StackOverflow2013

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POOpaque OpenGL textures have transparent border
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copied!<p>My problem concerns rendering text with OpenGL -- the text is rendered into a texture, and then drawn onto a quad. The trouble is that the pixels on the edge of the texture are drawn partially transparent. The interior of the texture is fine.</p> <p>I'm calculating the texture coordinates to hit the center of my texels, using NEAREST (non-)interpolation, setting the texture wrapping to CLAMP_TO_EDGE, and setting the projection matrix to place my vertices at the center of the viewport pixels. Still seeing the issue.</p> <p>I'm working on VTK with their texture utilities. These are the GL calls that are used to load the texture, as determined by stepping through with a debugger:</p> <pre><code>glGenTextures(1, &id); glBindTexture(GL_TEXTURE_2D, id); glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST ); glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST ); glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE); glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE); // Create and bind pixel buffer object here (not shown, lots of indirection in VTK)... glTexImage2D( GL_TEXTURE_2D, 0 , GL_RGBA, xsize, ysize, 0, format, GL_UNSIGNED_BYTE, 0); // Unbind PBO -- also omitted glBindTexture(GL_TEXTURE_2D, id); glAlphaFunc (GL_GREATER, static_cast<GLclampf>(0)); glEnable (GL_ALPHA_TEST); // I've also tried doing this here for premultiplied alpha, but it made no difference: glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA); glMatrixMode(GL_TEXTURE); glLoadIdentity(); </code></pre> <p>The rendering code:</p> <pre><code> float p[2] = ...; // point to render text at int imgDims[2] = ...; // Actual dimensions of image float width = ...; // Width of texture in image float height = ...; // Height of texture in image // Prepare the quad float xmin = p[0]; float xmax = xmin + width - 1; float ymin = p[1]; float ymax = ymin + height - 1; float quad[] = { xmin, ymin, xmax, ymin, xmax, ymax, xmin, ymax }; // Calculate the texture coordinates. float smin = 1.0f / (2.0f * (imgDims[0])); float smax = (2.0 * width - 1.0f) / (2.0f * imgDims[0]); float tmin = 1.0f / (2.0f * imgDims[1]); float tmax = (2.0f * height - 1.0f) / (2.0f * imgDims[1]); float texCoord[] = { smin, tmin, smax, tmin, smax, tmax, smin, tmax }; // Set projection matrix to map object coords to pixel centers // (modelview is identity) GLint vp[4]; glGetIntegerv(GL_VIEWPORT, vp); glMatrixMode(GL_PROJECTION); glPushMatrix(); glLoadIdentity(); float offset = 0.5; glOrtho(offset, vp[2] + offset, offset, vp[3] + offset, -1, 1); // Disable polygon smoothing. Why not, I've tried everything else? glDisable(GL_POLYGON_SMOOTH); // Draw the quad glColor4ub(255, 255, 255, 255); glEnableClientState(GL_VERTEX_ARRAY); glEnableClientState(GL_TEXTURE_COORD_ARRAY); glVertexPointer(2, GL_FLOAT, 0, points); glTexCoordPointer(2, GL_FLOAT, 0, texCoord); glDrawArrays(GL_QUADS, 0, 4); glDisableClientState(GL_TEXTURE_COORD_ARRAY); glDisableClientState(GL_VERTEX_ARRAY); // Restore projection matrix glMatrixMode(GL_PROJECTION); glPopMatrix(); </code></pre> <p>For debugging purposes, I've overwritten the outermost texels with red, and the next inner layer of texels with green (otherwise it's hard to see what's going on in the mostly-white text image).</p> <p>I've inspected the texture in-memory using gDEBugger, and it looks as expected -- bright red and green borders around the texture area (the extra empty space is padding to make its size a power of two). For reference:</p> <p><img src="https://i.stack.imgur.com/tKpyq.png" alt="Good pixels!"></p> <p>Here's what the final rendered image looks like (magnified 20x -- the black pixels are remnants of the text that was rendered under the debugging borders). Pale red border, but still a bold green inner border:</p> <p><img src="https://i.stack.imgur.com/iJT7i.png" alt="Bad pixels!"></p> <p>So it is just the outer edge of pixels that is affected. I'm not sure if it's color-blending or alpha-blending that's screwing things up, I'm at a loss. I've noticed that the corner pixels are twice as pale as the edge pixels, perhaps that's significant... Maybe someone here can spot the error?</p>

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