindTexture(GL_TEXTURE_2D, image_id)
glTexImage2D(GL_TEXTURE_2D, 0, GL_ALPHA, width, height, 0,
             GL_ALPHA, GL_FLOAT, f_image)
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST)
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST)
glActiveTexture(GL_TEXTURE0)

# palette
f_palette = (palette / float32(255))

palette_id = glGenTextures(1)
glEnable(GL_TEXTURE_1D)
glBindTexture(GL_TEXTURE_1D, palette_id)
glTexImage1D(GL_TEXTURE_1D, 0, GL_RGBA, 256, 0,
             GL_RGBA, GL_FLOAT, f_palette)
glTexParameterf(GL_TEXTURE_1D, GL_TEXTURE_MAG_FILTER, GL_NEAREST)
glTexParameterf(GL_TEXTURE_1D, GL_TEXTURE_MIN_FILTER, GL_NEAREST)
#glActiveTexture(GL_TEXTURE1)

# shaders
VERTEX_SHADER = shaders.compileShader("""#version 330

layout(location = 0) in vec4 position;
uniform vec2 offset ;

void main()
{
      gl_FrontColor = gl_Color;
      gl_TexCoord[0].xy = gl_MultiTexCoord0.xy;
      gl_Position = vec4((offset.x + position.x - 256) / 256, (256 - offset.y - position.y)/256, 0.0, 1.0);
}""", GL_VERTEX_SHADER)

FRAGMENT_SHADER = shaders.compileShader("""#version 330
    uniform sampler2D texture; 
    uniform sampler1D palette;

    void main() 
    { 
        vec2 uv = gl_TexCoord[0].xy; 
        vec4 color = texture2D(texture, uv); 
        gl_FragColor = texture1D(palette, color.a) ;
    }""", GL_FRAGMENT_SHADER)

shader = shaders.compileProgram(VERTEX_SHADER,FRAGMENT_SHADER)

# uniform variables
offset_uniform_loc = glGetUniformLocation(shader, "offset")
texture_uniform_loc = glGetUniformLocation(shader, 'texture' )
palette_uniform_loc = glGetUniformLocation(shader, 'palette' )


def display(w, h):
    """Render the geometry for the scene."""
    glViewport(0, 0, w, h)
    glMatrixMode(GL_PROJECTION)
    glLoadIdentity()
    glOrtho(0, w, 0, h, -1, 1)

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity()

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT)

    glEnable( GL_TEXTURE_2D )
    glActiveTexture( GL_TEXTURE0 )
    glBindTexture( GL_TEXTURE_2D, image_id)

    glEnable( GL_TEXTURE_1D )
    glActiveTexture( GL_TEXTURE1 )

    shaders.glUseProgram(shader)
    shaders.glUniform1i(texture_uniform_loc, 0)
    shaders.glUniform1i(palette_uniform_loc, 1)
    shaders.glUniform2f(offset_uniform_loc, 0, 0)

    try:
        my_vbo.bind()
        try:
            glEnableClientState(GL_VERTEX_ARRAY)
            glEnableClientState(GL_TEXTURE_COORD_ARRAY)
            glVertexPointer(3, GL_FLOAT, 20, my_vbo)
            glTexCoordPointer(2, GL_FLOAT, 20, my_vbo + 12)

            glBindTexture( GL_TEXTURE_1D, palette_id)
            glDrawArrays(GL_QUADS, 0, 4)
        finally:
            my_vbo.unbind()
            glDisableClientState(GL_TEXTURE_COORD_ARRAY)
            glDisableClientState(GL_VERTEX_ARRAY)
    finally:
        shaders.glUseProgram( 0 )

    glutSwapBuffers()


glutReshapeFunc(reshape)
glutIdleFunc(glutPostRedisplay)
glutMainLoop()

Answer 1

Use GL_CLAMP_TO_EDGE for your texture's GL_TEXTURE_WRAP_S.

This problem is occurring partly because the texture coordinate 1.0 references a location that is beyond the last texel's center. Clamp to edge will make it so that your coordinates are clamped to the centers of the edge texels in the S direction.

When textures are sampled, if a coordinate does not refer to an exact texel center then the filter mode and wrap behavior will dictate where the texel(s) are fetched from. By default OpenGL uses a repeat mode, so the nearest neighbor (closest texel center) to a texture coordinate close to 1.0 may come from the other side of your texture (repeat). With ordinary images you might not even notice this behavior, but when you wrap around to the other side of a lookup table the discontinuity can be very obvious.

Here is a diagram that illustrates this:

http://i.msdn.microsoft.com/dynimg/IC83860.gif

Notice how texture coordinate 1.0 actually refers to the boundary between palette entry 3 and 0?

The short version is if your range is from 0.0 to 1.0 then none of your texture coordinates are referencing texel centers and you can easily wind up sampling the wrong texel. You need to adjust your coordinates so that you are not using the texel boundary for each palette entry.

---

Alternatively, since you are using GLSL ≥ 130 you can use texelFetch (...) and skip all of this normalized texture coordinate nonsense altogether. If you want to get the palette entry for your texture using its a component, then try something like this:

gl_FragColor = texelFetch (palette, (int)(texture2D (texture, uv).a * 255.0), 0);

This will explicitly fetch the texel given by the integer index. You do not have to worry about nearest neighbor(s), wrap modes, filtering, etc.