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Simple texture example

I loaded three simple bmp’s and blended them with a GLSL fragment shader:
out_Color = texture(sam0, ex_TexCoord)/3.0 + texture(sam1, ex_TexCoord)/3.0 + texture(sam2, ex_TexCoord)/3.0;

This was just to make sure I had a cross-platform texture example for SDL2 with OpenGL 4 (OpenGL ES 2 or 3) on (Windows, Linux, OS X, iOS, Android).  I loaded pixels from bmp file with this simple code ( ).  SLD2 doesn’t include any image loading functionality; it’s FAQ ( ) references SDL_image.  The uncompressed source code for SDL2_image-2.0.0 is 27.7 MB (31.3 MB on disk).  I may later integrate SDL2_image (or some other image loading library), but for now I am just using bmp’s.  Another option would be to use KTX file format (an official Khronos OpenGL & OpenGL ES texture file format) or DDS; this link ( ) says NVIDIA OpenGL driver supports GPU acceleration for compressed textures with DDS but not for KTX (as of Aug 14 2013).  Or I could even write code that converts from BMP (etc) to a custom format file, and code that loads the texture from that custom format file.

image image

Another relevant texture issue is “texture atlas” and “array texture”.  This is relevant to the example of a thumbnail viewer, because the folder could contain an arbitrary number of image files (png bmp etc).

My AMD Radeon HD 6520G (HP laptop) has a GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS value of 32 (checked on Windows 7).  According to, the Asus Nexus 7 has a value of 16 for GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS.  A naïve approach of using one texture unit per thumbnail would limit you to only one GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS thumbnails (loaded at a time).  To render more thumbnails (than GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS), you would require state changes.

A more sane approach would be to fit all the thumbnails on one texture with a texture atlas, or to use hardware support for array textures.  My AMD Radeon HD 6520G (HP laptop) has a value of 8192 for GL_MAX_ARRAY_TEXTURE_LAYERS.  According to ( ), array textures (EXT_texture_array) was core since OpenGL 3.0.  OpenGL ES 3.0 lists array textures as a new feature.  Unfortunately, OpenGL ES 2.0 does not appear to support array textures, except as an extension – see ( ).

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