Pass merge shaders combine multiple pass files to a single pass file or image. They have access to all pass files, but only a single sample location at a time. One of the input file may be the currently rendered scene instead of a pass file on disk; in this case the merge shader is called every time the renderer has completed one sample, such as when a lens shader or primary material shader returns.
Here is an example of a simple merge shader that loops over all samples and selects the frontmost, based on its Z depth:
DLLEXPORT int passmerge_version(void) {return(1);} DLLEXPORT miBoolean passmerge( miColor *dummy, miState *state, void *paras) { int best_p = -1, p; miScalar best_z = 1e10, *zp; miColor *src, *tar; for (p=0;; p++) { zp = mi_renderpass_access(state, p, miRC_IMAGE_Z); if (!zp) break; if (*zp > 0 && *zp < best_z) { best_z = *zp; best_p = p; } } if (best_p >= 0) { tar = mi_renderpass_access(state, -1, miRC_IMAGE_RGBA); src = mi_renderpass_access(state, best_p, miRC_IMAGE_RGBA); *tar = *src; if ((int)state->raster_x/3%5 == (int)state->raster_y/3%5) switch(best_p) { case 0: tar->a = tar->r = 1; break; case 1: tar->a = tar->g = 1; break; case 2: tar->a = tar->b = 1; break; default: tar->a = tar->r = tar->g = tar->b = 1; } } return(miTRUE); }
The shader first loops over all passes, reading the Z distance from the depth frame buffer ( miRC_IMAGE_RGBA). In pass rendering mode, the color and depth frame buffers always exist. When the lookup fails, the end of the pass list was reached and the loop terminates. Otherwise, for non-infinite depths (*zp = 0), the one closest to the camera is saved in the best_ variables.
When the closest sample was found, assuming one non-infinite distance was found, its color value is copied to the output ( tar), and diagonal white stripes are painted on top. Only the color frame buffer is written to. A better merge shader would write the entire sample, and perform a correct depth-sorted alpha blending loop. This is what the built-in default merge function does. It is very rarely necessary to override the built-in merge function with a custom one like this.
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