Question

The reason I’m asking this is that our app (The Elements) runs fine on a Droid and a Nexus One, our two test phones, but not correctly on our recently acquired Atrix 4G. What draws is a skewed version of what should draw, with all the colors being replaced with alternating lines of cyan, magenta, and yellow (approximately), which leads us to believe that one of the primary colors for the sand particles that should show up is missing based on which line it’s on. I’m sorry for the unclear description, we had images but since this account doesn’t have 10 reputation we couldn’t post them.

Here is the code of our gl.c file, which does the texturing and rendering:

/*
 * gl.c
 * --------------------------
 * Defines the gl rendering and initialization
 * functions appInit, appDeinit, and appRender.
 */

#include "gl.h"
#include <android/log.h>

unsigned int textureID;

float vertices[] =
{0.0f, 0.0f, 1.0f, 0.0f, 0.0f, 1.0f, 1.0f, 1.0f};
float texture[] =
{0.0f, 0.0f, 1.0f, 0.0f, 0.0f, 1.0f, 1.0f, 1.0f};
unsigned char indices[] =
{0, 1, 3, 0, 3, 2};
int texWidth = 1, texHeight = 1;


void glInit()
{
    //Set some properties
    glShadeModel(GL_FLAT);
    glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_FASTEST);

    //Generate the new texture
    glGenTextures(1, &textureID);
    //Bind the texture
    glBindTexture(GL_TEXTURE_2D, textureID);

    //Enable 2D texturing
    glEnable(GL_TEXTURE_2D);
    //Disable depth testing
    glDisable(GL_DEPTH_TEST);

    //Enable the vertex and coord arrays
    glEnableClientState(GL_VERTEX_ARRAY);
    glEnableClientState(GL_TEXTURE_COORD_ARRAY);


    //Set tex params
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);

    //Set up texWidth and texHeight texHeight, 0, GL_RGB, GL_UNSIGNED_BYTE, emptyPixels);
    //Free the dummy array
    free(emptyPixels);

    //Set the pointers
    glVertexPointer(2, GL_FLOAT, 0, vertices);
    glTexCoordPointer(2, GL_FLOAT, 0, texture);
}

void glRender()
{
    //Check for changes in screen dimensions or work dimensions and handle them
    if(dimensionsChanged)
    {
        vertices[2] = (float) screenWidth;
        vertices[5] = (float) screenHeight;
        vertices[6] = (float) screenWidth;
        vertices[7] = (float) screenHeight;

        texture[2] = (float) workWidth/texWidth;
        texture[5] = (float) workHeight/texHeight;
        texture[6] = (float) workWidth/texWidth;
        texture[7] = (float) workHeight/texHeight;

        glMatrixMode(GL_PROJECTION);
        glLoadIdentity();
        if (!flipped)
        {
            glOrthof(0, screenWidth, screenHeight, 0, -1, 1); //--Device
        }
        else
        {
            glOrthof(0, screenWidth, 0, -screenHeight, -1, 1); //--Emulator
        }

        dimensionsChanged = FALSE;
        zoomChanged = FALSE;
    }
    else if(zoomChanged)
    {
        texture[2] = (float) workWidth/texWidth;
        texture[5] = (float) workHeight/texHeight;
        texture[6] = (float) workWidth/texWidth;
        texture[7] = (float) workHeight/texHeight;

        zoomChanged = FALSE;
    }
    //__android_log_write(ANDROID_LOG_INFO, "TheElements", "updateview begin");
    UpdateView();
    //__android_log_write(ANDROID_LOG_INFO, "TheElements", "updateview end");

    //Clear the screen
    glClear(GL_COLOR_BUFFER_BIT);

    //Sub the work portion of the tex
    glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, workWidth, workHeight, GL_RGB, GL_UNSIGNED_BYTE, colors);

    //Actually draw the rectangle with the text on it
    glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_BYTE, indices);
}

Any ideas as to what the difference is between the Atrix 4G and other phones in terms of OpenGL or why our app is doing what it is in general are much appreciated! Thanks in advance.

Here is an example of what it looks like: https://i.stack.imgur.com/noX4z.jpg

Answer 1

I know it’s a bit late to reply, but the real reason you’re seeing the mixed colors is due to OpenGL’s scanline packing alignment — not due to any driver bug or power-of-two sized texture issue.

Your glTexSubImage2D call is sending data in GL_RGB format, so I’m guessing your colors buffer is 3 bytes per pixel. Odds are the Droid and Nexus One phones have a default pack alignment of 1, but the Tegra 2 defaults to an alignment of 4. This means your 3 byte array can become misaligned with what the driver expects after every scanline, and a byte or two will be skipped for the next scanline, resulting in the colors you see. The reason why this works with a power-of-two sized texture is because your buffer just happens to be aligned properly for the next scanline. Basically this is the same issue as loading BMPs, where each scanline has to be padded to 4 bytes, regardless of the bit depth of the image.

You can explicitly disable any alignment packing by calling glPixelStorei(GL_PACK_ALIGNMENT, 1);. Note that changing this only affects the way OpenGL interprets your texture data, so there is no rendering performance penalty for changing this value. When the texture is sent to the graphics subsystem, the scanlines are stored in whatever format is optimal for the hardware, but the driver still has to know how to unpack your data properly. However, since you are changing the texture data every frame, instead of the driver being able to do one memcpy() to upload the entire texture, it will have to do TextureHeight *memcpy()s in order to upload it. This is not likely to be a major bottleneck, but if you are looking for the best performance, you may want to query the driver’s default pack alignment on startup using glGetIntegerv(GL_PACK_ALIGNMENT, &align); and adjust your buffer accordingly at runtime.

Here’s the specification on glPixelStore() for reference.